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Uprated Nexus + Gas Core Nuclear Second Stage

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Convair concept for a reusable Uprated Nexus SSTO with a gaseous core second stage. Circa 1964. The Uprated Nexus is the larger version of Convair’s reusable booster, see my diagram post here Nexus SSTO Booster Comparison.

In 1962 the NASA Future Projects Office called for a post-Saturn launch vehicle for the 1975-2000 time frame. NASA's forward looking plans for the 1970’s and 1980’s called for lunar bases with permanent crews of 100 or more and large scale manned exploration of the solar system. This vision helped drive the size of Nexus; manned interplanetary spacecraft means large liquid hydrogen tanks, nuclear engines were also foreseen … not just solid-core nuclear thermal engines like NERVA but gas-core nuclear rocket engines and nuclear-pulse (Orion) engines. These are massive systems and require massive boosters.

In this variant the first stage remains an all-chemical reusable booster stage powered by a hydrogen/oxygen plug-nozzle engine.

Second stage is a nuclear stage powered by two six million pound gas core open-cycle nuclear thermal rockets. Payload to low Earth orbit: 3.2 million pounds.

Scott Lowther of Aerospace Projects Review expended the time, doing the research to dig up the information on Nexus, and the data in this post is credited to his hard work. What I've included here is just the barest essential detail, for the full story on Nexus, along with diagrams and facts covering area's of the program not available elsewhere, I highly recommend  Scott Lowther's complete article on Nexus, available here V3N1 of Aerospace Projects Review.

The Uprated Nexus + gas core nukes is a launch vehicle that definitely falls under the heading of living dangerously. The limit on NTR-Solid exhaust velocities is the melting point of the reactor, in order to achieve higher exhaust velocities some enterprising engineer(‘s) designed a reactor to operate in a molten state.

From Winchell Chung’s Atomic Rockets site:

“Gaseous uranium is injected into the reaction chamber until there is enough to start a furious chain reaction. Hydrogen is then injected from the chamber walls into the center of this nuclear inferno where it flash heats and shoots out the exhaust nozzle.

The trouble is the uranium shoots out the exhaust as well.”

As Winchell Chung points out “An exhaust plume containing radioactive uranium is harmless in space but catastrophic in Earth's atmosphere.”

Presumably the booster would stage at sufficient altitude to prevent radioactive exhaust plumes from the twin second stage gas core engines from hitting the atmosphere – but the risk is certainly present and worth consideration.  

In theory: the reaction is maintained in a vortex tailored to minimize loss of uranium out the nozzle. Fuel is uranium hexaflouride (U235F6), propellant is hydrogen.

In some designs the reaction chamber is spun like a centrifuge. This encourages the heavier uranium to stay in the chamber instead of leaking into the exhaust. Failure mode, in which bearings on the chambers seize, would be, as Winchell notes, *spectacular.*

Model is based on diagram from Scott Lowther’s Unwanted blog, found here: Nexus + Gas Core Nuclear Second Stage.

Dimensions appearing in red on the diagram – those for the gas core second stage – are approximated based on the scale on Scott Lowther’s diagram (at the link immediately above).

I’ve extrapolated some details – such as the reaction control system – which do not appear in the diagram, based on the logical need for their existence – an argument could be made that gimbaling the gas core engines is sufficient. I based the RCS design on the Apollo service module’s reaction control jets which were designed around the same time period. The thrust structure on the bottom of the stage is my own extrapolation (since it is not visible in the diagram, but must logically be present) and may differ from the actual item designed by Convair.  

On Winchell Chung’sAtomic Rockets site this spacecraft is found here: Uprated GCNR Nexus which features an earlier 3D model I built of the gas core engines.
Gas Core (Open Cycle) NTR Data

I've used the first generation gas core engine data available on Winchell Chung’s Atomic Rockets, since gas core engines were highly theoretical at the time of this design.

Convair Nexus Reference Links, courtesy of Scott Lowether’s Unwanted Blog:

Convair Nexus SSTO

Convair Nexus 1million Lb Payload/2 million Lb Payload Comparison

Nexus + Gas Core Nuclear Second Stage

Nexus Gas Core Nuclear SSTO

Related Images

Convair Nexus SSTO

Nexus SSTO Booster Comparison
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tingmarco's avatar
Been thinking about it with Magitek. if Boosting a fuel's energy content to 2x doubled the Isp.. Well. I am not exactly how sure the 2 would go in hand.. Maybe the Fuel sort of carries its own oxidizer and combusts as well? Boosting compounds are energized and release their energy to accelerate propellant even further? (The payload that a 6000-sec Nexus would lift is nothing short of outright ridiculous)

A second note. Would Boosted Kerosene fuels increase thrust in a Turbojet in part due to higher energy per kg? (a 2x increase is 1.5x increase in thrust.etc) Could the same be true for Piston engines like the Merlin?
JESzasz's avatar
 You mention that Winchell Chung notes that if the bearings of a reaction chamber were to fail for a centrifuge type reaction chamber, the results would be "spectacular". I take that in mean "very, very, very bad, like perhaps even explode into a fireball bad"?

 And I know nothing but the bare basics of nuclear energy, but I'm guessing that operating any nuclear reactor in a molten state must be very difficult, and very dangerous. Is this in any way related to a nuclear reactor melting down, or completely different?

 It is amazing that such a vehicle was thought up ~50 years ago, and some of the ideas sound a little crazy. Which is probably why they've only stayed on the drawing board.
William-Black's avatar
Failure mode for any rocket is spectacular, usually.

The centrifuge design was an approach to limit uranium loss to the propellant stream and manage vortex stability in the reactor. If the bearings seized it would probably blow the fission core, a 55,000 K nuclear fireball, right out the nozzle. Or the engine might just glow white hot and vaporize, hard to say which.
Open Cycle Gas Core studies continue because the engineering challenges are within reach, and it would be the next best thing behind Orion nuclear pulse and fusion rockets.

Open Cycle Gas Core: Thrust 196,600 newtons (44, 200 Lbs). Specific impulse: 4400 seconds, Reactor power 6,000 megawatts.

An engine like this is capable of very fast 80-day manned Mars round trip (Mars courier mission) and just to repeat, that's 40 days each way with an engine having a total weight of about 100,000 kilograms.

For the past several months I've been working up designs from NASA TM X-67823, which describes fast manned Mars missions (called currier missions) with durations of 80, 100, 150, and 2OO days. Manned Jupiter and Saturn missions are also possible with this kind of propulsion system.

You can see the propulsion bus for this type of manned interplanetary spacecraft here Open Cycle Gas Core Nuclear Thermal Rocket.

Gas Core Radiation simulation is here Gas Core Rocket New Radiation Simulation.
Reactor-Axe-Man's avatar
I'm not sure why they opted for an off-axis thrust vector with the engines. You lose efficiency and what happens with a single-engine failure mode?
William-Black's avatar

I was talking to Winchell Chung about the gas-core rocket mountings – apparently in one of the source documents Scott Lowther dug up (after a decade of FOIA requests – the results of which are to be found in APR V3N1) Post-Saturn Launch Vehicles Study Phase III - Class IV Vehicles, General Dynamics|Astronautics report GD|A-AOK 65-009, Contract NAS 8-5022, May, 1964 (Five volumes.) Shows alternate mountings – now these were for solid core NTR, but looking at another document NASA TX X-53200 Advanced Post-Saturn Vehicle Study Executive Summary Report, which Winchell was kind enough to provide, these same alternate mountings are also shown for the gas core rockets, but with with no explanation –apparently in  GD|A-AOK 65-009  they are labeled "canted engine cluster" and "parallel engine cluster". But there is no explanatory text.


Your question is interesting – and definitely has me curious as to the reason.

I can definitely rule out my earlier speculation: the engines are not shown gimbaled in the source diagram, apparently this is the intended mounting.

Winchell is also unsure of the rationale for this mounting scheme.

Reactor-Axe-Man's avatar
There must have been some reason for it. Were there any production rockets that featured the canted engine design? I thought maybe the Titan II was like that, but every photograph I've been able to find of it shows a parallel configuration.
William-Black's avatar

“There must be some reason for it.”


My thoughts exactly.


I continued to look for references and I believe I’ve found the answer in a reference work dealing with the basic principles of rocket design; in short the explanation is this:


In cluster engine configurations the weight of multiple engines moves the center of gravity – this is problematic because there is a specific relationship between the center of gravity and the center of pressure, move CG too far in either direction and the rocket becomes unstable in flight. Shifting CG forward is accomplished by adding weight to the nose, shifting the CG aft is accomplished by canting the rocket engines.  
Thrust is still through the center of mass in the event of single engine failure. What this will do to your mission I can't say, but it is actual redundancy as long as the failure doesn't destroy the other engine.
Reactor-Axe-Man's avatar
All right, that makes sense. Bit of a kludge, but if the rest of the design is already frozen you do what you have to.
William-Black's avatar
Nuclear thermal rockets need to have a separation,  mount them too close together, directly side by side, or close enough to touch, and the neutrons from each reactor over-drive the other. These NTR's appear to be mounted as close as they possibly could be. Actually I am sort of surprised that there is no neutron moderator (at leas none was indicated in the diagram) mounted between them (or at least on the sides of the engines which face one another). I imagine this is why the nozzles are angled to point away from each other as well—because the path back up the nozzle would have no shielding, nothing to obstruct neutron flow (neutrons don't care about the force of thrust blasting out the nozzle, this will not stop them).

Losing a fraction of thrust efficiency isn't always a deal-killer, even with chemical rockets. The descent vehicle which managed to hover while lowering Curiosity  is a good example.

In the case of engine failure during climb to orbit (presuming this was not catastrophic enough to destroy the vehicle) you could gimbal the other engine to compensate.
Reactor-Axe-Man's avatar
I get the need to maintain separation due to neutron flux, I just wonder why they couldn't have been separated in such a way as to make their axis of thrust parallel to the long axis of the spacecraft.  It just seems needlessly complicated to me, since there doesn't seem to be any support machinery other than propellant piping and control conduit that services the reactors that would require such a mounting.
William-Black's avatar
It occurs to me that the diagram (which is one of the very few that are freely available) might just show the engines gimbaled to their outermost extreme. If I can dig up more information I'll add it to the post, and add the indications of the gimbal arc into the diagram.
cw3brett's avatar
Astonishingly huge bomb rocket.  I would honestly feel much safer watching an Orion ground-launch, as I assume a crash or pad-blowup would result in a much smaller fireball. Although, I've read that the Nexus would have been a perfect vehicle for lofting an Orion battleship.

Excellent work on detail and research once again!
William-Black's avatar

It’s one of those concepts that make you admire it — just for the sheer audacity of the thing, the scale of it. Those Future Projects Office guys. They didn’t think small.

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