Author Topic: Nuclear power the solution to RLV  (Read 39177 times)

Offline douglas100

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Re: Nuclear power the solution to RLV
« Reply #20 on: 08/12/2012 09:16 am »
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.

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.

I remember these! But, yes, the bottom line is that operation of a flying nuclear reactor in the atmosphere is a complete non-starter for safety reasons .
Douglas Clark

Offline mmeijeri

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Re: Nuclear power the solution to RLV
« Reply #21 on: 08/12/2012 10:17 am »
The rocket as a whole needs to have a T/W ratio of >1 in order to reach orbit.

Not with horizontal takeoff.
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Offline mmeijeri

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Re: Nuclear power the solution to RLV
« Reply #22 on: 08/12/2012 10:20 am »
Using a more mass-efficient grooved-ring core could potentially get you much higher T/W, maybe 20-30 or better.

And using ammonia instead of hydrogen could further increase T / W. So would LOX augmentation.

Quote
  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...

Ammonia on the other hand is much denser. And with horizontal takeoff the need for high T/W is reduced.
« Last Edit: 08/12/2012 10:21 am by mmeijeri »
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Offline mmeijeri

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Re: Nuclear power the solution to RLV
« Reply #23 on: 08/12/2012 10:26 am »
I 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.
« Last Edit: 08/12/2012 10:29 am by mmeijeri »
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Offline cambrianera

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Re: Nuclear power the solution to RLV
« Reply #24 on: 08/12/2012 11:59 am »
Very enlightening articles from Kirk Sorensen.
http://selenianboondocks.com/2010/06/ssto-ntr-bad/
http://selenianboondocks.com/2010/02/pf-expressions-example/
http://selenianboondocks.com/2010/02/payload-fraction-example-proof/

To everyone talking about higher T/W, what's the amount of investment needed to make a working nuclear engine with high T/W?
I bet with that money you can have a complete chemical SSTO RLV.
Oh to be young again. . .

Offline A_M_Swallow

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Re: Nuclear power the solution to RLV
« Reply #25 on: 08/12/2012 12:11 pm »

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.

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.

Offline 93143

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Re: Nuclear power the solution to RLV
« Reply #26 on: 08/13/2012 03:12 am »
And using ammonia instead of hydrogen could further increase T / W. So would LOX augmentation.

Yeah, but they seriously eat into the Isp advantage.

A hydrogen NTR with a chamber at 3100 K and 30 bar, with a 250:1 expansion ratio, gets a theoretical Isp in excess of 1000 seconds.  With the same parameters, ammonia looks unlikely to reach 500.  And 30 bar is already too low for a ground launch engine even with a much shorter nozzle; trying to boost dissociation by dropping the pressure is a losing proposition.

Note that while the tanks and propellant feed systems should be smaller and lighter for ammonia, the nozzle won't be; ammonia seems to gain maybe one or two percent on hydrogen's areal thrust density with the above parameters.  Not sure how the reactor itself would differ because I'm not a nuclear engineer...

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.

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.

Type of shielding depends on type of radiation.  And in this case, several hundred tons would be enough to cover the entire vacuum chamber of a large Polywell with more than a foot of lead, so as to knock out the extremely high-energy gamma rays Wikipedia says p-¹¹B can produce.  A shadow shield for a nuclear thermal rocket would weigh a tiny fraction of that.
« Last Edit: 08/13/2012 04:34 am by 93143 »

Offline monstermaschine

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Re: Nuclear power the solution to RLV
« Reply #27 on: 08/13/2012 09:00 am »
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.

Offline Carreidas 160

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Re: Nuclear power the solution to RLV
« Reply #28 on: 08/13/2012 10:20 am »

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.


Dare I suggest an air breathing NTR? (Nuclear Skylon perhaps?) ;D

Offline mmeijeri

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Re: Nuclear power the solution to RLV
« Reply #29 on: 08/13/2012 05:09 pm »
And 30 bar is already too low for a ground launch engine even with a much shorter nozzle;

Why are you looking at a maximum chamber pressure of 30 bar?

And we still have the option of horizontal launch.

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Offline monstermaschine

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Re: Nuclear power the solution to RLV
« Reply #30 on: 08/13/2012 05:18 pm »
Dare I suggest an air breathing NTR? (Nuclear Skylon perhaps?) ;D

Similar problem. If you leave athmosphere you loose your cooling fluid and the reactor will melt down, so you have to cool it actively with water, helium, hydrogen until it has shut down.

Offline 93143

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Re: Nuclear power the solution to RLV
« Reply #31 on: 08/13/2012 05:40 pm »
Why are you looking at a maximum chamber pressure of 30 bar?

Because that's the chamber pressure in the CEA file I had lying around.  I'm just trying to illustrate the magnitude of the performance disparity between H2 and NH3.  Thermal dissociation happens more at lower pressure, so the theoretical vacuum Isp for ammonia is only going to get worse if you crank the chamber pressure.

Frankly, I don't think an ammonia NTR, with or without LOX injection, is going to be enough better than an extended nozzle LOX/HC-TAN RS-25 variant to be worth the extra weight, cost, and operational issues.  But that's just an opinion, before detailed analysis...

Horizontal launch is a good option if you have a heavy, very high-Isp engine, so that the mass penalty for wings and so forth is less that the mass penalty for T/W>1, but you can still make orbit with a positive payload.  In this case, though, the engine may be fairly heavy but it isn't all that much better than an SSME Isp-wise, so unless you airbreathe I'm not certain it will work at all (though it is granted that ammonia is almost twice as dense as straight hydrolox, so there is that.)

...keep in mind that 3100 K is actually quite high for an NTR.  Bleeding-edge, really; if Kirk Sorensen were here he would dismiss it as gullible DRM 3.0 engineers believing those lying Russians...
« Last Edit: 08/14/2012 12:02 am by 93143 »

Offline randomly

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Re: Nuclear power the solution to RLV
« Reply #32 on: 08/13/2012 05:42 pm »
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.

Offline ARD

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Re: Nuclear power the solution to RLV
« Reply #33 on: 08/13/2012 05:43 pm »

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.


Dare I suggest an air breathing NTR? (Nuclear Skylon perhaps?) ;D

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. 

Offline Carreidas 160

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Re: Nuclear power the solution to RLV
« Reply #34 on: 08/13/2012 06:32 pm »

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.


Dare I suggest an air breathing NTR? (Nuclear Skylon perhaps?) ;D

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. 

Yeah, that was part of Project Pluto I believe. But in this case with "air breathing" I mean taking oxygen out of the atmosphere and use it to augment the H2 exhaust as was suggested. That way Nuclear Skylon won't need any LOX tanks. Don't know whether that reduces thrust compared to SLAM.

(It's just a hypothetical thought, in no way do I think a nuclear powered, oxygen augmented, HTHL SSTO is feasible)

Offline monstermaschine

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Re: Nuclear power the solution to RLV
« Reply #35 on: 08/13/2012 06:42 pm »
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.

Where does this 15% come from? Is it your estimation or from a former upper stage study?

Offline john smith 19

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Re: Nuclear power the solution to RLV
« Reply #36 on: 08/13/2012 09:06 pm »
I 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.

Wonder no more. The book  is called "Thrust into Space" and is by Max Hunter, a key developer of the Thor IRBM and later proposer of what would become DC-X.

The technical detail was sparse but IIRC he was talking about a molten salt reactor, which was the kind Oak Ridge developed for the Nuclear Bomber programme. Key features being compact design and ability to bleed off the the main poison a Xe isotope.

Building an effective heat exchanger to turn LH2 into GH2 with a molten fluoride salt mix on the other side was left as an exercise for the student :)
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Offline indaco1

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Re: Nuclear power the solution to RLV
« Reply #37 on: 08/14/2012 02:35 am »
..
And we still have the option of horizontal launch.

IMHO:

Additional weight and complexity for wings.  You have to stay in relatively dense air for a significant part of the trajectory, with huge tanks that creates terrific drag losses and probably a very high dynamic pressure and structural strenght requirements.  Besides in the atmosphere rocket efficiency (chemical or nuclear) is worst compared to a vertical launch even with a good altitude compensation.

If horizontal launch worked in reducing T/W and, as an effect, increase mass ratio it could have been used for chemical horizontal take off spaceplanes. There must be a reason they didn't made many horizontal take off launchers.

Maybe I miss something, but the only launcher I've heard that uses wing lift is Pegasus. But it must do it because is horizontally air launched, and despite the assist the payload fraction achieved is not that better than vertical take off launchers.

Even Maks carrier plane was intendet to perform a zoom manoeuvre at launch to make the spaceplane exit the atmosphere as soon as possible with a negligible use of wing lift.

Maybe somebody here has access to trajectory optimizators or data to confirm, but I really think horizontal launch for rockets is not a good idea.

If the veichle breaths air or it uses air as reaction mass it could be another story, of course.
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Offline DarkenedOne

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Re: Nuclear power the solution to RLV
« Reply #38 on: 08/14/2012 03:06 am »
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.

Im sure radiators can take care of the residual heat.  Remember high power nuclear reactors have been orbited before and they had no problem with using radiators to get rid of the heat.

Offline DarkenedOne

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Re: Nuclear power the solution to RLV
« Reply #39 on: 08/14/2012 03:38 am »
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.


Use radiators.  High power nuclear reactors have been flown in space before. 

Personally I was always thinking of a hybrid nuclear thermal / electric system.  When high thrust is required it would operate as a NTR. Than when low thrust high impulse is more appropriate than it would generate electric power with heat being disappated by the radiators.

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