"High-Powered" Plasma Drives

[Whiskey144]

This is something I'm curious about, as for a setting I'm working on almost all ships invariably use plasma-based propulsion, with the ideal propellants being something like xenon, argon, and occasionally helium. Also, I'm posting this here as, since it has to do with my particular setting, I don't think it really belongs in SLAM.

My question is, is it possible for a plasma-based engine to deliver performance good enough to push a ship massing several kilotonnes, and in some cases, multiple megatonnes, around at anywhere from 0.1-0.5 Gs?

If so, what kind of thrust force, exhaust velocity, and mass ratios for delta-vs in the several hundred km/s category look like?

Further, would building such ships to withstand, say, up to 50 Gs of acceleration be wasteful, as in-setting refitting a ship with a high performance fusion or antimatter drive is a mechanically simple, though very expensive, affair.

[Sarevok]

What kind of plasma drive ? Like nuclear thermal where the hot plasma is ejected at high speed as reaction mass ? Or ion propulsion where it's mostly minute quanties of gas essentially leaking via charge repulsion ?

Further, would building such ships to withstand, say, up to 50 Gs of acceleration be wasteful, as in-setting refitting a ship with a high performance fusion or antimatter drive is a mechanically simple, though very expensive, affair

Unless you are building a missile that accelerates to maximum velocity why would you want that kind of acceleration ?

[Whiskey144]

What kind of plasma drive ? Like nuclear thermal where the hot plasma is ejected at high speed as reaction mass ? Or ion propulsion where it's mostly minute quanties of gas essentially leaking via charge repulsion ?

I'd thought that noting the propellant would provide enough insight into the matter. To clarify, I envision something somewhat analogous to the currently in-development VASIMR drive. I'll clarify further by noting that most ships mount 2-6 drives, with the average vessel typically being a 3-4 engine ship.

Unless you are building a missile that accelerates to maximum velocity why would you want that kind of acceleration ?

I can envision at least two usages, that of a pursuit craft or a blockade runner. I'll further note that in-setting missiles tend to have on the order of nearly one hundred Gs of acceleration; however, missiles are multi-stage and this is for the cruise-phase stage of the munition, and it can only sustain such boosts for a limited time, typically 10-60 seconds, depending on a variety of factors.

[Sarevok]

I'd thought that noting the propellant would provide enough insight into the matter. To clarify, I envision something somewhat analogous to the currently in-development VASIMR drive. I'll clarify further by noting that most ships mount 2-6 drives, with the average vessel typically being a 3-4 engine ship.

VASMIR type drives have very low thrust. You are not getting 0.5 G worth of acceleration out of them. They are also big and heavy so you cant mount enough them to offset the drives inherent weaknesses.

[Whiskey144]

VASMIR type drives have very low thrust. You are not getting 0.5 G worth of acceleration out of them. They are also big and heavy so you cant mount enough them to offset the drives inherent weaknesses.

*sigh* Okay, I'll break this down:

-The drives are analogous to VASIMR, but are most certainly not VASIMR-type drives.
-I'm willing to settle for 0.1-0.2 Gs.
-In-setting, the drives are comparatively lightweight, to say, current standards. They've still got quite a bit of mass, but by % of o/a ship tonnage, it's quite negligible, compared to say, propellant tankage or cargo capacity.

My question still stands; could you get 0.1-0.2 Gs from 3-4 plasma drives mounted on a ship massing nearly half a million tonnes?

[Whiskey144]

EDIT: Inadvertent double post. Please ignore.

[Simon_Jester]

If the drive doesn't match any known type of engine in terms of its performance specs and operating principles, how would we go about answering the question? I don't see a way to give a yes or no on this- if the operating mechanism is arbitrary, the only question is whether there are theoretical limits on the potential exhaust velocity or something.

(I am the wrong person to answer this; I'm just trying to get a sense for how to find an answer)

[Whiskey144]

If the drive doesn't match any known type of engine in terms of its performance specs and operating principles, how would we go about answering the question? I don't see a way to give a yes or no on this- if the operating mechanism is arbitrary, the only question is whether there are theoretical limits on the potential exhaust velocity or something.

(I am the wrong person to answer this; I'm just trying to get a sense for how to find an answer)

The principles of operation are somewhat similar to a VASIMR; the way I envision it is as the effective equivalent of a large coilgun-type system, continuously accelerating a stream of plasma. There are limits, in-universe; for example, I intend for the drive to provide no more than 0.5 Gs of thrust, and probably an exhaust velocity of no more than 0.5c.

The types of ships that it pushes around are quite hefty compared to most scifi settings; for example, the protaganist's 252-meter long light frigate-type that I'm working out masses around 3.4 kilotonnes dry; most ships are in the 250-400 meter range, and mass around 300 thousand to just over a million metric tonnes.

If the question can't be answered, though, would it still be possible to work out the thrust and exhaust velocity requirements for a group of four engines that can push around said 3.4 kilotonne ship at accelerations of, at most, 0.5 Gs?

[Covenant]

The real answer to your question is yes, such designs are inherently wasteful because of the amount of energy you're putting into "go faster now" rather than "go faster efficiently." You can't do both, generally.

Furthermore, the numbers being discussed are extreme. Your faster vessel would reduce any living person inside it into paste, and even the lighter vessels are greater than the expected accelerations of the stated type of rocket motor. However there is one grand solution to this:

Show science some credit and either a) reduce your numbers, b) alter the need for it to be a coilgun pumped plasma motor or c) make the drive not conform to reality and stop pretending to be a futurist.

If you want it to be realistic, then you need to either alter your expectations or alter your parameters, or say "Screw this, the most important part for the story is the functionality, so I'll just write in a hand-wave and move on." Nobody is going to want to read anything of yours for a limp technical analysis of rocket motors, so it's not important, really. And if you want scientific integrity then stop fighting the science and write around it. Too many amateur science writers think that we the audience really care if the science is accurate to the letter. It's enough that it is consistent and not brazenly ignoring established physics.

[Whiskey144]

Furthermore, the numbers being discussed are extreme. Your faster vessel would reduce any living person inside it into paste, and even the lighter vessels are greater than the expected accelerations of the stated type of rocket motor. However there is one grand solution to this:

WRT the faster ships, I was under the impression that the human body, in certain positions, could withstand that much.....for short periods of time, though. I remember looking at a table on Atomic Rockets that said something about the g forces a human could take before various unpleasant things happened, but alas I cannot find it.

Show science some credit and either a) reduce your numbers, b) alter the need for it to be a coilgun pumped plasma motor or c) make the drive not conform to reality and stop pretending to be a futurist.

The main reason I want it to perform in this way and be of this type is:

1) The accelerations are usable for puttering around a star system's inner planets in suitably convenient amounts of time
2) Said accelerations and the drives that produce them are (relatively) low-powered, and do not have the unfortunate problems of a fusion torch and a ship trying to jockey for a spot in the 'traffic jam' of a crowded dockyard or orbital spaceport.

I'm also not trying to be a 'futurist'; the setting in question has the plot twist of Earth being a borderline myth. What I wanted was a drive system that could push you around in convenient amounts of time, produce enough thrust that skyscraper-internal ships could have a modicum of internal gravity, and also not run into Jon's Law.

If you want it to be realistic, then you need to either alter your expectations or alter your parameters, or say "Screw this, the most important part for the story is the functionality, so I'll just write in a hand-wave and move on." Nobody is going to want to read anything of yours for a limp technical analysis of rocket motors, so it's not important, really. And if you want scientific integrity then stop fighting the science and write around it. Too many amateur science writers think that we the audience really care if the science is accurate to the letter. It's enough that it is consistent and not brazenly ignoring established physics.

Since it seems I'll have to handwave this, what kind of handwavium would be a good way to run this? I'm not going for mass lightening, that's for certain.

[Covenant]

50g's are not literally going to turn you into paste, mind you, but this is not the sort of thing you can do while taking maneuvers or doing anything other than hanging on for grim death. In fact, 50 gravities is about the area at which people will flat out die or begin suffering massive physical damage if you really do much of anything. So there's no value to a 50g acceleration unless it's some kind of emergency overload red-line button that you turn a key to access and perform only while in autopilot mode.

I think for your needs you'd be better off having two or three different kinds of motors per vessel. You can use them as a non-detachable multi-stage system, or a geared-motor system without having them be the same thing. Having ALL your needs covered by duplicates of the same mechanism is less likely than having a set of short duration boosters (used for lifting-body work or rapid acceleration) and then a cruising engine that handles the boring task of making you continually accelerate to a higher speed with greater delta-v. Plus, it lets one of them get knocked out by combat or mechanical failure and create interesting plot points.

You also need to remember that any kind of powerful rocket motor like this is going to spray so much horrible shit out it's back-end that any form of 'jockeying' for position is likely to end with the complete and total destruction of mankind at the end of a grim, plasma-laser induced obliteration of the planet. Again, not literally, but it's not like these motors will share much of anything with the kinds of things you recognize as a rocket--the forces are amazingly powerful, so you wouldn't want to 'fire it up' anywhere near orbit unless you want to fly off from the planet with a pair of one-finger salutes. In accordance with the Kzinti-lesson, you should envision it akin to taking off from a planet by pointing all your biggest guns down and firing until you're out of orbit.

If you want internal gravity, just rotate them. Keeping a sustained 1g propulsion is not going to be advisable for a craft the size you're talking about, unless it has 3/4ths of that space taken up by fuel and reaction mass. It also reduces the amount of questions you have to account for by a huge amount. You'll be able to have all ships of all sizes with a comfortable gravity due to spinning, with just a few of the 'fast ships' designed for 1g or higher acceleration in mind. Describing the physical difference between a bloated whale civilian ship and the angry, sinister little dart of a fast-moving military ship could be a nice scene too.

As for the kind of handwave, you're probably looking into a torch-drive of some sort. These are widely recognized as the "I don't know, just have fun with it," option amongst harder sci-fis and most people will hear "Torch" and go "ahh, okay." And if any of your characters ask what a Torch is, let the other person scratch their head and say it's really complex, and then give the most cursory explanation of how a Torch differs from the long-endurance slow-acceleration engines that most everyone else is used to encountering.

If your goal is to have a believable setting, that's close enough. The rocket motor is there to enable the story, not to be the story. You're way better off inventing something ridiculous and then having a realistic examination of the implications of this ridiculous thing. That's way more interesting, and "hard sci-fi ish" than someone who wants to tell a good story and has to contort physics to fit around it. Hell, most of us here are fine with Star Wars and Star Trek level of "ignore it, it's not that important" hand-waving. The bar is not that high unless you go out of your way to claim it is literally real.

[Whiskey144]

50g's are not literally going to turn you into paste, mind you, but this is not the sort of thing you can do while taking maneuvers or doing anything other than hanging on for grim death. In fact, 50 gravities is about the area at which people will flat out die or begin suffering massive physical damage if you really do much of anything. So there's no value to a 50g acceleration unless it's some kind of emergency overload red-line button that you turn a key to access and perform only while in autopilot mode.

That's actually the kind of thing I envisioned it used for; a "PANIC" button that gives you a maximum of maybe one or two hours of acceleration (not that you'd need that much in one go) at that kind of "hold on for dear life and hope we don't crash into something" kind of boost.

I think for your needs you'd be better off having two or three different kinds of motors per vessel. You can use them as a non-detachable multi-stage system, or a geared-motor system without having them be the same thing. Having ALL your needs covered by duplicates of the same mechanism is less likely than having a set of short duration boosters (used for lifting-body work or rapid acceleration) and then a cruising engine that handles the boring task of making you continually accelerate to a higher speed with greater delta-v. Plus, it lets one of them get knocked out by combat or mechanical failure and create interesting plot points.

This is actually what most military vessels have in-setting; a set of "plasma drives" for most operations, and a fusion or antimatter torch that's used for high-G battle maneuvers or sprints to chase down or run away from something. OTOH, most civil ships simply don't need that kind of power, and thus lack the torches of military vessels.

EDIT: I should also clarify what I said in the OP; it's mechanically simple for a ship to replace it's plasma drives with more robust fusion torches or antimatter engines, but a ship typically doesn't carry all of those engines; at most, you'll have a few plasma drives and a torch of some sort.

You also need to remember that any kind of powerful rocket motor like this is going to spray so much horrible shit out it's back-end that any form of 'jockeying' for position is likely to end with the complete and total destruction of mankind at the end of a grim, plasma-laser induced obliteration of the planet. Again, not literally, but it's not like these motors will share much of anything with the kinds of things you recognize as a rocket--the forces are amazingly powerful, so you wouldn't want to 'fire it up' anywhere near orbit unless you want to fly off from the planet with a pair of one-finger salutes. In accordance with the Kzinti-lesson, you should envision it akin to taking off from a planet by pointing all your biggest guns down and firing until you're out of orbit.

That's the kind of thing I'm trying to avoid, actually. Jon's Law is closely related to the Kzinti Lesson, hence my mentioning it, so I'd say we're on the same page here. Though I'm curious; do you mean that the plasma drive I'm positing that pushes ships around at maybe half a G would do this, or are you referring to the engine plume of something like a nuclear or antimatter engine?

If you want internal gravity, just rotate them. Keeping a sustained 1g propulsion is not going to be advisable for a craft the size you're talking about, unless it has 3/4ths of that space taken up by fuel and reaction mass. It also reduces the amount of questions you have to account for by a huge amount. You'll be able to have all ships of all sizes with a comfortable gravity due to spinning, with just a few of the 'fast ships' designed for 1g or higher acceleration in mind. Describing the physical difference between a bloated whale civilian ship and the angry, sinister little dart of a fast-moving military ship could be a nice scene too.

Well, I'm pretty much thinking "1G continuous is nuts" right now, so I'm going for something that can be lived in, but isn't huge, like Lunar-level 0.1-0.2 Gs. Though you do have an interesting point with that scene bit. Food for thought, then.

As for the kind of handwave, you're probably looking into a torch-drive of some sort. These are widely recognized as the "I don't know, just have fun with it," option amongst harder sci-fis and most people will hear "Torch" and go "ahh, okay." And if any of your characters ask what a Torch is, let the other person scratch their head and say it's really complex, and then give the most cursory explanation of how a Torch differs from the long-endurance slow-acceleration engines that most everyone else is used to encountering.

Would you consider a plasma drive-derived torch to be believable, assuming the following characteristics:

1) no more than 0.5 G acceleration capability.
2) Exhaust velocity is around 80% of cee.

If your goal is to have a believable setting, that's close enough. The rocket motor is there to enable the story, not to be the story. You're way better off inventing something ridiculous and then having a realistic examination of the implications of this ridiculous thing. That's way more interesting, and "hard sci-fi ish" than someone who wants to tell a good story and has to contort physics to fit around it. Hell, most of us here are fine with Star Wars and Star Trek level of "ignore it, it's not that important" hand-waving. The bar is not that high unless you go out of your way to claim it is literally real.

Yeah, that last sentence is definitely something I want to avoid. I think with some aspects, I've been kind of going for not just being believable but also somewhat plausible.

[Simon_Jester]

Furthermore, the numbers being discussed are extreme. Your faster vessel would reduce any living person inside it into paste, and even the lighter vessels are greater than the expected accelerations of the stated type of rocket motor. However there is one grand solution to this:

WRT the faster ships, I was under the impression that the human body, in certain positions, could withstand that much.....for short periods of time, though. I remember looking at a table on Atomic Rockets that said something about the g forces a human could take before various unpleasant things happened, but alas I cannot find it.

If 50g is indeed the figure, then "short periods of time" is measured, at best, in seconds. And that is going to be literally bone-breaking force- I mean, the average adult male would weigh the equivalent of something like four metric tons under that kind of acceleration. What happens to a man who has four tons of weight applied to him? He winds up pretty thoroughly crushed, I'd expect.

The main reason I want it to perform in this way and be of this type is:

1) The accelerations are usable for puttering around a star system's inner planets in suitably convenient amounts of time
2) Said accelerations and the drives that produce them are (relatively) low-powered, and do not have the unfortunate problems of a fusion torch and a ship trying to jockey for a spot in the 'traffic jam' of a crowded dockyard or orbital spaceport.

I'm also not trying to be a 'futurist'; the setting in question has the plot twist of Earth being a borderline myth. What I wanted was a drive system that could push you around in convenient amounts of time, produce enough thrust that skyscraper-internal ships could have a modicum of internal gravity, and also not run into Jon's Law.

Those are incompatible goals- accelerating several thousand tons of matter at rates of several meters per second per second inevitably implies power output on the order of gigawatts. Looking at realistic ion engines you're looking at a power-to-thrust ratio of something roughly around 1 kW per newton; you need millions of newtons of force to achieve the desired acceleration of the desired mass. You do the math.

So Jon's Law hits you by default.

Since it seems I'll have to handwave this, what kind of handwavium would be a good way to run this? I'm not going for mass lightening, that's for certain.

If you want reaction drives, and you want those accelerations, you need to accept Jon's Law. Spacecraft will need to be designed with that in mind- hulls should be as lightly built as feasible to maximize acceleration. Ships that go in for planet landings will need to land in areas (reinforced pads or bodies of water) that can survive having many billions of joules dumped into them in a short time.

50g's are not literally going to turn you into paste, mind you, but this is not the sort of thing you can do while taking maneuvers or doing anything other than hanging on for grim death. In fact, 50 gravities is about the area at which people will flat out die or begin suffering massive physical damage if you really do much of anything. So there's no value to a 50g acceleration unless it's some kind of emergency overload red-line button that you turn a key to access and perform only while in autopilot mode.

That's actually the kind of thing I envisioned it used for; a "PANIC" button that gives you a maximum of maybe one or two hours of acceleration (not that you'd need that much in one go) at that kind of "hold on for dear life and hope we don't crash into something" kind of boost.

An hour at fifty gravities isn't remotely survivable- again, your body will weigh four tons during that engine burn. The front of your head will be like a fifty kilogram weight pressing down on the back of your head. Your heart cannot begin to pump blood against that kind of acceleration. And so on.

If you want acceleration in that range, you need magic artificial gravity- and, in all probability, magic engines. The only real life things that accelerate at that rate are solid fuel rockets that burn out in a matter of seconds- essentially just a long stick of explosives that burns from one end to the other.

That's the kind of thing I'm trying to avoid, actually. Jon's Law is closely related to the Kzinti Lesson, hence my mentioning it, so I'd say we're on the same page here. Though I'm curious; do you mean that the plasma drive I'm positing that pushes ships around at maybe half a G would do this, or are you referring to the engine plume of something like a nuclear or antimatter engine?

Both, but the half-g engine does it to a lesser degree.

Put it this way- suppose you are currently moving at 5 km/s in orbit around a planet, parallel to the ground. You wish to accelerate your craft by 5 meters per second in one second, from 5000 m/s to 5005 m/s.

Calculate the amount by which your thousand-ton ship's kinetic energy must increase in order for this to happen.

Well, I'm pretty much thinking "1G continuous is nuts" right now, so I'm going for something that can be lived in, but isn't huge, like Lunar-level 0.1-0.2 Gs.

It's remotely possible, but for comfort and living conditions, spin is far and away the better choice. Your ships are big enough that spinning for gravity is actually (somewhat) possible, too... and could be built bigger without being more massive.

Warships might be more compact, or at least have a more compact crew module so that said module can be more easily radiation-shielded and the like. But then, the crew of a warship is going to receive and be expected to handle zero-g training.

Would you consider a plasma drive-derived torch to be believable, assuming the following characteristics:

1) no more than 0.5 G acceleration capability.
2) Exhaust velocity is around 80% of cee.

Why do you care? Don't obsess over the numbers; you are not a rocket scientist or you wouldn't be asking us this question, which means you can't make the design plausible. If you can't tell the story without explaining how the rocket works, you're going about this the wrong way.

[Whiskey144]

If 50g is indeed the figure, then "short periods of time" is measured, at best, in seconds. And that is going to be literally bone-breaking force- I mean, the average adult male would weigh the equivalent of something like four metric tons under that kind of acceleration. What happens to a man who has four tons of weight applied to him? He winds up pretty thoroughly crushed, I'd expect.

When I say an hour or two "total", I mean that's the total duration that the engines can handle spitting out that much energy before either failing or running out of gas. Probably both.

I don't really mean that it's the amount of time that a powered dash would last.

Those are incompatible goals- accelerating several thousand tons of matter at rates of several meters per second per second inevitably implies power output on the order of gigawatts. Looking at realistic ion engines you're looking at a power-to-thrust ratio of something roughly around 1 kW per newton; you need millions of newtons of force to achieve the desired acceleration of the desired mass. You do the math.

Oh. Well crap, I didn't expect something of that magnitude. Though would a 0.1 G capability be within an order of magnitude of that or would it be a lot less?

It's remotely possible, but for comfort and living conditions, spin is far and away the better choice. Your ships are big enough that spinning for gravity is actually (somewhat) possible, too... and could be built bigger without being more massive.

Alright then....... Two things; one more of a comment and the other a question. Question: how well do you expect an internal spin-hab would work? Comment: I wasn't really too keen on the spin-hab ideas because of the fact that it adds more complexity; OTOH, the way I've got things worked out it's far less complexity, than say, the way I'm working out the FTL system.

Why do you care? Don't obsess over the numbers; you are not a rocket scientist or you wouldn't be asking us this question, which means you can't make the design plausible. If you can't tell the story without explaining how the rocket works, you're going about this the wrong way.

You have a point there, I suppose. It's a bit of a pet peeve of mine, I guess; should probably just get over it, call it a plasma drive, say it spits out plasma and call it a day.

[Simon_Jester]

Oh. Well crap, I didn't expect something of that magnitude. Though would a 0.1 G capability be within an order of magnitude of that or would it be a lot less?

You can calculate the force you need to accelerate a given mass at a given rate easily enough; that's algebra.

You're unlikely to get power requirements much lower than an ion drive in terms of power consumed per newton of thrust generated, so there's really no getting around this problem. Not unless you're willing to accept really low accelerations. You could probably accelerate a few thousand tons at 0.001g without creating a truly devastating exhaust plume, for instance- but such a drive would not be useful for the kind of purpose you desire.

Alright then....... Two things; one more of a comment and the other a question. Question: how well do you expect an internal spin-hab would work? Comment: I wasn't really too keen on the spin-hab ideas because of the fact that it adds more complexity; OTOH, the way I've got things worked out it's far less complexity, than say, the way I'm working out the FTL system.

Mechanically, spinning things is very easy. For a stationary habitat, trivially so- you spin it up and just let it keep going. For a ship with components not being spun for gravity, you need some bearings and such, but it is really not a problem compared to the enormous power and engineering requirements to get the ship working at all.

[Whiskey144]

You're unlikely to get power requirements much lower than an ion drive in terms of power consumed per newton of thrust generated, so there's really no getting around this problem. Not unless you're willing to accept really low accelerations. You could probably accelerate a few thousand tons at 0.001g without creating a truly devastating exhaust plume, for instance- but such a drive would not be useful for the kind of purpose you desire.

Hmm. Raises an interesting question, in relation to the spinhab unit that I think I'll go with. *pauses and does math* Yeah, a 50,000 second 0.01G burn gives a terminal velocity of nearly 5 km/s, and distance covered of about 12,000 kilometers.

Well, at least I get continuously-thrusting ships.

[Whiskey144]

Ghetto Edit: I've been unable to find equations that would allow me to determine the needed thrust force and exhaust velocity/specific impulse that would be needed to push around a ship of x tonnes at y m/s^2 or Gs.

Would you happen to know where I might be able to find such things?

Side note: would you consider a fusion engine a good idea for an SSTO spaceplane type vehicle?

[Simon_Jester]

Look up Newton's Second Law. That gives you thrust. You can, in principle, calculate the exhaust velocity backward from the rate at which you throw mass out the back of the rocket, or the rate of fuel expenditure from the exhaust velocity- conservation of momentum.

Really, this should be on Atomic Rocket somewhere; I never went looking for it because I never needed it that badly, but it's there.

And hell no a fusion engine is a terrible idea for an SSTO, because of the huge trail of superheated exhaust. You do not want something like that taking off from ground level; chemical rockets are bad enough.

[Whiskey144]

Look up Newton's Second Law. That gives you thrust. You can, in principle, calculate the exhaust velocity backward from the rate at which you throw mass out the back of the rocket, or the rate of fuel expenditure from the exhaust velocity- conservation of momentum.

Thank 'e. I should be able to go from here then. Time for some Google-fu!
EDIT: Not sure why I didn't think of this earlier as well. Still, that's why asking questions is great.

Really, this should be on Atomic Rocket somewhere; I never went looking for it because I never needed it that badly, but it's there.

*facepalms* Why did I not think to go to the most obvious place? Ah well, better late than never I suppose.

And hell no a fusion engine is a terrible idea for an SSTO, because of the huge trail of superheated exhaust. You do not want something like that taking off from ground level; chemical rockets are bad enough.

O_o. In that case, I think I'll probably settle for nuke thermal; we know it could work pretty well, and the DUMBO variant of the NTR that, sadly, never flew, is estimated to be fully capable of liftoff. Though, I'm curious, what engines do you think would be suitable for an SSTO spaceplane?

[Simon_Jester]

Chemical rockets, ramjets, scramjets... those are the big ones that come to mind. Most proposals I'm familiar with use a large, purely air-breathing mothership to get up to some multiple of the speed of sound and a high enough altitude that atmosphere isn't a serious problem, then take off from there on some high-speed engine (like a scramjet). The scramjet gets them to high enough speed that a relatively modest boost of delta-v can get them all the way into orbit.

[Whiskey144]

Chemical rockets, ramjets, scramjets... those are the big ones that come to mind. Most proposals I'm familiar with use a large, purely air-breathing mothership to get up to some multiple of the speed of sound and a high enough altitude that atmosphere isn't a serious problem, then take off from there on some high-speed engine (like a scramjet). The scramjet gets them to high enough speed that a relatively modest boost of delta-v can get them all the way into orbit.

Hmmm. The thing is, other things I've read indicate that a chemfuel rocket would be.....unsuitable, to say the least, for an SSTO vehicle.

I do remember reading a proposal about a close-cycle gas core nuke thermal rocket that was SSTO, powered descent, and could heft an insane two million (!) pounds to LEO! OTOH, I'm thinking that that's no where near what I need for the average cargo spaceplane used by a something like a free trader ship (I envision an SSTO with a 100 tonne payload to be perfectly sufficient, especially as the average merchant ship has a good 1000 tonnes of cargo capacity, on average).

And, as I have just run the numbers for a ~210,000 tonne ship kicking around at 0.01 Gs, I find I need around 20 meganewtons of thrust. Ouch......it just sounds like it'll be spitting out a Plume of Doom. Pushing around at 0.001 Gs, it drops by an order of magnitude, but has the issue in that it now takes for-freaking-ever to get somewhere. Reducing mass seems to have only marginal effect; dropping some 40-50 thousand tonnes and I still need around 16.5 meganewtons of thrust force.

[Simon_Jester]

Hmmm. The thing is, other things I've read indicate that a chemfuel rocket would be.....unsuitable, to say the least, for an SSTO vehicle.

Rockets are good for certain purposes and bad for others; whether you want them (and for what) depends on many details that are too complicated to be properly analyzed by amateurs. There are a lot of spaceplane designs kicking around; you'll have to go looking on your own.

The main problem with chemical rocketry is that tanks of rocket fuel add a lot of mass, which creates huge mass penalties for staging: if you want to send something to the moon and it needs 20% of its own weight in fuel to make the needed engine burns to get back to Earth, that increases the mass of each stage that goes behind it by a large fraction. It adds up. Moreover, you need additional fuel purely to accelerate the fuel consumed later in the burn, causing the percent of the rocket's takeoff weight devoted to fuel to grow geometrically (exponentially? Not sure).

But if you only use the chemical rocket boosters once, or twice (once to get the final push up to orbital velocity after your scramjet peters out around Mach 15-20, having already climbed above nearly the whole atmosphere, and once for reentry burn)... the mass of fuel required decreases, and the objections against chemical rocketry decrease too.

And, as I have just run the numbers for a ~210,000 tonne ship kicking around at 0.01 Gs, I find I need around 20 meganewtons of thrust. Ouch......it just sounds like it'll be spitting out a Plume of Doom. Pushing around at 0.001 Gs, it drops by an order of magnitude, but has the issue in that it now takes for-freaking-ever to get somewhere. Reducing mass seems to have only marginal effect; dropping some 40-50 thousand tonnes and I still need around 16.5 meganewtons of thrust force.

...Excuse me. 0.01g is 0.1 meters per second squared. Applying Newton's second law, you should need... yes, 21 million newtons of thrust. This is not necessarily a Plume of Doom; it depends on how far away you are and how high the exhaust velocity is. But it's certainly not to be taken lightly.

...But how did you get 16.5 million newtons of thrust for a ship of 20 to 25% the mass?

Still, though, this definitely falls under the heading of things to be handled with care.

[Whiskey144]

But if you only use the chemical rocket boosters once, or twice (once to get the final push up to orbital velocity after your scramjet peters out around Mach 15-20, having already climbed above nearly the whole atmosphere, and once for reentry burn)... the mass of fuel required decreases, and the objections against chemical rocketry decrease too.

Yeah...... I think the thing is that I want something with oomph, but also crazy-good multipurpose reusability. OTOH, using a solid-core nuke thermal seems like a good compromise, assuming suitably robust......or unpopulated, areas.

...Excuse me. 0.01g is 0.1 meters per second squared. Applying Newton's second law, you should need... yes, 21 million newtons of thrust. This is not necessarily a Plume of Doom; it depends on how far away you are and how high the exhaust velocity is. But it's certainly not to be taken lightly.

Well, considering I'm thinking this are pretty much (relatively) high-thrust plasma drives, I'm thinking exhaust velocity is somewhere in the region of several thousand to tens of thousands of km/s.

...But how did you get 16.5 million newtons of thrust for a ship of 20 to 25% the mass?

What I have so far is a decided volume for the ship in question; from there I simply go quick-and-dirty by deciding on an average density. The 210 kilotonne figure is for 0.25 tonnes/m^3; dropping it to 0.2 tonnes/m^3 gives me about 168 kilotonnes, a 20% decrease in tonnage. Doing the math gives me 16.5 meganewtons of thrust for the same acceleration; about a 21.5% decrease in thrust requirement. So it's roughly proportional, I think.

Still, though, this definitely falls under the heading of things to be handled with care.

Well, "handle with care" is a lot better than "do not use in close proximity to other objects". Considering how I'm intending to work everything together, it sounds pretty reasonable, actually; in fact, it matches up quite closely with the "huge amounts of paperwork for taking captaincy of a small-ish frigate".

[Simon_Jester]

...But how did you get 16.5 million newtons of thrust for a ship of 20 to 25% the mass?

What I have so far is a decided volume for the ship in question; from there I simply go quick-and-dirty by deciding on an average density. The 210 kilotonne figure is for 0.25 tonnes/m^3; dropping it to 0.2 tonnes/m^3 gives me about 168 kilotonnes, a 20% decrease in tonnage. Doing the math gives me 16.5 meganewtons of thrust for the same acceleration; about a 21.5% decrease in thrust requirement. So it's roughly proportional, I think.

...Okay, before you said 40 to 50 thousand tons. Not the same thing.

Still, though, this definitely falls under the heading of things to be handled with care.

Well, "handle with care" is a lot better than "do not use in close proximity to other objects". Considering how I'm intending to work everything together, it sounds pretty reasonable, actually; in fact, it matches up quite closely with the "huge amounts of paperwork for taking captaincy of a small-ish frigate".

Anything armed with nuclear-equivalent weapons (and pretty much any spacegoing warship will be) will require a LOT of vetting before you can take control, unless you've got technomagic shielding to deal with the consequences of someone going apeshit at the trigger. Maybe even then.

[Whiskey144]

...Okay, before you said 40 to 50 thousand tons. Not the same thing.

Yeah... I should have been more specific.

Anything armed with nuclear-equivalent weapons (and pretty much any spacegoing warship will be) will require a LOT of vetting before you can take control, unless you've got technomagic shielding to deal with the consequences of someone going apeshit at the trigger. Maybe even then.

Well, in-setting, power generation for ships is onboard fusion reactors; a good set of anywhere from two to six will give you up to a terawatt of usable power, less waste heat. Factoring in other inefficiencies, and the requirements of the electric propulsion, then weapons, which tend to be laser armament, is going to be sub-kiloton range.

OTOH I do have the technomagic shields, so meh, it all depends. 'Course, the average civil ship tend to lack the most basic armament; generally, a few PD lasers might be carried. Most companies simply consider armament to be "extraneous cost", though the operating crews will mount whatever they can if they tend to operate the ship in the seedier regions.

Though the catch is that non-corporate ship owners and captains tend to have ships that are well-armed.......sometimes even of comparable to military vessels of similar tonnage range. But yeah, the idea is that because of the ship specs, there's a lot of paperwork involved.

Ironically, things like antimatter storage and transporation equipment are barely regulated.........but antimatter production and propulsion devices/systems are.

EDIT: about how damaging would a group of 4 drives producing 16.5 meganewtons be, with an top exhaust velocity of 100,000 km/s? Doing out the math, a mass ratio of only three would be required for a delta-v of around 110,000 km/s. Not too shabby, though it's probably a whole lot more than is necessary.....