NASA Looking At Nuclear Thermal Rockets To Explore the Solar System

MarkWhittington writes: Officially, NASA has been charged with sending astronauts to Mars sometime in the 2030s. Toward that end, according to a story in Universe Today, space agency engineers at the Marshall Space Flight Center are looking at an old concept for interplanetary travel, nuclear thermal engines. "...according to the report (cached), an NTP rocket could generate 200 kWt of power using a single kilogram of uranium for a period of 13 years – which works out of to a fuel efficiency rating of about 45 grams per 1000 MW-hr. In addition, a nuclear-powered engine could also provide superior thrust relative to the amount of propellant used." However, some doubts have been expressed whether NASA will be granted the budget to develop such engines.

Comments

Ion Thruster

[Brett Buck]

An Ion thruster (of any variety) is not *remotely* a replacement for a nuclear thermal engine. The ISP is great but the thrust levels are (and always will be, at rational sizes) feeble. And it's very likely that massively clustering them to get the thrust up will required a nuclear reactor to power them. 6/10ths of an *ounce* of thrust for 4 kW power input.

Ion thrusters have their uses, like in gently nudging things over long periods. They are not going to replace chemical rocket or NTP engines for any sort of high-thrust application.

About time

[mbone]

At the Europa day on the Hill last summer, I ran into a 90 yr old Harry Finger (the former head of NERVA) who remains absolutely convinced that this technology (which was ready for flight tests back in the Apollo period) is essential for human travel to the planets, and needs to be revived.

Looking at the delta-V requirements for a human Mars mission, I can't say I disagree with him.

Re:Some potential, but hardly for a genuine leap

[Baloroth]

No other mode of transportation has to carry its own reaction mass and throw it away. Not bicycles, cars, trains, ships, submarines, or airplanes.

Quite right. Because no other form of transportation takes place in a vacuum. Unless you know of some radical new physics, standard reaction-mass engines will be necessary for spaceflight for... well, forever, so, I'm not sure exactly what your point is. And yes, they've worked on the idea before with NERVA. We have, believe it or not, made a few technological and engineering breakthroughs since then (mind you: NERVA worked. It worked very well. It was canceled for political reasons, not practical ones).

Re:OMG

[Anonymous Coward]

If only someone had been launching small quantities of radioisotopes into space for many decades and perfected the containment vessels... Oh wait, they have. They're called RTGs and they're absolutely designed to survive the rocket exploding on launch pad or free-fall from space after a failed launch. There has never been an incident where an RTG has leaked radioactive material into the environment. Not that it would matter - the amount we're talking about here is equal to the amount of uranium released into the environment by a coal-fired power plant every two hours in normal operation.

Re:Some potential, but hardly for a genuine leap

[brambus]

Whoever the hell moded this tripe Insightful needs to have their head examined, along with the author.

ancient discredited NERVA/ROVER program which began in 1956 and dragged on to a miserable failed end in 1973

You mean the discredited program that produced working engines and test-fired them on vacuum stands, proving they are practical and work? You might also note another program that was terminated in 1972: Apollo. Oh my, what an abominable failure that one was...

the fact that any rocket has to carry and throw away a vast load of reaction mass

And how else would you propose to move in space? Mr Newton might have something to say here.

But the actual raw energy needed to lift 118 tonnes to 200 km is...

If you think the difficulty in achieving orbit is just lifting something sufficiently high up, you're more dense than I thought... Here's an idea, first learn about something, then start lecturing about it.

No other mode of transportation has to carry its own reaction mass and throw it away. Not bicycles, cars, trains, ships, submarines, or airplanes.

Please note that all of the above modes of transportation have one thing in common: they only work on the Earth. Or when was the last time you last saw a car drive through outer space?

"Getting into orbit" requires a big rocket.

[robbak]

That big rocket is mostly just to put the payload into orbit. Once in a low earth orbit, it doesn't take that much more to take it from there to a different orbit.

This xkcd is probably the best way to grasp the difficulties of 'getting into space".

https://what-if.xkcd.com/58/

Re:"Getting into orbit" requires a big rocket.

[Anonymous Coward]

Once in a low earth orbit, it doesn't take that much more to take it from there to a different orbit.

This isn't really true. Getting from Earth surface to low Earth orbit takes a delta-v of 9.4 km/s, and getting from there to geostationary orbit takes another 3.9 km/s (see this map). So, in terms of delta-v, you need to go another 3.9/9.4 ~ 40% as far as you already have.

Okay, that's not that much further. But that doesn't mean that the size of the rocket you need just goes up by 40%. The required rocket size is *exponential* as a function of delta-v. To launch 1 tonne into low Earth orbit, you need a rocket that weighs ~30 tonnes - so, to launch 1 tonne into geostationary orbit, you need a rocket that weighs 30^1.4 ~ 120 tonnes. That's four times as much rocket, just to go that little bit further from one Earth orbit to another.

Most of that rocket is still for putting that payload into orbit, as you said. But instead of a 30-tonne rocket putting a 1-tonne payload in orbit, it's a 120-tonne rocket putting a 4-tonne rocket in orbit, and that 4-tonne rocket putting a 1-tonne payload in a higher orbit.

This is why more fuel-efficient engines, like nuclear or ion rockets, would be a great help even if they didn't have enough thrust to launch directly from the ground. If you can get 1 tonne from low Earth orbit to geostationary orbit with a 2-tonne nuclear rocket instead of a 4-tonne chemical rocket, the chemical rocket that launches you from the ground only needs to be 60 tonnes instead of 120 tonnes. That's a big advantage.

If you're just tooling about in low/medium Earth orbit, sure, chemical rockets are all you need. But if you want to go to geostationary orbit or the moon, nuclear/ion rockets can make it more efficient; and if you want to to Mars/Venus and back, they're almost essential.