The 3 kilometer question...

[Guest]

We can calculate that, actually. Let's assume a mass ratio of 2, Isp of 800ks, thrust of 1G at 9200T, that is 90MN. That means 4600T of propellant. F/(g0*Isp)=mdot (from the definition of Isp). Plugging in numbers... Your propellant consumption is about 11 kilograms per second.  Given our 4600T, that gives you endurance of about four days of constant thrusting. Should be enough for a minimum distance constant acceleration Earth-Mars cruise. 

Now let's figure out antimatter, while we're at it. Thrust power is (F*g0*Isp)/2. Plugging in numbers... 300TW. Quite a beast, definitely torchship-level output. Over 4 days, that thing gives out 141000PJ. That's a lot, but how much antimatter? m=E/(2c^2), from the famous equation (divided by 2, because only half of the mass is antimatter). This gives three quarters of a ton. Behold the power of math(ematica).  Given that your civilization can play around with gravity, this is probably not an unreasonable amount to put on a 9200T ship. 

Of course, it's still a planet killer. Any torchship is, nevermind anything interstellar. However, one might ask if you really do need that kind of endurance. Rather, try to calculate what kind of trajectories you really need for the story to work, and work out what kind of drive you need from there. None of this requires more than high school maths and a table of physical constants.

Edited July 11, 2019 by Guest

[Spacescifi]

19 minutes ago, Dragon01 said:

We can calculate that, actually. Let's assume a mass ratio of 2, Isp of 800ks, thrust of 1G at 9200T, that is 90MN. That means 4600T of propellant. F/(g0*Isp)=mdot (from the definition of Isp). Plugging in numbers... Your propellant consumption is about 11 kilograms per second.  Given our 4600T, that gives you endurance of about four days of constant thrusting. Should be enough for a minimum distance constant acceleration Earth-Mars cruise. 

Now let's figure out antimatter, while we're at it. Thrust power is (F*g0*Isp)/2. Plugging in numbers... 300TW. Quite a beast, definitely torchship-level output. Over 4 days, that thing gives out 141000PJ. That's a lot, but how much antimatter? m=E/(2c^2), from the famous equation (divided by 2, because only half of the mass is antimatter). This gives three quarters of a ton. Behold the power of math(ematica).  Given that your civilization can play around with gravity, this is probably not an unreasonable amount to put on a 9200T ship. 

Of course, it's still a planet killer. Any torchship is, nevermind anything interstellar. However, one might ask if you really do need that kind of endurance. Rather, try to calculate what kind of trajectories you really need for the story to work, and work out what kind of drive you need from there. None of this requires more than high school maths and a table of physical constants.

 

Wow! Thanks.

Did not realize how antimatter could give thrust for days on end.

Which is more than good enough for orbiting planets and rendezvous.

See, I do not have to worry about slowly cruising between worlds at all. That is what the translation jump drive is for. Getting close to planets in vacuum. The grav-inverters can be used to just hover in space above a planet, but if you wanna orbit you have to burn propellant.

Yet about half my ship's mass is methane propellant, which brings the efficiency down from liquid hydrogen, but not by much. Since we are injecting antimatter into it!

 

Math is my weakest subject, but it is nothing I could not fix by studying.

 

EDIT: An exploration ship never knows what it will face, so yeah it needs the endurance/propellant.

Liners on regular routes could get by with less, as they do not plan on the unexpected happening.

Edited July 11, 2019 by Spacescifi

[Guest]

Actually, those Isp numbers are for LH2. For LCH4, you may need to slightly adjust them. There are equations for that, too. TBH, at those energies, the exact propellant used is kind of secondary. However, keep in mind that this particular example has too low mass flow to allow proper open-cycle cooling. This tends to be a problem with extreme examples of a torch drive (the Expanse fusion drives are the same). That said, I think that using a transparent thrust chamber (fused quartz or something), a secondary active cooling system (which you may as well use to generate electricity) and a few hundred tons of some sort of plasma radiator, it might be possible to handle that kind of heat. Liquid hydrogen might be a good coolant medium, once again thanks to its transparency. Of course, the rest of the ship will need to be shielded against all that radiated heat.

Definitely put some effort into understanding the equations behind all this. The exact numbers are not important, but the orders of magnitude let you get the general picture of what to expect from your craft. For example, if your ship is not supposed to "slowly" (going from Earth to Mars in less time than it takes a sailboat to cross the Baltic sea can't really be called that) flying between planets, you definitely do not need an 800ks drive and antimatter loads on order of a ton. If you want to go from Earth to Moon at 1G, requirements drop dramatically. In fact, you could probably get away with gas core NTRs. If you give me some idea of what exactly do you want it to do, I can run the numbers for you.

That said, I'd say 1G travel is overrated. I also used to think that you'd need at least that for planetary liftoff, but you really don't. Here's the trick: air is heavy. Hydrogen has MW of 2, methane 16, a single oxygen molecule 32, a nitrogen molecule 28. By using air as propellant, you can actually gain a good bit of thrust, especially if your primary engine mode runs on hydrogen. Not only that, your Isp doesn't matter with air. You can dump a lot of it into the engine, which will cause Isp to plummet (and exhaust velocity - very good thing for not cutting the launch pad in half), while greatly boosting thrust. By the time you're high enough that air runs out, you can probably switch to your primary propellant, which can have TWR of a fraction of G. Unless you're interested in airless bodies like Mercury (and even then, it's only 0.38G or so), you don't need a high thrust space drive. Since you have gravity manipulation, generating it by thrust would be redundant.

Edited July 11, 2019 by Guest

[Spacescifi]

29 minutes ago, Dragon01 said:

Actually, those Isp numbers are for LH2. For LCH4, you may need to slightly adjust them. There are equations for that, too. TBH, at those energies, the exact propellant used is kind of secondary. However, keep in mind that this particular example has too low mass flow to allow proper open-cycle cooling. This tends to be a problem with extreme examples of a torch drive (the Expanse fusion drives are the same). That said, I think that using a transparent thrust chamber (fused quartz or something), a secondary active cooling system (which you may as well use to generate electricity) and a few hundred tons of some sort of plasma radiator, it might be possible to handle that kind of heat. Liquid hydrogen might be a good coolant medium, once again thanks to its transparency. Of course, the rest of the ship will need to be shielded against all that radiated heat.

Definitely put some effort into understanding the equations behind all this. The exact numbers are not important, but the orders of magnitude let you get the general picture of what to expect from your craft. For example, if your ship is not supposed to "slowly" (going from Earth to Mars in less time than it takes a sailboat to cross the Baltic sea can't really be called that) flying between planets, you definitely do not need an 800ks drive and antimatter loads on order of a ton. If you want to go from Earth to Moon at 1G, requirements drop dramatically. In fact, you could probably get away with gas core NTRs. If you give me some idea of what exactly do you want it to do, I can run the numbers for you.

That said, I'd say 1G travel is overrated. I also used to think that you'd need at least that for planetary liftoff, but you really don't. Here's the trick: air is heavy. Hydrogen has MW of 2, methane 16, a single oxygen molecule 32, a nitrogen molecule 28. By using air as propellant, you can actually gain a good bit of thrust, especially if your primary engine mode runs on hydrogen. Not only that, your Isp doesn't matter with air. You can dump a lot of it into the engine, which will cause Isp to plummet (and exhaust velocity - very good thing for not cutting the launch pad in half), while greatly boosting thrust. By the time you're high enough that air runs out, you can probably switch to your primary propellant, which can have TWR of a fraction of G. Unless you're interested in airless bodies like Mercury (and even then, it's only 0.38G or so), you don't need a high thrust space drive. Since you have gravity manipulation, generating it by thrust would be redundant.

 

Given how high the exhaust is moving, I doubt you would even see much of a plume in space.

Not a long one anyway.

What do you think?

[Guest]

A plume from that kind of engine would resemble a glowing beam, regardless of what it was made of. It would likely be quite long, as well, exactly because of the particles' velocity. Extreme performance drives generally tend to be like that. That, incidentally, makes it quite a good weapon. However, if you have that kind of energy at your disposal, I would expect purpose-built weapons to be even better.

[Spacescifi]

8 minutes ago, Dragon01 said:

A plume from that kind of engine would resemble a glowing beam, regardless of what it was made of. It would likely be quite long, as well, exactly because of the particles' velocity. Extreme performance drives generally tend to be like that. That, incidentally, makes it quite a good weapon. However, if you have that kind of energy at your disposal, I would expect purpose-built weapons to be even better.

Are you saying it would not funnel out like normal plumes (get wider at the end)?

Since space is a vacuum I am rather certain that it would fan out. Just a longer plume perhaps?

However if they have scifi plasma bolt weaponry, then the plume could actually look like a jet fighter's.

 

How in space?

Scifi forcefields would be the only way I imagine to keep the exhaust plume from spreading out, which you can do if you have plasma bolt weapons, meaning shaped propelled plasma is possible.

The benefit of beam exhaust is that more of the thrust goest into your forward momentum rather than being spread out to the sides.

Edited July 11, 2019 by Spacescifi

[Guest]

12 minutes ago, Spacescifi said:

Are you saying it would not funnel out like normal plumes (get wider at the end)?

Since space is a vacuum I am rather certain that it would fan out. Just a longer plume perhaps?

It would funnel out. However, the effect would only be visible at extreme (meaning: interplanetary) ranges, because the particles are moving just that fast. At the Isp I've given, that's two and a half percent of speed of light. It would not be very useful to keep it straight, either. It will likely diffuse and become invisible by the time it starts to visibly spread out. Also, it will probably not be very visible at all, because there aren't many particles in it. Remember, your mass flow rate is rather small, and it's moving incredibly fast. 

That said, it would give impressive visuals: the engine itself would glow like a star. It should be possible to engineer a design that could withstand that (lots of active cooling, high melting points, and transparency to let the plasma itself act as a radiator), but even at very high efficiency it would be blindingly bright, outputting gigawatts of heat.

Edited July 11, 2019 by Guest

[Spacescifi]

13 minutes ago, Dragon01 said:

It would funnel out. However, the effect would only be visible at extreme (meaning: interplanetary) ranges, because the particles are moving just that fast. At the Isp I've given, that's two and a half percent of speed of light. It would not be very useful to keep it straight, either. It will likely diffuse and become invisible by the time it starts to visibly spread out. Also, it will probably not be very visible at all, because there aren't many particles in it. Remember, your mass flow rate is rather small, and it's moving incredibly fast. 

That said, it would give impressive visuals: the engine itself would glow like a star. It should be possible to engineer a design that could withstand that (lots of active cooling, high melting points, and transparency to let the plasma itself act as a radiator), but even at very high efficiency it would be blindingly bright, outputting gigawatts of heat.

 

One last question. Shoud I use bell shaped nozzles or jet fighter nozzles that can expand or narrow?

Since the vessel is capable of hovering in the air or in space above a planet thanks to grav-inverter scifi tech?

I was thinking due to the high velocity the nozzle type may not even matter in vacuum.

But I was also thinking that the jet fighter nozzles would be optimal in an atmosphere.

Mixing even a tiny amount of atmosphere in the air would make for a great propulsion system to move forward while hovering.

Barring antimatter use inside atmospheres, metallic hydrogen could be used instead, as it gives plenty of thrust to make a good thermal jet too.

Edited July 11, 2019 by Spacescifi
Planet

[Guest]

Metallic hydrogen is much more dubious than antimatter and just about as dangerous. Might as well go with antimatter. For atmospheric use, go with bell nozzles. They're light, and there is no point investing in altitude compensation, since those are just steering jets, and air is free anyway. You're carrying so much energy that you don't really need to fiddle around with altitude optimization. 

If outputting antimatter exhaust directly into atmosphere bothers you, put it into a generator instead and use electric ducted fans. They're heavy and work best at low speeds, but you can use them to loft your ship clear of the planet, especially with the help of an antigravity device.

[Spacescifi]

6 minutes ago, Dragon01 said:

Metallic hydrogen is much more dubious than antimatter and just about as dangerous. Might as well go with antimatter. For atmospheric use, go with bell nozzles. They're light, and there is no point investing in altitude compensation, since those are just steering jets, and air is free anyway. You're carrying so much energy that you don't really need to fiddle around with altitude optimization. 

If outputting antimatter exhaust directly into atmosphere bothers you, put it into a generator instead and use electric ducted fans. They're heavy and work best at low speeds, but you can use them to loft your ship clear of the planet, especially with the help of an antigravity device.

Lofting up the big ship to space is not an issue, it literally inverts gravity's effect on the vessel so that it falls up at 1g instead of down.

Hovering is just another mode of the same device.

I could use ducted fans, that would be simple.

Or I could just put the antimatter energy into the electrical capacitors, and shunt the electrical power into the air to make a thermal jet using the same rocket nozzles. I think I will do that.

Last question ( can't think of anymore).

Barring scifi tech, how long would the nozzlws last before they need to be replaced?

And would mahnetic nozzles be preferable? In space yes.

In atmosphere? Not so much. Since the plume is going flame all over it in the atmosphere.

Come to think of it, a ducted fan may be my only option to avoid wrecking the nozzle earlier than expected... assuming it is one of those fancy yet fragile magnetic nozzles.

 

[Guest]

That depends. Your main engine will need a magnetic nozzle, at least if it's a torch drive of the type I outlined. For atmospheric use, you will want a fused quartz support structure. Nothing else will survive the heat. 

If your main engine is space only, then it doesn't matter all that much what you make your secondary nozzles out of. However, carbon works very well, especially amorphous allotrope. It's light, can withstand impressive temperatures and is mechanically tough. If you can make a nozzle out of that, it will last forever. For an air resistojet or arcjet, graphite is perfect, because it's conductive and, once again, can withstand the heat. Arcjet will have electrode wear, but that will happen no matter what you use. 

Since you have gravity neutralization, arcjets that can use either air or main engine propellant are a good idea. You can use them as RCS in space and as lift jets/RCS in atmosphere.

Edited July 11, 2019 by Guest

[Spacescifi]

2 minutes ago, Dragon01 said:

That depends. Your main engine will need a magnetic nozzle, at least if it's a torch drive of the type I outlined. For atmospheric use, you will want a fused quartz support structure. Nothing else will survive the heat. 

If your main engine is space only, then it doesn't matter all that much what you make your secondary nozzles out of. However, carbon works very well, especially amorphous allotrope. It's light, can withstand impressive temperatures and is mechanically tough. If you can make a nozzle out of that, it will last forever. For an air resistojet or arcjet, graphite is perfect, because it's conductive and, once again, can withstand the heat. Arcjet will have electrode wear, but that will happen no matter what you use. 

Since you have gravity neutralization, arcjets that can use either air or main engine propellant are a good idea. You can use them as RCS in space and as lift jets/RCS in atmosphere.

 

I do not think KSP has a magnetic nozzle mod. Nor do I think they would show a realistic antimatter injected propellant plume (not much plume, just a bright light).

Don't magnetic nozzles take thermal damage in vacuum? Or does the lack of air and the direction of the exhaust make thermal heating less of an issue than what it would be for conventional nozzles?

[Guest]

Magnetic fields don't melt.  Coils that produce them might, which is why they need to be actively cooled. The point of a magnetic combustion chamber and nozzle is that there is no physical contact between superheated plasma and the ship's structure. For a space-only drive, you can probably work with a lattice of generating coils with little structural support, and just retract them into the ship when entering atmosphere. If you want it to work in atmosphere a casing will be needed.

No, you won't see accurate visuals for those things in KSP, at least not yet. You can try Nertea's Far Future Technologies mod for some somewhat more toned down antimatter engines, including one with a plasma core (albeit with a solid chamber). Many of those engines do, in fact, have a magnetic nozzle. It looks like a roughly nozzle-shaped lattice structure. I recommend you check it out, it's still in development, but it's an excellent mod.

[Spacescifi]

4 minutes ago, Dragon01 said:

Magnetic fields don't melt.  Coils that produce them might, which is why they need to be actively cooled. The point of a magnetic combustion chamber and nozzle is that there is no physical contact between superheated plasma and the ship's structure. For a space-only drive, you can probably work with a lattice of generating coils with little structural support, and just retract them into the ship when entering atmosphere. If you want it to work in atmosphere a casing will be needed.

No, you won't see accurate visuals for those things in KSP, at least not yet. You can try Nertea's Far Future Technologies mod for some somewhat more toned down antimatter engines, including one with a plasma core (albeit with a solid chamber). Many of those engines do, in fact, have a magnetic nozzle. It looks like a roughly nozzle-shaped lattice structure. I recommend you check it out, it's still in development, but it's an excellent mod.

Thanks for the insight.

Well... you persuaded me. Antimatter it is! I can use it afterall for scifi.

I was at first set on using vacuum jets that could not be throttled (1g only, 2g only and so forth).

[Guest]

If you're writing SF, you should use the minimum of "miracle tech" that allows your story to work.  The more subtle it is, the less likely it is to raise questions. A heat radiator that's probably too good for its stated area (but don't forget to have radiators!) is better than a magic engine that handwaves the energy issues away. While nearly a ton of antimatter does make a superweapon, it's somewhat easier to justify it not being an issue. For example, the storage units may be self-contained, self-powered (that is, powered by antimatter in them, so that if they ever fail, this means they no longer have antimatter), very though and designed by an independent organization, so it's practically impossible to cause an uncontrolled antimatter release and the containers themselves are black boxes. Remember to be consistent with other pieces of tech you use, too, and to consider any unintended side effects they might have. This can also provide inspiration for the plot, someone using a previously established device in a unique, but logical way.

This is why science in general is great, BTW. It doesn't so much "forbid" most things, but just dictates certain ways those things can be done, and consequences they have. It provides solutions just as if not more often as it creates problems, and even those problems can usually be solved.  If you pay attention to it and do your research, you can end up with some really interesting things that you could make a part of your universe.