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Post by darthroach on Feb 7, 2017 0:12:15 GMT
So it's a NERVA that can go toe to toe with interplantery Orions. What's more, it could conceivably get approved by some bureaucrats somewhere? Astonishing. Hope they make this work.
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Post by darthroach on Jan 27, 2017 14:22:56 GMT
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Post by darthroach on Jan 23, 2017 0:32:30 GMT
Yes, we did see roach's post. Well, I did. Besides, I think LANTRs don't burn the LH2-LOX, they just use it as an additive. I would call whatever the LOx and H2 are doing in the exhaust stream, "burning", all right. It's an idea lifted straight from jet engine afterburners. Or rather, that's exactly what it is, just with oxidizer instead of fuel. The difference here is that the combustion is not the main energy source.
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Post by darthroach on Jan 23, 2017 0:29:54 GMT
Saturn INT-21 would have had 75 tonnes capacity to LEO. A LEO-Mars Surface-Earth Surface trip, assuming that all velocities that can be reduced with aerobraking are fully reduced with aerobraking, required about 10 km/s of delta-V. Assuming a 6 km/s methane NTR, we can get away with a mass ratio of 5.3, i.e. we can have 14 tonnes of payload+structural mass - with an 8 km/s hydrogen NTR, we can instead get a mass ratio of 3.5 and ~21 tonnes payload+structural mass - though I'd be a bit more doubtful about that being able to develop TWR>1 on Mars's surface - and that is without any ISRU, of course I am not sure direct launch is the best mission architecture for Mars exploration, let alone colonization. The current idea, courtesy of the ever active mr Zubrin and featured in the book/movie "the Martian", is to have separate cargo and crew launches, with the possibility of using a separate high-speed interplanetary shuttle for crewed use introduced in the latter. You launch the hab, supplies and return vehicle ahead of time, and send the crew with the next mission's start supplies in the next launch window. Allows lighter launches, because the return vehicle spends the 2 years on Mars gathering propellant. Either way, we should really stop thinking in terms of what will get a crew of scientists over there one time, and more in terms of stable, quick and safe transport infrastructure for colonization. We landed on the moon with chemical rockets, but it's unlikely any moon colony will be sustained without long-use refuellable NTR shuttles. Same with Mars.
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Post by darthroach on Jan 22, 2017 10:34:13 GMT
I was wondering, is there any reason why some sort of Chemical/Nuclear rocket wont work? For example you could burn LOX Decane and then pass the resulting H2O and CO2 through an NTR. This seems like a way to use the chemical energy of propellent that would normally be wasted in an NTR. If you burn it before passing it through the reactor, all you're going to do is get way less energy out of the reactor, ending up with a very heavy, slightly hotter chemical rocket. If it even works. What you're proposing has already been considered, btw. LANTR, or LOx-augmented NTR, proposes injecting LOx in the exhaust stream. Increases thrust at the expense of about 1/3 of the exhaust velocity. Essentially a more efficient form of dumping remass into the exhaust to increase thrust since the remass recovers some energy from chemical combustion.
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Post by darthroach on Jan 22, 2017 10:29:08 GMT
Solar energy based propulsion systems may work well enough out to the orbit of Mars, but we're never going to go any further with them. If we're serious about space exploration, we're going to have to go nuclear at some point. The only way nuclear powerplant and propulsion tech is going to advance is if it's used. So I'm for either NTR or at least NEP. And as to the type of NEP, I'd suggest using the Fusion Driven Rocket - Magneto-Inertial Fusion. It may well be the first type of fusion reactor we get going and would probably end up paying dividends in many other areas of technology. And the best part is, it will probably work if we build it. www.projectrho.com/public_html/rocket/enginelist.php#id--Pulse--Inertial_Confinement--Magneto_Inertial_Fusion
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Post by darthroach on Jan 21, 2017 16:46:55 GMT
This thing isn't even blackbox, we can for the most part model it quite accurately.
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Post by darthroach on Jan 20, 2017 15:20:29 GMT
Actually, do resistojets have better Isps than NTRs? They should be similar, since they both use the same basic idea of heating a gas to the melting point of a chamber and spewing it out a de laval nozzle. Resistojets have a higher velocity since they can operate at higher temperatures since they aren't held back by Uranium Oxides. Efficient warfare is usually not fun... I love using drones and missiles as much as I hate launching and planning each and every one of their burns. Indeed, fissile fuels rarely make for the best structural materials. Now, if only there was a way to circumvent that pesky temperature limit - we could use atomic rockets the way god intended them! 🤔
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Post by darthroach on Jan 19, 2017 9:36:52 GMT
okay general question here, is there any advantage to very high exhaust velocity? Depends on what you mean by "really high" and what the mission profile is. Ve is inversely correlated with thrust and therefore acceleration, because given some constant amount of energy, choosing to accelerate the remass to a higher velocity massively reduces the mass flow rate. Same kinetic energy but diminishing change in momentum (Wk = mv^2 ; p = mv ; forgive eastern european notation). Chemical rockets have an overabundance of thrust but have very low dela v, so higher Isp is always welcome. But once you reach certain levels - enough to do partial brachistochrone trajectories, for example - it becomes a tradeoff. You can accelerate for longer, but you also take much longer to accelerate. Not only does this mean you will take months or years to get anywhere, it also makes maneuvers like orbital insertions difficult. So rule of thumb: the further you want to go, the more Isp you want. You don't want too much.
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Post by darthroach on Jan 18, 2017 14:58:15 GMT
Definitely, yes - I think a closed cycled GCNTR, i.e. nuclear lightbulb, might actually be somewhat easier to implement in game - as you do not have to worry about fuel loss. And might be preferred for military use anyways, if possible, since I am not sure if you could maintain fuel containment in an open-cycle GCNTR under high acceleration 1. While the open cycle engine requires some tinkering with fluid flow to work, the closed circle one requires quartz "lightbulbs" that can actually withstand the intense heat of fissioning gas. Overall a much more complicated system. While it does not eject any fuel out the back, it runs into several far bigger problems. How do you cycle the 20 thousand K or hotter gas and replace the spent fuel with unspent? How exactly do you cool the lightbulbs? How do you even build them? How strong can you make the thing before the lightbulb walls absorb too much heat? The only real argument I see for this system is that it could be used inside the atmosphere. With the open cycle version at least the heat management is rather straightforward. 2. Define high acceleration. Something makes me think if we're using high-pressure expanding gas inside the combustion chamber of a rocket - at tens of thousands of degrees kelvin no less - a couple hundred miligees should really be fine. I mean, if you can calculate a way to confine the core under no acceleration, you can probably figure out a way to do it with acceleration too.
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Post by darthroach on Jan 18, 2017 13:55:14 GMT
The main advantage of Orion is that it is the only remotely feasible engine with torchlike performance - high exhaust velocity AND high thrust, so you can use it to get a TWR>1 even for very large payloads with high double or triple digit km/s delta-V, allowing you to boost straight from Terra to Mars on a brachistochrone trajectory. Of course, even when using pure fusion detonators (and thus having more or less absolutely no fallout) you would still want to keep the launch site away from population. Though, Orion wouldn't be nearly as environmentally bad as usually thought, because it only uses very small, sub-kiloton devices. Orion is a separate topic, though. Orion will also take a lot more work to implement in the game than the Gas Core rocket, since the latter shares many properties with the NTRs already present. The biggest difference is the need for a cooling loop and pressure shell. The specifics of the fluid flow needed to contain the core are not actually needed, what matters is the mass flow, core temperature, fuel leak rate (biggest blackbox function involved, seems) and propellant heat absorption. From the combustion chamber into the nozzle and out it's like any other rocket.
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Post by darthroach on Jan 17, 2017 20:32:35 GMT
Might be interesting if GCNTRs can be scaled down to missile sizes. Then turning the engines off is a question for the enemy to deal with after the 30km/s missile says hi. As far as I can tell, no. In order to reach criticality, the core needs to be of either sufficient mass and sufficient density. If you wanna make it smaller, you will have to increase the working pressure - and there is only so much that real materials can take. Not to mention a smaller radius means far, far more heat getting dumped per unit of combustion chamber surface area. Then again I only have the vaguest idea of how nuclear reactions work so I might be wrong - but I believe it was mentioned in one of the papers cited on AtomicRockets. ETA: even if you could, 30km/s missiles don't seem like the best idea. At that kind of relative velocity, they're basically dumb projectiles as far as evasion is concerned. And evasion will be concerned because the exhaust will light up the sky like Vegas on New Years.
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Post by darthroach on Jan 17, 2017 14:26:11 GMT
True enough, but not every ship can or needs to be gigantic. Put it whichever way you like, Orion would probably not fill the same niche as the average NTR-propelled ship. The only thing that seems more brute-force than Orion is skyrocketing mass ratios. Which is yet more reason to include both these propulsion systems into the simulation - so we can see what they are like up against each other Honestly, if either of these systems is workable (which they probably are), that's what the admirals of Sol system would pay an arm and a leg to get. The performance gain is just too much not to look into.
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Post by darthroach on Jan 17, 2017 12:23:01 GMT
I ean you could run your gaseous nuclear fuel through a cooling loop... its only 4000K. Edit: stick a thermocouple or two on that and you make power too It's actually far higher than 4000K, so you might be thinking some of the weird liquid-or-vapor core designs. No coolant loops here I'm afraid. The entire point is that the fuel itself does not need to be cooled, since it's not in direct contact with any bits and pieces that need to remain solid to work. To get a perspective on the amount of heat that this thing puts out you just have to consider the fact that even though only about 7-10% of the radiation hits the combustion chamber walls (in theory, anyway), it requires a radiator cooling system. Well, when running at lower temperatures you could make do with regenerative cooling, but might as well go for broke. The upper limit on the Isp is defined by the amount of heat the shell can take before being compromised instantly. This paper offers a fairly good estimate of the heat properties of this engine: High Specific Impulse Gas-Core ReactorsAccording to this, even for a modest 2500s of Isp you'd need to exhaust THE PROPELLANT (hydrogen) at 8300K. For 5000 - at 22'000K. Which means the core has to be at least that hot, and in reality hotter.
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Post by darthroach on Jan 17, 2017 7:41:47 GMT
Orion drives seem more feasible, true, though they require massive shock absorbers and are not exactly efficient. A gas core NTR offers roughly similar performance while also keeping a far more traditional ship architecture. NSWR would give us torchship-tier performance if Zubrin is right, but it also has some very, very shiny failure modes. So far no one has gotten around the problem of fuel storage, and there is no way I would take that thing into a battle. As for metallic hydrogen, there is the problem of producing metallic hydrogen and even then we're looking at only 1/2 to 1/3 of the Isp. Considering the fact that at any given time only a fraction of your fuel will be in the combustion chamber, dumping it is not exactly a be-all end-all problem. It does, however, make firing the engine in short bursts rather wasteful. The question is, does that really outweigh the advantage in sheer power? A ship with 5000s of Isp can afford to have a very reasonable mass ratio of 3 and still have around 50km/s of delta v. Hell, you could run the thing with a mass ratio of 2 and still have about 35 km/s! Carrying a bit more fuel for times when you might need to dump the combustion chamber seems like a bargain.
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