Quote from: DarkenedOne on 08/12/2012 01:05 amQuote from: butters on 08/12/2012 12:58 amNobody is going to be allowed to land a nuclear fission reactor on Earth, no matter how credible their promise that it will never ever crash. That's crazy talk.Sorry buddy, but its already been done. The US nuclear airplane program tested and flew airplane reactors. Same thing with the Soviets. Guess its not as crazy as you think.Both programs were canceled long ago. There are lots of crazy things we used to do that we will never do again. Above-ground nuclear weapons testing. X-ray machines in shoe stores.
Quote from: butters on 08/12/2012 12:58 amNobody is going to be allowed to land a nuclear fission reactor on Earth, no matter how credible their promise that it will never ever crash. That's crazy talk.Sorry buddy, but its already been done. The US nuclear airplane program tested and flew airplane reactors. Same thing with the Soviets. Guess its not as crazy as you think.
Nobody is going to be allowed to land a nuclear fission reactor on Earth, no matter how credible their promise that it will never ever crash. That's crazy talk.
The rocket as a whole needs to have a T/W ratio of >1 in order to reach orbit.
Using a more mass-efficient grooved-ring core could potentially get you much higher T/W, maybe 20-30 or better.
That's starting to get into the range where ground launch is technically possible, though it's not as vast an improvement as it seems like it would be because LH2 tanks are so huge...
I think Elon is on the right track. Make a reusable booster stage first, then work on making the other parts reusable.
Incorrect.The rocket as a whole needs to have a T/W ratio of >1 in order to reach orbit. That is the engine, the fuel, the payload, and the rest of the rocket. Chemical rockets engines have exhaust velocities significantly less than the orbital velocity they must achieve. For a rocket like the Falcon 9 with an ISP of 300s the rocket needs 20 times its mass in propellant in order to achieve a delta -V of 9000 m/s for LEO. Thus just counting the engine and the fuel the engine would have to have a T/W ratio of at least 21 in order for the entire system to have a T/W >1. In order to carry so much fuel with such large tanks chemical rocket engines require high T/W ratios. For a NTR with an ISP of 900s the rocket equation dictates that the rocket equation dictates that the rocket carry 2.7 times its own mass in propellant. Just counting the engine and the fuel again the NTR engine only needs a T/W of >3.7 in order to reach orbit. A NTR with a T/W ratio of 30 and an ISP of 900 would definitely make a good RLV. It would have a payload faction many times more than a chemical RLV.
And using ammonia instead of hydrogen could further increase T / W. So would LOX augmentation.
When performing the calculations add several hundred tons of shielding to the dry weight of the LV. Modern shielding may be plastic rather than lead but it is still massive.
LOX injection could be a good idea for ground launch of an NTR, if ground launch of an NTR were a good idea. Start with a high level of injection, then taper it off as you gain speed and altitude. It should go to zero partway through the burn. This way you have high thrust when you need it and high Isp when you need it, and the majority of your propellant mass is LOX.
And 30 bar is already too low for a ground launch engine even with a much shorter nozzle;
Dare I suggest an air breathing NTR? (Nuclear Skylon perhaps?)
Why are you looking at a maximum chamber pressure of 30 bar?
Did someone mention the shutdown difficulty?
... and the need to carry an extra 15% fuel mass to cool the reactor after shutdown to keep it from melting from the residual decay heat.
Quote from: 93143 on 08/13/2012 03:12 amLOX injection could be a good idea for ground launch of an NTR, if ground launch of an NTR were a good idea. Start with a high level of injection, then taper it off as you gain speed and altitude. It should go to zero partway through the burn. This way you have high thrust when you need it and high Isp when you need it, and the majority of your propellant mass is LOX.Dare I suggest an air breathing NTR? (Nuclear Skylon perhaps?)
Quote from: Carreidas 160 on 08/13/2012 10:20 amQuote from: 93143 on 08/13/2012 03:12 amLOX injection could be a good idea for ground launch of an NTR, if ground launch of an NTR were a good idea. Start with a high level of injection, then taper it off as you gain speed and altitude. It should go to zero partway through the burn. This way you have high thrust when you need it and high Isp when you need it, and the majority of your propellant mass is LOX.Dare I suggest an air breathing NTR? (Nuclear Skylon perhaps?) The USAF looked into something like that in the 1950s--Supersonic Low Altitude Missile, an atomic ramjet. It got as far as reactor tests at 170kN of thrust--within an order of magnitude of SABRE's planned performance.
Quote from: monstermaschine on 08/13/2012 09:00 amDid someone mention the shutdown difficulty? Quote from: randomly on 08/11/2012 11:55 pm... and the need to carry an extra 15% fuel mass to cool the reactor after shutdown to keep it from melting from the residual decay heat.
Quote from: randomly on 08/11/2012 11:55 pmI think Elon is on the right track. Make a reusable booster stage first, then work on making the other parts reusable.Sure. Still, it's interesting to wonder whether NTR might be what would make an SSTO RLV possible. Sounds plausible to me, but I don't think you would want to use it for launch from Earth. For Mars (or the moon) it could be enormously useful however. Zubrin has proposed something like that.
..And we still have the option of horizontal launch.
Did someone mention the shutdown difficulty? You can't simply switch off such reactors like NERVA. They will suddenly blow up, if you stop cooling them by hydrogen. You have to shut them down like any other power reactor (first the control rods to stop fission, then cooling down). During the shutdown time the Isp becomes very low. So it is very difficult to induce a certain delta v at a desirable Isp. The problem becomes lesser if You can burn hours and cool down quarters of hours, as it would be possible for interplanetary flights. But if You only have a quarter of an hour for your complete manouver like in an upper stage, you have to be content with a very low medium Isp.