NASA is exploring nuclear propulsion — a project started by the agency 60 years ago

[Elderberry]

Houston Chronicle by  Alex Stuckey Oct. 14, 2019

NASA is exploring nuclear propulsion — a project started by the agency 60 years ago

With America’s eyes trained on the impending moon mission in the 1960s, NASA officials began quietly working on a project that could transform space travel.

They were trying to build a nuclear rocket engine, capable of getting astronauts to Mars in four months — about half the time of a conventional chemical rocket.

Though the program showed early promise, it died due to inadequate funding.

Almost 60 years later, NASA is picking up where it left off, working again to develop a nuclear propulsion system that could carry humans farther than ever. President Donald Trump recently issued an executive order calling on NASA to develop and launch nuclear space systems, and Congress has provided $100 million to study the cost and feasibility of building such a rocket over the next year.

“The main rationale for pursuing this again is human Mars missions,” said Jeff Sheehy, chief engineer in the Space Technology Mission Directorate at NASA headquarters in Washington, D.C. “There’s been a re-emphasis on going to the moon and doing those things on the moon that are necessary to develop the systems and demonstrate the capability to go to Mars.”

But there still is a long way to go before NASA personnel can even begin to think about building these systems.

“We’re trying to determine how many billions it’s going to cost through the first set of flight(s) … so we can go to decision makers and say this is what it will cost, these are the benefits,” Sheehy said. “It’s up to the senior-most people in the agency to determine whether it’s worth the cost of developing the system.”

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[Snarknado]

Any ideas on how it would work? A nuclear reactor is a great source of heat, but how do you translate heat to propulsion in empty space?

[thackney]

Any ideas on how it would work? A nuclear reactor is a great source of heat, but how do you translate heat to propulsion in empty space?

During the sixth meeting of the National Space Council (NSC) today (Aug. 20), the NASA chief lauded the potential of nuclear thermal propulsion, which would harness the heat thrown off by fission reactions to accelerate propellants such as hydrogen to tremendous speeds.

https://www.space.com/nuclear-propulsion-future-spacecraft-nasa-chief.html

[Joe Wooten]

Any ideas on how it would work? A nuclear reactor is a great source of heat, but how do you translate heat to propulsion in empty space?

Many different proposals are out there. The one we KNOW works is the old NERVA tested out at Jackass Fats NV back in the late 60's and early 70's. It used a fission reactor with fuel rods like a nuke power plant, but much more robust, and liquid hydrogen as a working fluid. The version NASA/AEC was going to test in orbit would have required at east two Saturn 5's to get it up there. One for the reactor and one for the rest of the ship. For a trip to Mars, I think water would be a better working fluid as you don't have to keep it cryogenic and it's density allows you to use smaller volume tanks.

There are other proposals like this one
https://www.bwxt.com/what-we-do/nuclear-thermal-propulsion-ntp
http://astronautix.com/n/nerva.html

[Snarknado]

Interesting. If hydrogen tanks were shaded from the sun, wouldn't they'd stay pretty cold on their own in interplanetary space?

I wonder if at some point it might be feasible to use nuclear-generated electricity to power particle acceleration. If you could accelerate to near c I think it would maximize the force per kg of propellant. And they've already used the nuclear-electric technology successfully to power the instruments on Cassini...

[Joe Wooten]

Interesting. If hydrogen tanks were shaded from the sun, wouldn't they'd stay pretty cold on their own in interplanetary space?

I wonder if at some point it might be feasible to use nuclear-generated electricity to power particle acceleration. If you could accelerate to near c I think it would maximize the force per kg of propellant. And they've already used the nuclear-electric technology successfully to power the instruments on Cassini...

They would still require power to keep the hydrogen liquid. Water can be used in the closed cycle power generation mode, which will keep it from freezing. It is also an excellent radiation shield.

There is no way current nuclear propulsion systems could even get a respectable fraction of light speed. The exhaust velocities are way too low. Not even a fusion drive using bombs (Orion) has enough exhaust velocity to get much more than about 0.1c. Someone did a calculation that even a Bussard Ramjet could probably get only to about 0.3c to 0.5c.

The thermoelectric power sources used in Cassini, Voyager, and New Horizons are too small for a manned ship. Even back in the 70's the idea was to use a Stirling engine for power generation and the reactor as a heat source to run it in closed or open cycle. The best bet for a long term drive would be a hybrid where you use the drive to get out of and in to orbit around a planet, then go closed cycle for power generation at a much lower reactor power.