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The project, in concert with US government agency DARPA, aims to develop pioneering propulsion system for space travel as soon as 2027:
The project is intended to develop a pioneering propulsion system for space travel far different from the chemical systems prevalent since the modern era of rocketry dawned almost a century ago.
"Using a nuclear thermal rocket allows for faster transit time, reducing risk for astronauts," Nasa said in a press release.
[...] Using current technology, Nasa says, the 300m-mile journey to Mars would take about seven months. Engineers do not yet know how much time could be shaved off using nuclear technology, but Bill Nelson, the Nasa administrator, said it would allow spacecraft, and humans, to travel in deep space at record speed.
[...] Using low thrust efficiently, nuclear electric propulsion systems accelerate spacecraft for extended periods and can propel a Mars mission for a fraction of the propellant of high-thrust systems.
Also at CNN and Engadget. Link to Nasa press release.
(Score: 1) by Runaway1956 on Friday January 27, @10:39AM (9 children)
No matter the source of energy, the ships need reaction mass. TFA mentions that the nuclear rocket will use a fraction of the reaction mass that conventional rockets use, but doesn't offer details. Then TFA devolved into a personal attack on Musk, and I stopped reading. I'm suspicious that these rockets are so much vaporware.
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(Score: 1, Disagree) by Anonymous Coward on Friday January 27, @10:50AM (4 children)
(Score: 2, Informative) by Anonymous Coward on Friday January 27, @01:37PM (2 children)
Acceleration is related to the change in momentum, which goes as the velocity of the ejecta, not the velocity squared.
(Score: 2) by HiThere on Friday January 27, @02:28PM (1 child)
True, it's velocity that is related to the square of the speed of the ejecta. It's still a quite significant difference. (OK, momentum too. But for figuring transit time velocity is the relevant one.)
I'm not quite clear on the limitations of the "nuclear-electric rocket" that they're proposing, though. My first guess was some fancy development of the ion-rocket, in which case it could run for a long time, but wouldn't produce that much thrust. However if they're ejecting reactor mass, you likely wouldn't want to use it where the exhaust could enter an atmosphere.
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(Score: 2) by crafoo on Friday January 27, @02:50PM
well, the nuclear rocket project was pretty cool, but yeah I think it was a vacuum engine not designed for atmosphere use. https://www1.grc.nasa.gov/historic-facilities/rockets-systems-area/7911-2/ [nasa.gov]
Pretty sure this new project is something new, but we have tried to use nuclear power for rockets and we even prototyped it out many decades ago.
(Score: 1, Informative) by Anonymous Coward on Friday January 27, @09:10PM
No it isn't. Momentum is linear. Kinetic energy is squared.
The relevant factor that makes this sort of nuclear rocket useful is that the exhaust velocity is determined by both the temperature of the exhaust and inversely by the molecular mass. H2 - O2 rockets are currently the best and they have an exhaust molecular mass of 18, giving a maximum Isp of about 420 before they melt the engine. Nuclear rockets can just heat up hydrogen and that has a mass of 2.
The NERVA rocket they developed back in the sixties and seventies had a tested Isp of 840 and was expected to reach about 1200 with further development.
https://en.wikipedia.org/wiki/NERVA [wikipedia.org]
(Score: 5, Interesting) by Immerman on Friday January 27, @03:20PM (3 children)
Probably because the details are entirely context-dependent, the rocket equation is sort of messy.
As a rule, nuclear-thermal rockets to date have a specific impulse (Isp) in the 500-1000 second range, compared to the 250-400 seconds of chemical rockets. So probably safe to assume about twice the specific impulse. To put that in context the massive increase in engineering complexity for the Raptor full-flow staged combustion engine design gained, as I recall, less than a 10% increase, and that's considered a dramatic improvement.
The rocket equation is
delta_v = g*Isp*ln(m_initial/m_final)
or
m_initial/m_final = e^(delta_V/(g*Isp))
So twice the Isp means twice the delta-V from a given amount of propellant, but you don't usually care about "what will this get me?", you want to know "what do I need to get this?", and the change in propellant needed for the same delta-V depends entirely on where you started on the exponential curve.
(Score: 2) by Immerman on Friday January 27, @03:36PM (2 children)
As a sample comparison, to get the ~11km/s delta-V to escape from Earth's surface into interplanetary space is
mi/mf @ 380seconds Isp (=SpaceX Raptor) = 19
mi/mf @ 800seconds Isp (some nuclear engine) = 4
So roughly a 5x reduction in propellant mass needed
On the other hand, I believe to get between LEO and Mars is closer to 4km/s, in which case the difference is
mi/mf @ 380seconds Isp (=SpaceX Raptor) = 2.9
mi/mf @ 800seconds Isp (some nuclear engine) = 1.7
Much less dramatic. However that's the minimum delta-V required - in practice the whole point is to get there faster, which means a bunch more delta-V added symmetrically at both ends (speeding up then slowing down)... and I suppose "what will this get me" really is the key question.
(Score: 2) by mcgrew on Saturday January 28, @08:08PM (1 child)
Directly from Earth? Not likely! It would take off from Earth with a chemical rocket as its first stage. Once it's in space it can spew all the radioactivity it can, which is the very reason for using nukes in the first place, to have them in the gamma ray machine that is outer space in as short a time as possible.
You still have to do a hell of a lot more than just get there to be in any way useful.
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(Score: 2) by Immerman on Saturday January 28, @09:38PM
I certainly hope so! I just grabbed a couple of the delta-V's I could recall offhand to demonstrate how the benefit is highly delta-V dependent
Though if I recall correctly some of the NTR designs should theoretically be safe for use in the atmosphere...so long as nothing goes wrong...