Various reaction drives in M-Space and their fuel

[Thot]

M-Space has spaceship construction rules, but they do seem to be lacking fuel tanks... which is particularly sad if you wish to use chemical rockets or even hard SF reaction drives in your setting.

Has anyone done any work on this in order to get good numbers of Delta V and fuel percentage out of the M-Space spaceship construction system?

 

 

 

 

 

 

 

 

[Lloyd Dupont]

Chemical rocket won't get you anywhere in just one life time in the space between stars....

 

Other than that, for interplanetary near space, here is some interesting and relevant reading about chemical propulsion:
https://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html

 

And here some hard science speculation about space travel:
https://www.youtube.com/watch?v=EzZGPCyrpSU

 

Edited September 24, 2019 by Lloyd Dupont

[g33k]

10 hours ago, Lloyd Dupont said:

Chemical rocket won't get you anywhere in just one life time in the space between stars....

No, but ships needing separate drives (one for FTL and another for in-system maneuvering) is one of the frequent tropes of sci-fi.

  

[Lloyd Dupont]

Oh, yes, my bad, obviously!

You can be inspired by the NASA link, they say that 1/3rd of fighter mass is fuel.... That can give you some inspiration perhaps?

[Joerg]

49 minutes ago, g33k said:

No, but ships needing separate drives (one for FTL and another for in-system maneuvering) is one of the frequent tropes of sci-fi.

Yes, and reaction drives are common, but chemical rocket drives are extremely rare as they have bad exhaust temperatures/velocities compared to plasma - whether electromagnetically, direct fusion exhaust, or other such means. Then there are ion drives, gouvod for long term acceleration at low thrust rates, like system insertions to aid solar sail braking, or possibly braking by collecting solar wind with a buzzard ramjet.

Maneuver drives using chemical drives or "going teakettle" exhausting e.g. superheated steam or a hot flame might be useful in those applications, but the reaction mass to acceleration ratio is simply abysmal.

For a setting of my own, I have been looking at a slower-than-light application of the Alcubierre drive, using the warp-tunnel effect to "accelerate" to stellar speeds with manageable/survivable inertial accelerations.

[Thot]

15 hours ago, Joerg said:

 chemical rocket drives are extremely rare as they have bad exhaust temperatures/velocities compared to plasma - whether electromagnetically, direct fusion exhaust, or other such means. Then there are ion drives, gouvod for long term acceleration at low thrust rates, like system insertions to aid solar sail braking, or possibly braking by collecting solar wind with a buzzard ramjet.

Maneuver drives using chemical drives or "going teakettle" exhausting e.g. superheated steam or a hot flame might be useful in those applications, but the reaction mass to acceleration ratio is simply abysmal.

[...]

Given the heating and power problems all the alternatives produce, chemical rockets still are the best option for almost any application , even IF we ever solve the fusion problem on the ground. For a campaign of mine, I created an entire galactic setting where even the most advanced civilizations use chemical rockets, because the advantages of all the other systems aren't just worth the complications - but that was with a different system. (And yes,that implies renting rockets to lift you into space at the spaceports.)
But yes, it's terribly hard to do space travel. That's a given.

 

Anyway, does anyone have stats for fuel tanks (be they for chemical drives, fusion rockets, ion engines or whatever) for use in M-Space? I can do the math myself, but before I spend time on that, I'll rather check out what all the other people did.

 

Or do you guys just wing it? (But then, why bother with a spacecraft design system at all?)

 

 

 

 

Edited September 25, 2019 by Thot

[Joerg]

1 hour ago, Thot said:

Given the heating and power problems all the alternatives produce, chemical rockets still are the best option for almost any application , even IF we ever solve the fusion problem on the ground. For a campaign of mine, I created an entire galactic setting where even the most advanced civilizations use chemical rockets, because the advantages of all the other systems aren't just worth the complications - but that was with a different system. (And yes,that implies renting rockets to lift you into space at the spaceports.)
But yes, it's terribly hard to do space travel. That's a given.

Chemical rockets can be used as on-board attitude drives for passive propulsion (solar sails, probably powered by laser light, in case of doubt created by using the photosphere of a star as laser medium), but electrical drives (whether plasma or ion) are vastly superior for prolonged periods of acceleration or deceleration.

The equivalent of a railgun for high velocity plasma is little different from chemical thrusters. A one component plasma (or ion cloud) is easier to produce and to maintain than a mixture of chemical educts and products. Flames are messy (I use them e.g. in atomic absorption spectroscopy, under very regulated conditions, and they still cause trouble). Hot flames are messier.

 

1 hour ago, Thot said:

Anyway, does anyone have stats for fuel tanks (be they for chemical drives, fusion rockets, ion engines or whatever) for use in M-Space? I can do the math myself, but before I spend time on that, I'll rather check out what all the other people did.

Fuel and reaction mass tanks are cargo modules with piping. M-Space has no rules for external cargo pods (like massive fuel tanks for planetary take-offs that can be dumped once outside of the worst of the gravity well), but there is no conceptual reason not to allow them.

Fuel and/or reaction mass consumption isn't part of the M-Space ship design.

 

1 hour ago, Thot said:

Or do you guys just wing it? (But then, why bother with a spacecraft design system at all?)

The design system is there to provide some abstract concept about designing a major stage for a space opera game. The result is for providing a consensus among the players about the dimensions and capabilities of their spacecraft.

And yes, I wing a lot of it all the time, much like I wing e.g. horse stats.

[Thot]

32 minutes ago, Joerg said:

Fuel and/or reaction mass consumption isn't part of the M-Space ship design.

That was indeed my impression, but that's disappointing. Fuel is THE big issue with spacecraft.

Well, then I'll just have to use GURPS Spaceships, converted to metric. The stats will work with BRP with only slight conversion, I believe.

Edited September 25, 2019 by Thot

[Prinz Slasar]

In the setting ELEVATION for M-SPACE there are some rough assumptions for fuel and transport ranges.

The starships in this setting are fueled by Nebulium.

1. The nebulium orbs losing 3 grams of material for every LY the wormhole stretches. [p 42]

2. The fuel is enough for a travel up to 800 light years before the nebulium needs refueled [p 16]; the more grittier version of the setting reduces this to 5 wormholes at maximum jump distance.

3. ELEVATION adds fuel values to the hyperspace stat of its starships: hyperspace 1 [fuel for 100 LY per jump], hyperspace 2 [fuel for 400 LY per jump] and hyperspace 3 [fuel for 800 LY per jump]

 

 

[Joerg]

24 minutes ago, Thot said:

That was indeed my impression, but that's disappointing. Fuel is THE big issue with spacecraft.

The big issue with reaction drive spacecraft completely relying on internal energy sources.

The technology of M-Space clearly has soft SF gimmicks like artificial gravity and as a consequence to that opens to concepts like inertial dampeners, reactionless drives (or rather the dark matter equivalent of atmospheric drives*), or photonic drives.

I feel that the request is similar to a game about modern navy warfare to provide the logistics for trireme sailing.

24 minutes ago, Thot said:

Well, then I'll just have to use GURPS Starships, converted to metric. The stats will work with BRP with only slight conversion, I believe.

What kind of chemical fuel were you thinking about, anyway? The best reaction mass to mass and volume ratio comes with metallic hydrogen. Oxygen as reaction partner has rather bad performance as reaction mass, as has the ensuing H2O molecules or the OH radical (or molecule ion). You get the best force out of your hydrogen by shooting out the protons and electrons separately along linear accelerators. The trouble with metallic hydrogen is that you want to keep it really cool, and in a place where unplanned sudden expansion won't explode all of your ship. (Only fusion plasma and anti-matter containment are worse in this regard...)

For fluid media, bags of graphene might be sufficient to keep the stuff together at reasonable pressure, and they might be compressed with the help of magnets distributed over the membrane that allow to compress the bubble. These would require either maintenance drones or some multi layer concept to deal with micrometeorite impacts or similar punctures.

 

Another possibility for in-system maneuvering are tugs rendezvousing with your FTL ship, providing a form of tether and all the fuel for deceleration or acceleration, allowing your ship to retain that precious reaction mass for systems lacking such services. Other passive drives or wireless energy transfer would address a lot of fuel concerns, too.

 

*If the dark matter in the aggregation disk consists of point sources of gravity, those point sources could be targeted by artificial gravitation, and provide the medium/reaction mass against this the ship's actio=reactio push or pull.

Without artificial gravity, deck plans only make sense Expanse-style, with vertical ship designs. Rotating habitat rings need to slow down for course alterations and don't make sense for warships (sorry, Babylon 5 Terran Fleet) due to gyro effects (unless brutally used as gyros, which makes them unusable as habitats, at least during maneuvering).

 

 

 

[Thot]

Quote

I feel that the request is similar to a game about modern navy warfare to provide the logistics for trireme sailing.

Well, one could argue that a generic SF RPG should include support for more than one type of SF setting. Besides, it doesn't only lack chemical rocket fuel but also fusion rocket fuel, antimatter rocket fuel, even just reactor fuel for the average magic space drives. I had hoped someone might have done some work there. No problem if that isn't within the system's abilities, I'll just import a design system from another source.

Quote

What kind of chemical fuel were you thinking about, anyway?

Hydrogen, methane, kerosene or hydrazine, combined with liquid oxygen. Enough to give a ship a few thousand m/s of delta v. If you want faster travel than what this will allow for, use the FTL drive. If you don't have one, coast a lot, and do swing-by maneuvers, and possibly use the Oberth effect to your benefit.

Of course, in such a universe, space battles are boring artillery duels with little to no actual tactics once the shooting starts.

Quote

Without artificial gravity, deck plans only make sense Expanse-style, with vertical ship designs. Rotating habitat rings need to slow down for course alterations and don't make sense for warships (sorry, Babylon 5 Terran Fleet) due to gyro effects (unless brutally used as gyros, which makes them unusable as habitats, at least during maneuvering).

There are centrifugal artificial gravity system ideas that do work better, like rotating the entire ship along one axis, or enclosed centrifuges that are encased in stationary armor on the outside, or even using tethered fuel tanks as counterweights to rotate around a common center of mass with the main ship (this is particularly useful for ships in the classic "player ship size" range). As your chemical rocket ships ( I maintain they are the most likely ship to ever actually be built in significant numbers) coast a lot anyway, having to stop rotation for a while while changing course isn't a big deal, because  you won't do that often.

 

 

Edited September 25, 2019 by Thot

[Thot]

53 minutes ago, prinz.slasar said:

In the setting ELEVATION for M-SPACE there are some rough assumptions for fuel and transport ranges.

The starships in this setting are fueled by Nebulium.

1. The nebulium orbs losing 3 grams of material for every LY the wormhole stretches. [p 42]

2. The fuel is enough for a travel up to 800 light years before the nebulium needs refueled [p 16]; the more grittier version of the setting reduces this to 5 wormholes at maximum jump distance.

3. ELEVATION adds fuel values to the hyperspace stat of its starships: hyperspace 1 [fuel for 100 LY per jump], hyperspace 2 [fuel for 400 LY per jump] and hyperspace 3 [fuel for 800 LY per jump]

 

 

But that is for the FTL side of things. That's "magic" anyway, so one can easily make up any number one likes. Which is cool, but not helping with the question.

[pachristian]

If you are looking for science in your science fiction, and want somewhat realistic spacecraft, check out this web site:

http://www.projectrho.com/public_html/rocket/

[Joerg]

13 hours ago, Thot said:

Well, one could argue that a generic SF RPG should include support for more than one type of SF setting. Besides, it doesn't only lack chemical rocket fuel but also fusion rocket fuel, antimatter rocket fuel, even just reactor fuel for the average magic space drives.

You'll end up with vastly different ship designs when you don't allow for artificial gravity.

I think that acceleration assist is a standard service in developed systems, whether it is a railgun service to get ships at initial vees, lightsails and lasers, or pellets of fuel shot into a capture mechanism (by railgun) using the slowing of the pellet as additional delta v. If it uses containers, you might even shoot them back (by railgun) as reaction mass. (And yes, that makes for quite busy space lanes.)

Military vessels might wish to operate without these considerations, and exploration of new systems doesn't offer any such comforts (leaving this as a priority job to install some assist systems).

But then, entering a vessel that is getting shot at might be something one leaves for AIs which have backups if sentient.

 

13 hours ago, Thot said:

I had hoped someone might have done some work there. No problem if that isn't within the system's abilities, I'll just import a design system from another source.

The ship design is tailored to the tech tree assumption, and to the top end of the tech tree assumption, as such technology tends to be distributed to less advanced places of origin by trade and/or theft.

Fuel is just one aspect that may get glossed over. Heat management, armor (or shielding) against micrometeorites and cosmic radiation are another aspect usually neglected (especially the fact that any armor that is capable of stopping a micrometeorite will create significant amounts of hard braking radiation).

Force fields are known to science, but they have infinited range (if at extremely low intensity) and no discrete edges. If you want something like fields that extend only so far, you need a medium, e.g. a plasma (which could produce an electric flow around the vessel which might be sufficient to dampen the amount of cosmic radiation hitting the vessel somewhat, and will interact very briefly with incoming material objects, too.

 

 

13 hours ago, Thot said:

Hydrogen, methane, kerosene or hydrazine, combined with liquid oxygen. Enough to give a ship a few thousand m/s of delta v. If you want faster travel than what this will allow for, use the FTL drive. If you don't have one, coast a lot, and do swing-by maneuvers, and possibly use the Oberth effect to your benefit.

Like I said, hydrogen plasma exhaust gets better exhaust velocities, and exhaust velocity is where the rocket equation gets power from.

 

13 hours ago, Thot said:

Of course, in such a universe, space battles are boring artillery duels with little to no actual tactics once the shooting starts.

That changes when your ammunition gets "intelligent". The difference between the long range battleship combat of WW1 and WW2 and the Falkland conflict rocket attacks.

 

13 hours ago, Thot said:

There are centrifugal artificial gravity system ideas that do work better, like rotating the entire ship along one axis, or enclosed centrifuges that are encased in stationary armor on the outside, or even using tethered fuel tanks as counterweights to rotate around a common center of mass with the main ship (this is particularly useful for ships in the classic "player ship size" range).

The tethered countermass rotation is the only viable options for vehicles you can push to more than walking speed with chemical rockets, causing significant fuel expenditure to build up/slow down the rotation.

You don't want centrifugal acceleration with nauseous speed differentials just for lifting your head. Rotational velocities for O'Neill cylinders are expected to be tolerable in this respect (according to Isaac Arthur's excellent videos).

 

13 hours ago, Thot said:

As your chemical rocket ships ( I maintain they are the most likely ship to ever actually be built in significant numbers) coast a lot anyway, having to stop rotation for a while while changing course isn't a big deal, because  you won't do that often.

If you are in for long coasting, I maintain that Ion drives and assisted initial velocity is way more likely. Booster rockets are a form of assisted initial velocity source, btw. So are "ballistic" fuel tanks that go active for rendezvous with your craft, or use shorter range matter "beams" to transfer the payload.

I think that the fission-reactor powered atomic rocket will be built for interplanetary travel until ship-sized internal fusion reactors become available. And yes, these do use up reaction mass.

Otherwise, passive drives (light sails) are going to play a role.

 

[Thot]

15 hours ago, Joerg said:

You'll end up with vastly different ship designs when you don't allow for artificial gravity.

I don't quite see why this is a reply to the sentence you quoted immediately before that?

 

15 hours ago, Joerg said:

Fuel is just one aspect that may get glossed over.

It's extremely fundamental, in contrast to the other things you mention, as it limits the ship's core ability (velocity change) significantly and cannot be explained away without violating major physics laws, unlike with waste heat, for instance (because at least there are radiator concepts that could theoretically work up to some point; rectionless thrusters are flat out impossible if we are not talking about lamps photon drives).

15 hours ago, Joerg said:

Like I said, hydrogen plasma exhaust gets better exhaust velocities, and exhaust velocity is where the rocket equation gets power from.

But there are issues with higher specific impulse drives that, at close distance, look quite significant, to sy the least.

1. If you do the math for the waste heat, you find that your crew is boiled, or with some  thrusters, the ship is melted, regardless of what kind of cooling system you imagine.

2. Their accellerations are so incredibly low that crews don't notice them. Which also means it takes a boring amount of time to get to any desired speed.

3. Too much delta V in the hands of basically any civilian tramp freighter captain mean every doofus owns a weapon of mass destruction.  Chemical rockets don't really pose that problem.

4. All the higher specific impulse drives are incredibly complicated, which means expensive and prone to failure. Do you really want spacecraft to be a privilege of the top 0.001% in your interstellar empire?

5. What is the power source?

15 hours ago, Joerg said:

That changes when your ammunition gets "intelligent". The difference between the long range battleship combat of WW1 and WW2 and the Falkland conflict rocket attacks.

I don't quite see how the nature of the missile changes the fact that the ships basically don't move, so there is no maneuvering, merely a quite boring ECM battle which ends the moment one side is hit.

15 hours ago, Joerg said:

The tethered countermass rotation is the only viable options for vehicles you can push to more than walking speed with chemical rockets, causing significant fuel expenditure to build up/slow down the rotation.

They are all viable, depending on the size of the ship.

You can do rotation with a gyro core or similar installations, no need to spend delta V on that (though the total amount you'd spend wouldn't be high anyway).

15 hours ago, Joerg said:

If you are in for long coasting, I maintain that Ion drives and assisted initial velocity is way more likely.

No issues with renting a rocket for "assisted initial velocity", but ion drives are complicated, extremely low thrust and power hungry. See above.

15 hours ago, Joerg said:

I think that the fission-reactor powered atomic rocket will be built for interplanetary travel until ship-sized internal fusion reactors become available.

Right now, the big money is on chemical rockets.

15 hours ago, Joerg said:

Otherwise, passive drives (light sails) are going to play a role

For unmanned ships with unlimited time and as long as there is no FTL (which seems likely, though), perhaps.

[Joerg]

4 minutes ago, Thot said:

I don't quite see why this is a reply to the sentence you quoted immediately before that?

You somehow doubted the ability of the M-Space system to cater to chemical rockets, whereas I wanted to point out that the ship design is based on the tech tree M-Space inherited from Traveller, with weird spots of softness in the SF behind it.

 

4 minutes ago, Thot said:

It's extremely fundamental, in contrast to the other things you mention, as it limits the ship's core ability (velocity change) significantly and cannot be explained away without violating major physics laws, unlike with waste heat, for instance (because at least there are radiator concepts that could theoretically work up to some point; rectionless thrusters are flat out impossible if we are not talking about lamps photon drives).

Shielding the payload (whether the on-board AI / heuristic or organisms, or just organic or highly organized matter) from the hostile environment is quite essential. Even maintaining digital data integrity is a challenge under exposure to cosmic radiation. And structural components don't get better with such exposure either.

 

So what is the hurry to change course and velocity? It is not like anybody will be surprised that the target object comes into range.

 

Reactionless thrusters are impossible with current ability to manipulate gravity or spacetime.

Photon drives - especially infrared - are a possibility. Not for high thrust, but everything counts in large amounts. Another possibility is to send a lighter (easier to brake) module ahead to install a mirror array on the target, and then use light beams to brake multiple times as the beam is reflected between the vessel (or its parabolic light sail) and the target.

 

4 minutes ago, Thot said:

But there are issues with higher specific impulse drives that, at close distance, look quite significant, to sy the least.

1. If you do the math for the waste heat, you find that your crew is boiled, or with some  thrusters, the ship is melted, regardless of what kind of cooling system you imagine.

The main problem is to have an energy source that doesn't produce vast amounts of waste heat (i.e. wasted energy). Although adding some more energy, a medium and a mirror array allows to send defined IR laser beams as photon drive. I've seen calculations presented by David Brin which make a Sundiver possible - a combination of cooling and force generation.

 

4 minutes ago, Thot said:

2. Their accellerations are so incredibly low that crews don't notice them. Which also means it takes a boring amount of time to get to any desired speed.

Hence my cry for assisted acceleration. Including light sail technologies (which also can serve as energy generators using photovoltaics).

I am a chemist, and that makes me doubt chemical energy as a viable long range solution. Booster rockets to overcome the worst of the gravity well or to get on an initial vector are feasible, but remotely transfered push and illumination sounds like a way more feasible option.

 

4 minutes ago, Thot said:

3. Too much delta V in the hands of basically any civilian tramp freighter captain mean every doofus owns a weapon of mass destruction.  Chemical rockets don't really pose that problem.

Not delta V, just V. Releasing a payload on a swing-by course and (in case of atmospheric entry a heat shield) will produce enough terminal V to be very destructive and extremely hard to fend off, especially if the last swing comes from the direction of the sun.

 

4 minutes ago, Thot said:

4. All the higher specific impulse drives are incredibly complicated, which means expensive and prone to failure. Do you really want spacecraft to be a privilege of the top 0.001% in your interstellar empire?

Magnetic coils and electrodes aren't exactly rocket science. Chemical rockets are... by definition.

Personally, I think that a plasma technician is just a glorified electrician. I am a chemist, but I am deeply respectful of explosives (and rocket fuel is by definition a bomb waiting to happen). (And I have worked with plasma in the lab, and with high temperature lasers. Getting silicates to melt or at least sinter within non-geological time frames without evaporating the stove can be frustrating.)

Do you really think that uneducated blue-collar work will be able to maintain high-tech beyond their home culture? That's like putting wipers with oily rags in charge of a fusion reactor.

 

4 minutes ago, Thot said:

5. What is the power source?

Yes, that's the question. The energy source with the least waste heat would be the hydrogen oxygen fuel cell, leaving the waste product as reaction mass.

I still advocate external energy, photovoltaic cells, most likely focussed by a solar sail also providing thrust while braking towards the light source or accelerating away from it.

I don't see fusion reactors with a manageable waste heat solution on spaceships in our near future. Fission reactors are nothing but steam cookers, and you need to get rid of the heat. (Same problem with fusion reactors, too - although those temperatures at least allow a secondary energy collection with photovoltaics.)

Anti-matter and mass-energy conversion probably can reach the highest energy densities, but releasing that mass into energy results in hard radiation and heat. Storing the stuff requires quite creative or energy-intensive and not very stable solutions - ion traps near zero Kelvin, or possibly some 3D-printed clathrate crystals with strong enough electrostatic towards the cavities to immobilize anti-protons (or ideally anti-atom cores). I don't think that such a technology is much further away than a continuous artificial fusion process. The vast amounts of energy to produce the stuff is available as soon as we get mass-produced photo-voltaic collectors in near-Earth orbits around our primary.

 

4 minutes ago, Thot said:

I don't quite see how the nature of the missile changes the fact that the ships basically don't move, so there is no maneuvering, merely a quite boring ECM battle which ends the moment one side is hit.

That's the general problem with weapons powerful enough to bridge those distances and remain effective.

Self-destructing missiles reduce the Kessler syndrome, or at least make it more manageable. Misses with a rail gun remain in instable orbits for generations - imagine entering a hardly explored solar system and to be hit by a salvo of high velocity slugs from a space battle millennia in the past.

 

4 minutes ago, Thot said:

They are all viable, depending on the size of the ship.

Ship size is extremely limited if you want chemical propulsion for its sublight drive.

 

4 minutes ago, Thot said:

You can do rotation with a gyro core or similar installations, no need to spend delta V on that (though the total amount you'd spend wouldn't be high anyway).

Any rotating body is a gyro, and quite hard to convince to change direction. A space installation producing a palpable centrifugal force without nauseating anybody exposed to it is extremely likely to have a significant amount of angular momentum that needs to be overcome for altering the orientation of the rotational axis. (Not to mention that changes of that orientation will cause weird and disturbing effects on any organism subject to these changes (including plants grown to provide life support assistance).

 

4 minutes ago, Thot said:

No issues with renting a rocket for "assisted initial velocity", but ion drives are complicated, extremely low thrust and power hungry. See above.

Are ion drives that complicated? Ion beams in mass spectrometers are produced by various means from quite diverse sources, and once you have freed an ion, accelerating and directing it is fairly straightforward. Electrostatic solutions require less energy than magnetic ones.

 

4 minutes ago, Thot said:

Right now, the big money is on chemical rockets.

Yes, nobody in their right mind would use irradiated steam (or other such exhaust) within the atmosphere, and getting out of the gravity well is the main problem we are facing at them moment.

That's why each entry to space (other than reaching the 100km mark) requires travelers and payload to latch themselves to an undetonated fuel bomb guaranteed to rip you apart when fuel flow goes fatally wrong.

However, once you are out of the well, the fission rocket becomes quite attractive, as you can convert the superheated steam directly into propulsion without needing to run a heat-force engine.

Other forms of assisted take-off and landing are under consideration. I see the Skyhook concept as the most likely assist system for atmospheric entry and take-off, although other concepts (like active suspension railgun ramps as presented by Isaac Arthur) are feasible, too.

 

4 minutes ago, Thot said:

For unmanned ships with unlimited time and as long as there is no FTL (which seems likely, though), perhaps.

I disagree. Light sails in combination with solar powered lasers (or mirrored beams) can provide constant thrust in periods you would otherwise coast, and reduce a lot of fuel requirements. Sure, you want redundance, the ability to bridge fatal defects in your collectors, with on-board means, but that redundance is costly in terms of payload, and may be under-powered. (Compare the number of lifeboat-seats on the Titanic.)

 

I can see some attraction in making the planetary young adult novels of Heinlein featuring reusable chemical rockets the background for a space game. The technology modeled in M-Space is far beyond that level, though. Yes, it is possible to design lower tech-level vessels with the M-Space rules, or to alter the technology tree, but that requires alterations to the ship design rules.

[Thot]

On 9/27/2019 at 12:07 AM, Joerg said:

You somehow doubted the ability of the M-Space system to cater to chemical rockets, whereas I wanted to point out that the ship design is based on the tech tree M-Space inherited from Traveller, with weird spots of softness in the SF behind it.

Well, Traveller CAN do chemical rockets and other reaction thrusters. M_space doesn't seem to have that option at all.

On 9/27/2019 at 12:07 AM, Joerg said:

 

Shielding the payload (whether the on-board AI / heuristic or organisms, or just organic or highly organized matter) from the hostile environment is quite essential. Even maintaining digital data integrity is a challenge

Sure, but there is no had physical barrier preventing tech to solve these issues. Reaction mass is different in that you cannot work around the need for it.

 

On 9/27/2019 at 12:07 AM, Joerg said:

So what is the hurry to change course and velocity? It is not like anybody will be surprised that the target object comes into range.

Slow motion  is boring?

On 9/27/2019 at 12:07 AM, Joerg said:

Reactionless thrusters are impossible with current ability to manipulate gravity or spacetime.

No. Reactionless thrusters as depicted in Traveller and M-Space are flat-out impossible, full stop.

The only way it IS possible is with photons as your reaction mass, and that requires amounts of energy no foreseeable spaceship will ever have accessible. Even then, thrust would be so low that they'd not be worth it, except for interstellar voyages.

On 9/27/2019 at 12:07 AM, Joerg said:

The main problem is to have an energy source that doesn't produce vast amounts of waste heat

Every joule of energy you produce eventually becomes waste heat. You can get rid of some by putting it in the reaction drive's exhaust, but that is necessarily imperfect.

On 9/27/2019 at 12:07 AM, Joerg said:

I am a chemist, and that makes me doubt chemical energy as a viable long range solution.

Worked fine for all the deep space probes any group of humans has sent out, with few exceptions.

On 9/27/2019 at 12:07 AM, Joerg said:

Not delta V, just V.

V follows from delta V. A chemical rocket can at most have an impact velocity of something like 10km/s. Bad for the house or block it hits, but rather unimpressive on the city. Not so with a fusion rocket that gives you 200.000 km/s of delta V...

 

On 9/27/2019 at 12:07 AM, Joerg said:

Magnetic coils and electrodes aren't exactly rocket science. Chemical rockets are... by definition.

But you forgot more than half of the drive here: The power source. The additional cooling. The reaction mass that needs to be cooled even more.

 

On 9/27/2019 at 12:07 AM, Joerg said:

Do you really think that uneducated blue-collar work will be able to maintain high-tech beyond their home culture? That's like putting wipers with oily rags in charge of a fusion reactor.

 

No, I am saying that high Isp-drives are even more complicated than chemical rockets, and we hit a limit there, too.

 

On 9/27/2019 at 12:07 AM, Joerg said:

Yes, that's the question. The energy source with the least waste heat would be the hydrogen oxygen fuel cell, leaving the waste product as reaction mass.

See above: It is not about the efficiency of the power source. You will always have 100% of your stored energy as waste heat EVENTUALLY, except for that which you put out of your engine.

 

On 9/27/2019 at 12:07 AM, Joerg said:

I still advocate external energy, photovoltaic cells, most likely focussed by a solar sail also providing thrust while braking towards the light source or accelerating away from it.

Photovoltaics is great technology, but it won't power a multi-GW-spaceship. And that's what you need for high Isp ships..

On 9/27/2019 at 12:07 AM, Joerg said:

Ship size is extremely limited if you want chemical propulsion for its sublight drive.

Only by your ability to build stable hulls that can handle the acceleration stress. The size of a single rocket engine is limited, yes, but you can just use multiple engines.

On 9/27/2019 at 12:07 AM, Joerg said:

Any rotating body is a gyro, and quite hard to convince to change direction.

That wasn't my point. If you have gyro core (or just two countering centrifugges, all you need is the ability to create rotation (like a common motor), wihtout having to spend any delta V.

On 9/27/2019 at 12:07 AM, Joerg said:

Are ion drives that complicated?

Judging by the time and effort put into their development without having anything terribly impressive to show for yet, I would say yes.

 

On 9/27/2019 at 12:07 AM, Joerg said:

Light sails in combination with solar powered lasers (or mirrored beams) can provide constant thrust in periods you would otherwise coast, and reduce a lot of fuel requirements.

The amount of sail surface needed for that to be noticeable on a timescale relevant for manned space flight is prohibitive.

On 9/27/2019 at 12:07 AM, Joerg said:

I can see some attraction in making the planetary young adult novels of Heinlein featuring reusable chemical rockets the background for a space game. The technology modeled in M-Space is far beyond that level, though. Yes, it is possible to design lower tech-level vessels with the M-Space rules, or to alter the technology tree, but that requires alterations to the ship design rules.

And I had hoped someone had done the groundwork already.

[Joerg]

5 hours ago, Thot said:

Well, Traveller CAN do chemical rockets and other reaction thrusters. M_space doesn't seem to have that option at all.

M-Space doesn't spend much text on technologies from the real world. Digital computing doesn't receive much attention, neither do combustion engines.

I agree with an earlier comment by you that fuel consumption for FTL is arbitrary. Same for energy consumption of such technology.

5 hours ago, Thot said:

Sure, but there is no had physical barrier preventing tech to solve these issues. Reaction mass is different in that you cannot work around the need for it.

While there is no hard physical barrier preventing technology that produces magnetic shields able to ward off cosmic radiation, there is a problem with the energy consumption for such a device.

 

5 hours ago, Thot said:

Slow motion  is boring?

No argument here. So why your fascination with chemical rockets? 😋

But then, we aren't talking about slow motion, only slow deceleration.

 

5 hours ago, Thot said:

No. Reactionless thrusters as depicted in Traveller and M-Space are flat-out impossible, full stop.

Anything able to create artificial space curvature and localized gravity wells inside a vessel should be able to do so outside of the vessel, too.

If you assume an "ether" of Dark Matter pervading the galaxy, that source of gravity offers a reactio to your artificial gravity actio.

 

But I agree with your disagreement on reactionless action.

Yet, conservation of momentum poses a problem with all FTL travel.

 

5 hours ago, Thot said:

The only way it IS possible is with photons as your reaction mass, and that requires amounts of energy no foreseeable spaceship will ever have accessible.

Provided you procure that energy on board of your vessel. Mass-energy transformation has some potential (aka anti-matter), but handling that stuff even after wasting huge amounts of solar energy creating it in the first place remains problematic.

5 hours ago, Thot said:

Even then, thrust would be so low that they'd not be worth it, except for interstellar voyages.

No jet fighter dog fights this way, no.

 

5 hours ago, Thot said:

Every joule of energy you produce eventually becomes waste heat. You can get rid of some by putting it in the reaction drive's exhaust, but that is necessarily imperfect.

 

Imperfect - yes. But building a carnot engine to use stored potential energy to move distributed heat to concentrated heat is easy. Getting the source of stored potential energy for that is hard.

You need supraconducting magnet coils to avoid frying yourself warding off cosmic radiation (or counteracting the Jovian asteroid belt) if you plan to survive your space trip.

 

5 hours ago, Thot said:

Worked fine for all the deep space probes any group of humans has sent out, with few exceptions.

You mean ballistic objects after assisted acceleration (true, using chemical rockets) with minimal thrust for course correction?

This is mostly faulted to our starting point at the bottom of the gravity well, and the enormous surplus energy expended to overcome that. Most of the kinetic energy of our deep system probes comes from swing-by acceleration leeching kinetic energy from the planets.

 

5 hours ago, Thot said:

V follows from delta V. A chemical rocket can at most have an impact velocity of something like 10km/s. Bad for the house or block it hits, but rather unimpressive on the city. Not so with a fusion rocket that gives you 200.000 km/s of delta V...

You can build up velocity using gravitic assist of planets and planetoids, regardless whether you start your shot with a chemical rocket or a fusion rocket. An umpowered impactor is hard to detect until it is way too late to avoid it.

 

5 hours ago, Thot said:

But you forgot more than half of the drive here: The power source. The additional cooling. The reaction mass that needs to be cooled even more.

Power generation aboard a space ship... ok, that's an issue when you are far from the Goldilocks zone. As long as you are within collector range of a primary, you can collect sun light, and by skewing your collector sail you can use that collection for a bit of additional thrust. Photovoltaic collectors will generate electric energy, most likely to be stored chemically or in sufficiently dimensioned condensators. Rotational energy in gyros would be possible, too, but a bit self-defeating when what you want to use the energy for is to accelerate or decelerate mass.

 

Using the external fusion reactor as light source for your PV panels - if need be via an array of collector sails spread around the ship, possibly separate from the ship - sounds like the sensible way for power generation.

5 hours ago, Thot said:

No, I am saying that high Isp-drives are even more complicated than chemical rockets, and we hit a limit there, too.

Really? Getting the fuel flow right is a bitch - that's why Korolev's umpteen drive moon rocket didn't take off. The few big rocket exhausts of the Saturn were anything but blue collar friendly, either.

 

5 hours ago, Thot said:

See above: It is not about the efficiency of the power source. You will always have 100% of your stored energy as waste heat EVENTUALLY, except for that which you put out of your engine.

Eventually, you will have radiated off every Joule above cosmic background radiation, too...

Process heat put to a controlled use isn't waste heat. The problem with anything but chemically stored energy available for electrochemical processes is the release of heat over time, and in correlation to the work done. Heat pumps transfer heat where you can handle it, and concentrate it - by adding more heat from the work you put in, reducing your on-board energy storage, but when that heat concentration translates into propulsion, this is energy put out of your engine.

 

5 hours ago, Thot said:

Photovoltaics is great technology, but it won't power a multi-GW-spaceship. And that's what you need for high Isp ships..

Carnot engines don't work in Vacuum unless you can dump your coolant as reaction mass. That's the problem with nuclear reactors on board of space vessels - the slow plutonium pile powering thermocouples for those interstellar missions doesn't give you any gigawatts, either.

Gigawatts in terms of chemical energy translate as megatons of fuel, so there is no way you'll be happy with your chemical thrusters, either. On-board power generation is prohibitive.

Photovoltaics with beamed light is a different proposal than your rooftop panel relying on daylight. Put a collector array in orbit not too far from the sun, and provide a coherent beam of just the perfect wave length for your PV target, and your wattage won't suck.

Maybe a maser for energy transmission might be more useful?

 

 

5 hours ago, Thot said:

Only by your ability to build stable hulls that can handle the acceleration stress. The size of a single rocket engine is limited, yes, but you can just use multiple engines.

Building a stable frame (the hulls can be attached to) with sufficiently low mass, putting on a massive shield in flight direction to minimize collisions caused by your own vector, ... you'll need hectares of rocket engines to push this.

 

5 hours ago, Thot said:

That wasn't my point. If you have gyro core (or just two countering centrifugges, all you need is the ability to create rotation (like a common motor), wihtout having to spend any delta V.

That kind of gravity isn't habitable - tidal nausea will incapacitate your crew, rather than prevent bone degradation and fluid build-up.

What do you call "impressive" in terms of space drives? Your chemical rocket only leaves you coasting after a short boost, the ion drive doesn't offer short boosts but has a greater output during the journey.

 

 

5 hours ago, Thot said:

The amount of sail surface needed for that to be noticeable on a timescale relevant for manned space flight is prohibitive.

Light is cheap when you haven't reached Kardashev 2 yet. Combining sail pushback with PV energy generation will beat even fusion power plant efficiency if you have no heat dump to cool down your steam turbine.

 

5 hours ago, Thot said:

And I had hoped someone had done the groundwork already.

GURPS apparently has.

Covering highly asymmetric tech levels without huge doses of handwavium if you don't want to use the standard unobtainium is always a big demand for a slim SF rules set.

Just out of interest, if you want a gigawatt of propulsive force generation, how many Saturn rockets do you need to lash together, and how long can they maintain this output?

[Thot]

2 hours ago, Joerg said:

While there is no hard physical barrier preventing technology that produces magnetic shields able to ward off cosmic radiation, there is a problem with the energy consumption for such a device.

I can imagine some workaround by incredibly sophisticated magnetic field shapes. But there simply is no option whatsoever to get rid of the limitations of reaction mass.

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No argument here. So why your fascination with chemical rockets? 😋

Because they are real. We could build such ships right now. Elon Musk's SpaceX is beginning to. They do seem to be as close to the real thing as we'll ever be.

 

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If you assume an "ether" of Dark Matter pervading the galaxy,

Then of course everything is different. But it doesn't look like it.

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But I agree with your disagreement on reactionless action.

Yet, conservation of momentum poses a problem with all FTL travel.

You don't need FTL in order to have a space setting.

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No jet fighter dog fights this way, no.

Actually, that IS doable. With some 5000 m/s of delta v and an acceleration of 0.5g (similar to most atmospheric fighter jets today), you could do 20 minutes of constant burning. If you limit yourself to relatively low speeds, that can be enough to do traditional dogfighting. Only for half an hour or so (as you don't burn CONSTANTLY), but that's good enough for a carrier vs carrier battle.

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building a carnot engine to use stored potential energy to move distributed heat to concentrated heat is easy. Getting the source of stored potential energy for that is hard.

Even in the best case that will just add to your overall waste heat.

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You need supraconducting magnet coils to avoid frying yourself warding off cosmic radiation (or counteracting the Jovian asteroid belt) if you plan to survive your space trip.

No, storing enough H2 around you is actually an option. And as you need that for your rocket anyway... but sure, a magnetic field would be convenient.

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You mean ballistic objects after assisted acceleration (true, using chemical rockets) with minimal thrust for course correction?

Of course. That is what real space travel is: Some chemical rockets and lots of orbit calculations.

 

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Power generation aboard a space ship... ok, that's an issue when you are far from the Goldilocks zone. As long as you are within collector range of a primary, you can collect sun light, and by skewing your collector sail you can use that collection for a bit of additional thrust. Photovoltaic collectors will generate electric energy, most likely to be stored chemically or in sufficiently dimensioned condensators. Rotational energy in gyros would be possible, too, but a bit self-defeating when what you want to use the energy for is to accelerate or decelerate mass.

As a chemist, you will surely know how important it is to quantify such statements.

The amount of energy for a high Isp drive is... unsettling.

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Really? Getting the fuel flow right is a bitch - that's why Korolev's umpteen drive moon rocket didn't take off. The few big rocket exhausts of the Saturn were anything but blue collar friendly, either.

I never claimed it wasn't hard, just that other drives are even harder.

 

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Gigawatts in terms of chemical energy translate as megatons of fuel, so there is no way you'll be happy with your chemical thrusters, either.

The beauty of the chemical rocket is that it carries its power in its fuel, and that takes most of that out of the rear end of your spacecraft with it. Yes, you need really large amounts of fuel for high delta V, but it seems to be easier to just accept that and have a very large tank than to build overcomplicated drives that never quite achieve what you design them for.

We live under the tyranny of the rocket equation, and this is not going to change.

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That kind of gravity isn't habitable - tidal nausea will incapacitate your crew, rather than prevent bone degradation and fluid build-up.

Not at all. NASA has done tests with it - some people cannot stand it, yes, but others can. For my campaign, I just assumed all relevant characters had passed that test.

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What do you call "impressive" in terms of space drives? Your chemical rocket only leaves you coasting after a short boost, the ion drive doesn't offer short boosts but has a greater output during the journey.

Well, all that effort to just get some 10,000 m/s of delta V when you could as well just put more fuel on your spaceship... seems wasted.

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Just out of interest, if you want a gigawatt of propulsive force generation, how many Saturn rockets do you need to lash together, and how long can they maintain this output?

You could do that computation (based on E=1/2m*v²), but by putting it in terms of energy, you are making things complicated because of the rocket equation.

Let's just imagine a rocket ship, let us call it the Battlerocket Solaria. Without fuel, it is about as massive as a Nimitz class aircraft carrier, so about 100,000 tons.We want our Battlerocket Solaria to have significant delta V, so we give it a fuel tank with 900,000 tons of fuel. Therefore, the starting mass (fully loaded) is 1,000,000 tons. With a H2/O2 rocket, that will give us:

Delta V=4462*LN(1,000,000/100,000)

which equals 10,274 m/s of total Delta V.

How big the engine is that we use this fuel with is irrelevant for that, but we'll probably equip our Battlerocket Solaria with missiles and fighters for armament, so the mothership won't need to be terribly agile. The Saturn V rocket's engine had a thrust of 33,851 kN. let us give our battlerocket three of those, so we have about 100,000 kN of hrust. When the Battlerocket Solaria is fully loaded, her engines will accellerate her at  1 m/s², which goes up to 10m/s² when the tanks are almost dry. She could do this for about 2,000 seconds non-stop - but you'd never do that, of course.

 

 

 

 

 

 

Edited September 28, 2019 by Thot

[Joerg]

5 minutes ago, Thot said:

I can imagine some workaround by incredibly sophisticated magnetic field shapes. But there simply is no option whatsoever to get rid of the limitations of reaction mass.

Those incredibly sophisticated magnetic field shapes sound like a handful of unobtainium with discrete field borders, or some almost fractal array of magnetic coils around the hull. Neither will be feasible without an unobtainium internal energy source, or alternatively an externally induced magnetic reaction to electromagnetic radiation.

 

5 minutes ago, Thot said:

Because they are real. We could build such ships right now. Elon Musk's SpaceX is beginning to. They do seem to be as close to the real thing as we'll ever be.

Yes, but a setting like that is not anywhere close to the setting outlined by M-Space. That is an FTL setting with workable, highly efficient fusion reactors (or a similar kind of unobtainium internal reactor), consuming rather negligible amounts of fusion fuel probably doubling as highly energetic reaction mass after fusion.

 

5 minutes ago, Thot said:

Then of course everything is different. But it doesn't look like it.

Dark Matter is postulated to permeate the space almost everywhere baryonic matter (stuff like we know it) is found (I recently read about two satellite dwarf galaxies which may not have the uniform rotation of other galaxies like ours). If there is something influencing the inertia of the stars so that they don't drift individually, that something is available to be targeted by inertia-affecting hypothetical gravitics. I called that stuff "ether" because it provides an unseen medium between the stars which is a current hard science postulate.

 

5 minutes ago, Thot said:

You don't need FTL in order to have a space setting.

True. But M-Space provides the tools for an FTL setting similar to those of e.g. Classic Traveller. Applying it to the rocket age is similar to using Mythic Britain for a Clan of the Cave Bear setting as far as technology is involved.

 

5 minutes ago, Thot said:

Actually, that IS doable. With some 5000 m/s of delta v and an acceleration of 0.5g (similar to most atmospheric fighter jets today), you could do 20 minutes of constant burning. If you limit yourself to relatively low speeds, that can be enough to do traditional dogfighting. Only for half an hour or so (as you don't burn CONSTANTLY), but that's good enough for a carrier vs carrier battle.

Only you wouldn't want to sit inside those vessels if you can remote-control them at much higher G-forces than you yourself could withstand. Something like the drone ship-to-ship combat of Hamilton's Night's Dawn trilogy.

 

5 minutes ago, Thot said:

Even in the best case that will just add to your overall waste heat.

No, it transforms waste heat into infared laser exhaust generating thrust (in case of doubt by superheating hydrogen plasma to push the ship away reflected by low temperature plasma shields).

 

5 minutes ago, Thot said:

No, storing enough H2 around you is actually an option. And as you need that for your rocket anyway... but sure, a magnetic field would be convenient.

Liquid H2? You don't want to expose that to micro-meteorites providing outlets for lateral thrust... H2O has sufficient density to filter out some cosmic radiation if you give it a 10m radius or so, and you might use the inner part for growing food (algae, animal plankton, fish) and supporting your life support.

Anyway, using your fuel for shielding means that you are left without shielding for the last leg of your journey (if you plan for emergency fuel, plan on continuing the mission after having expended this emergency fuel). This is a bit hard on the crew and other sensitive payload, don't you think?

5 minutes ago, Thot said:

Of course. That is what real space travel is: Some chemical rockets and lots of orbit calculations.

Until we get a sky hook and a railgun into a useful orbit.

There are quite advanced passive take-off systems for atmospheric lift-off, with transfer of external energy to accelerate reaction mass, or even to generate thrust by reflection of what the craft's target is shot with. Getting a rocket assembly into an orbit where most of the heavy lifting out of our gravity well has already been paid appears to be the basic requirement for a crewed Mars mission to me, regardless whether that is in Earth orbit or on the Moon.

 

Apart from the protomolecule stuff, the main cases of unobtainium in the first two seasons/volumes of The Expanse are controlled fusion and the efficiency of the Epstein drive. (And the weird idea that an ice harvester ship would make the trip back to the customer when a railgun could send re-packaged ice cubes there in a constant stream of missiles).

 

5 minutes ago, Thot said:

The amount of energy for a high Isp drive is... unsettling.

The amount of energy (and fuel) to lift an intact, fueled Saturn V rocket equivalent into a near earth orbit to start a Mars voyage is as unsettling.

Chemical rockets allow a journey to the moon inside of 3 days. A trip to Mars at ideal constellation is in the order of 400 days, and in less than ideal constellations several times that. Unless you enjoy roleplaying camp fever, not much in the way of roleplaying opportunities for such a voyage. You can of course gloss over it, let the protagonists make a number of skill improvement and body deterioration rolls and say "two years have passed, and you have reached Mars orbit."

 

5 minutes ago, Thot said:

I never claimed it wasn't hard, just that other drives are even harder.

Rather you claim that other active shipboard drives are softer SF. I still disagree - the main advantage of rocket drives is that they can be tested outside a vacuum.

 

5 minutes ago, Thot said:

The beauty of the chemical rocket is that it carries its power in its fuel, and that takes most of that out of the rear end of your spacecraft with it. Yes, you need really large amounts of fuel for high delta V, but it seems to be easier to just accept that and have a very large tank than to build overcomplicated drives that never quite achieve what you design them for.

 

Or you skip the requirement for high delta V, and use external power for much of your acceleration.

 

Chemical drives are fine if you want to have a game in Earth orbit, and/or possible Earth/Moon L4 and L5 NEO capture sites. That's not quite a space opera setting, though.

 

5 minutes ago, Thot said:

We live under the tyranny of the rocket equation, and this is not going to change.

For interstellar drone missions, the light sail approach riding laser beams is a real alternative. It takes quite a bit of initial expenditure for the infrastructure, but the free electricity generated by that infrastructure might be enough to pay for that expenditure.

 

5 minutes ago, Thot said:

Not at all. NASA has done tests with it - some people cannot stand it, yes, but others can. For my campaign, I just assumed all relevant characters had passed that test.

And you said you didn't want to limit space travel to a privileged few?

 

5 minutes ago, Thot said:

Well, all that effort to just get some 10,000 m/s of delta V when you could as well just put more fuel on your spaceship... seems wasted.

Yes, no computer is ever going to need more than 640 KByte RAM.

When it comes to pushing Near Earth Objects with usable material into convenient collection sites like Earth orbit or Earth-Moon L4 and L5 points, an ion drive making use of a fraction of the payload and solar panels is vastly more efficient than sending chemical rockets and the fuel to do the job. But that's just big money you're glossing over.

 

5 minutes ago, Thot said:

You could do that computation (based on E=1/2m*v²), but by putting it in terms of energy, you are making things complicated because of the rocket equation.

Let's just imagine a rocket ship, let us call it the Battlerocket Solaria. Without fuel, it is about as massive as a Nimitz class aircraft carrier, so about 100,000 tons.We want our Battlerocket Solaria to have significant delta V, so we give it a fuel tank with 900,000 tons of fuel. Therefore, the starting mass (fully loaded) is 1,000,000 tons. With a H2/O2 rocket, that will give us:

Delta V=4462*LN(1,000,000/100,000)

which equals 10,274 m/s of total Delta V.

How big the engine is that we use this fuel with is irrelevant for that, but we'll probably equip our Battlerocket Solaria with missiles and fighters for armament, so the mothership won't need to be terribly agile. The Saturn V rocket's engine had a thrust of 33,851 kN. let us give our battlerocket three of those, so we have about 100,000 kN of hrust. When the Battlerocket Solaria is fully loaded, her engines will accellerate her at  1 m/s², which goes up to 10m/s² when the tanks are almost dry. She could do this for about 2,000 seconds non-stop - but you'd never do that, of course.

This vessel is of course sitting somewhere where it doesn't have to care about escape velocity any more.

A 9:1 fuel to payload ratio sounds extremely optimistic to me, anyway.

 

Rather than a battleship, let's discuss something like a small O'Neill cylinder (or just a pressurized donut that diameter) being pushed to say Ceres to establish a human colony on (or rather inside) that planetoid (probably in a sub-surface O'Neill cylinder, or attaching your travel cylinder to a pole of Ceres). You'll probably want to send two such vessels at the same time.

With a target like that, you need to provide enough fuel to brake at the target, then stabilize your rendezvous orbit for years. You don't even have to carry the fuel for the return trip - such reaction mass would be harvested from Ceres.

Assuming similar mass and getting the initial acceleration from Earth completely by booster modules (some of which you might harvest a few years after your arrival for return missions to Earth), you can slow down from 10km/s. The shortest possible distance between Earth and Ceres is 250 million kilometers, resulting in a journey of a bit less than 3000 days or a little under 8 years. In practical reality, your path would probably be at least twice as long. Still, 15 years of interplanetary journey sound remotely possible with chemical rockets, and with mostly female passengers and frozen embryos (again mostly female, and some frozen sperm) you might be able to establish a viable second or third generation population of colonists with enough bio-diversity.

 

So - an interplanetary journey of  8 to 15 years, and your entire fuel available for braking for less than an hour at full thrust. What's your hurry with acceleration and deceleration?

[Thot]

10 hours ago, Joerg said:

Yes, but a setting like that is not anywhere close to the setting outlined by M-Space.

 

Yes. My misunderstanding was, apparently, to see M-Space as a generic "SF guide for Mythras" instead of a more specific "this is Mythras Star Wars".

 

10 hours ago, Joerg said:

Only you wouldn't want to sit inside those vessels if you can remote-control them at much higher G-forces

5 m/s² is half a G. That's not something worth thinking about. And depending on your setting, there may be reasons to use human pilots over remote controls or Ai brains.

 

10 hours ago, Joerg said:

 

Liquid H2? You don't want to expose that to micro-meteorites providing outlets for lateral thrust...

We are talking about spaceships with the sturdiness of an airplane at most. If a micrometeorite hits you, you're dead anyway.

10 hours ago, Joerg said:

Anyway, using your fuel for shielding means that you are left without shielding for the last leg of your journey

That would be the landing - or the "we cut our tanks dry" emergency where radiation isn't your biggest concern.

10 hours ago, Joerg said:

Chemical drives are fine if you want to have a game in Earth orbit, and/or possible Earth/Moon L4 and L5 NEO capture sites. That's not quite a space opera setting, though.

Well, for my past campaign, I had to add FTL anyway, so I just decided to set the FTL drive's capabilities in such a way that chemical rockets sufficed.

10 hours ago, Joerg said:

And you said you didn't want to limit space travel to a privileged few?

Not to a hereditary class of oligarchs. If I can simply and plausibly make any PC and important NPC in my campaign a spacefarer just by declaring them so, without any other worldly consequences like fortune or fame, that's fine by me. Besides, it doesn't seem to be such a small share of the population who can stand it.

 

10 hours ago, Joerg said:

Yes, no computer is ever going to need more than 640 KByte RAM.

I wasn't saying that Ion drives had more delta V than necessary, but that they don't, in practice, have more delta V than chemical rockets.

10 hours ago, Joerg said:

When it comes to pushing Near Earth Objects with usable material into convenient collection sites like Earth orbit or Earth-Moon L4 and L5 points, an ion drive making use of a fraction of the payload and solar panels is vastly more efficient than sending chemical rockets and the fuel to do the job. But that's just big money you're glossing over.

For an ion drive, you need exotic reaction mass like Xenon right now - because it is freaking complicated to use non-noble gases.  There is basically no way you can mine xenon from an asteroid.

10 hours ago, Joerg said:

This vessel is of course sitting somewhere where it doesn't have to care about escape velocity any more.

Well, in orbit (at 8 km/s, roughly).

10 hours ago, Joerg said:

A 9:1 fuel to payload ratio sounds extremely optimistic to me, anyway.

Why? Once your are in orbit (and such a behemoth would be built there already), you just slap more fuel tanks onto it until you are at 90% fuel, or even more Add another 9 million tons and you get to 20,000 m/s of delta V.

10 hours ago, Joerg said:

Rather than a battleship, let's discuss something like a small O'Neill cylinder (or just a pressurized donut that diameter) being pushed to say Ceres to establish a human colony on (or rather inside) that planetoid (probably in a sub-surface O'Neill cylinder, or attaching your travel cylinder to a pole of Ceres). You'll probably want to send two such vessels at the same time.

Why would anyone want to build a colony on Ceres? There is nothing there that you cannot find elsewhere closer to the sun.

10 hours ago, Joerg said:

With a target like that, you need to provide enough fuel to brake at the target, then stabilize your rendezvous orbit for years. You don't even have to carry the fuel for the return trip - such reaction mass would be harvested from Ceres.

Assuming similar mass and getting the initial acceleration from Earth completely by booster modules (some of which you might harvest a few years after your arrival for return missions to Earth), you can slow down from 10km/s. The shortest possible distance between Earth and Ceres is 250 million kilometers, resulting in a journey of a bit less than 3000 days or a little under 8 years. In practical reality, your path would probably be at least twice as long. Still, 15 years of interplanetary journey sound remotely possible with chemical rockets

Cassini needed 7 years to get to Saturn. Kickstarted with a liftoff rocket (a Titan IV), and with 2000 m/s on-board delta V.

10 hours ago, Joerg said:

What's your hurry with acceleration and deceleration?

Ion drives and the even less powerful higher Isp drives are not noticeably accelerating, meaning there is no action going on, which is bad for tension at the game table. And they don't seem to give that impressive results in practice anyway. The space probe Dawn had only about 11,000 m/s of delta V. Compared to our Battlerocket Solaria, I can only say: Why bother with the ion thruster, plus the power source, plus the waste heat, plus the extremely rare reaction mass in the form of Xenon?

 

[Joerg]

44 minutes ago, Thot said:

Yes. My misunderstanding was, apparently, to see M-Space as a generic "SF guide for Mythras" instead of a more specific "this is Mythras Star Wars".

To me, it is more like Mythras Traveller or Space Opera, with a more naval than airborn feel to it. But yes, it is an FTL setting, or at least provides all the tools for one.

There is of course a generic SF guide involved, but ship design will always follow the assumptions of the tech tree - I know I have to adjust the system for the tech tree of the setting I am building.

 

44 minutes ago, Thot said:

5 m/s² is half a G. That's not something worth thinking about. And depending on your setting, there may be reasons to use human pilots over remote controls or Ai brains.

Sure. Once you have decided to use humans like ablative armor, there is nothing to stop you from crewing expandable missiles with them.

 

44 minutes ago, Thot said:

We are talking about spaceships with the sturdiness of an airplane at most. If a micrometeorite hits you, you're dead anyway.

Why? A micrometeorite creates a puncture and might require you to repair a number of vital systems or shut off certain systems (and activate the redundancy plans).

 

44 minutes ago, Thot said:

That would be the landing - or the "we cut our tanks dry" emergency where radiation isn't your biggest concern.

Rather reaching a stable parking or docking orbit.

44 minutes ago, Thot said:

Well, for my past campaign, I had to add FTL anyway, so I just decided to set the FTL drive's capabilities in such a way that chemical rockets sufficed.

So basically, you FTLed to your planetary destination and then used maneuver jets for the final docking or landing?

 

44 minutes ago, Thot said:

Not to a hereditary class of oligarchs.

If this is a genetic advantage, then you will witness the birth of a hereditary class of oligarchs.

 

44 minutes ago, Thot said:

If I can simply and plausibly make any PC and important NPC in my campaign a spacefarer just by declaring them so, without any other worldly consequences like fortune or fame, that's fine by me. Besides, it doesn't seem to be such a small share of the population who can stand it.

Since you are declaring these characters so, what is to stop you from declaring each and any of them to be a member of one of the spacefaring castes?

 

44 minutes ago, Thot said:

I wasn't saying that Ion drives had more delta V than necessary, but that they don't, in practice, have more delta V than chemical rockets.

For an ion drive, you need exotic reaction mass like Xenon right now - because it is freaking complicated to use non-noble gases.  There is basically no way you can mine xenon from an asteroid.

"Exotic" reaction mass like metal ions evaporated from an anode, as I regularly do in my AAS lamps in the lab. I can name a couple of ion sources used in mass spectroscopy, where the ion source and subsequent accelerator is essentially an ion drive(r). The technology of the ion source is about as "complicated" as a vacuum tube TV screen.

 

44 minutes ago, Thot said:

Why would anyone want to build a colony on Ceres? There is nothing there that you cannot find elsewhere closer to the sun.

It offers you a heat sink, raw material, reaction mass, microgravity, shielding... everything you need for heavy industry.

 

44 minutes ago, Thot said:

Cassini needed 7 years to get to Saturn. Kickstarted with a liftoff rocket (a Titan IV), and with 2000 m/s on-board delta V.

Doing a number of gravitational assist maneuvers, yes.

 

44 minutes ago, Thot said:

Ion drives and the even less powerful higher Isp drives are not noticeably accelerating, meaning there is no action going on, which is bad for tension at the game table.

It is almost like traveling on a raft in a current, yes. No false drama here.

Travel is boring. ("Are we there yet?") Hence the show runners write "encounters" such as micrometeorite strikes (requiring frantic repair activity), solar flares (basically a "take cover" exercise, with some ominous rolling for system failures resulting in more repair activity), resource shortages (due to contamination or some other made-up reason), etc. If you have NPCs, you can let one of them go haywire. (It works... I cannot count how many times I have seen those scenes. Which, on second thought, means that you probably shouldn't use this scenario unless you manage to subvert it somehow.)

"All stations - prepare for emergency thrust!" - another "take cover" excercise, plus not having secured your stuff creates a complication. In The Expanse, Amos had such a scene in Season 2 (IIRC).

 

 

A table-top rpg cannot realistically simulate all the things that need to be considered for any futuristic or even contemporary activity without handwaving. NPCs usually remain quite one- or two-dimensional.

Getting every aspect of technology right is impossible.

 

[g33k]

On 9/29/2019 at 11:24 AM, Joerg said:

Sure. Once you have decided to use humans like ablative armor, there is nothing to stop you from crewing expandable missiles with them.

I think that was an Asimov story.

The premise was (very expensive) AI's were doing so much, humanity did almost nothing but direct the AI's (including making war).

When some historian-geek, or something, discovered in old books that humans did basic math, it revolutionized their warfare -- cheap, self-reproducing, overpopulating humans replacing expensive AI's

Edited October 2, 2019 by g33k

[g33k]

On 9/28/2019 at 2:43 AM, Thot said:

... Reaction mass is different in that you cannot work around the need for it.

Your Unobtanium needs to be radioactive; that is to say you need incredibly-much incredibly-intense energy.

Accelerate your reaction mass to near-c velocities -- even small amounts of it -- and let God and Einstein give you more reaction-mass.

Yes, now your engine is a near-c WMD... <shrugs> wotcha gonna do?

Also a Clarketech device ... <shrugs> wotcha gonna do?

At least relativistic mass-increase agrees with known physics.

[Lloyd Dupont]

15 hours ago, Thot said:

Why would anyone want to build a colony on Ceres? There is nothing there that you cannot find elsewhere closer to the sun.

It's a question I sometimes ask myself...
If space travel is such a difficult matter, why bother? even today there is hardly anyone in Antarctica, and it's much easier!

I guess, it's a "realistic option" only if it's "relatively easy" (to go there, with the setting's technology) and "somewhat attractive"..

Let's see....

- science station
- biggest gravity well in the asteroid belt (gravity might help some industrial / chemical reaction, and be welcome by "local workers")
- hideout for pirate? although might be a worst hideout that some lonesome forgotten asteroid or just plan deep space...
- trade station, halfway between inner planet and outer planet, rest stop, refuel stop, etc...

mmmm.... it's all I can think of for now....