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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.
OMG nu-cle-er radiashun in space! Think of the environmental damage!
ok.../. cut off the "ket"....but roc is cool, too.
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.
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.
What an emotionally charged word :"explore". The Solar System's mostly empty. Big deal. What's to explore?
No kidding. They should have said browse the solar system. Everyone knows that explore has an inherent Microsoft bias.
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).
"None can love freedom heartily, but good men; the rest love not freedom, but license." --John Milton
Leaving aside how ridiculous your argument is, I'll poke some holes in your math instead. To paraphrase XKCD, space is not high, space is fast. LEO is 7.8 km/s. Accelerating 118 tonnes to 7.8 km/s takes 3590 GJ, significantly more than the 232 GJ you mention.
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?
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/
Prediction for end of Universe #42: Fencepost error in Quantum_bogosort.cpp
Which, of course, is exactly what they will be doing. Make the gas a plasma, and contain it with magnetic and electric fields.
Prediction for end of Universe #42: Fencepost error in Quantum_bogosort.cpp
Well, we know that the US had nuclear-armed B-52s and nuclear xommamd and control EC-135s airborne 24/7 until at least 1992. That led to a couple of scary accidents. Google "Chrome Dome" for more information. That was one leg of the nuclear triad - subs, missiles, and bombers on alert 24/7. The bombers periodically received a "do not attack" signal.
What the strategic command has been up to since 1992 we don't know. They keep such things secret when possible, for obvious reasons.
Space Balls had a lovely Winnebago that traveled through space quite fine thank you very much
In this case, it's absurdly beside the point as well. The expenditure of nuclear materials is utterly irrelevant to the problem.
The way to rocket works it to use nuclear-generated heat to expand and accelerate a working fluid (usually hydrogen) and shoot it out the nozzle. What matter is the mass of the working fluid expended per impulse (force x time) - the specific impulse (lb-sec/lb or kg-sec/kg, for units of seconds) or ISP. T
housands and thousands of lbs of the working fluid will be consumed, the fact that it also consume a few ounces of nuclear material, too, is utterly in the noise.
A very good chemical rocket will have an ISP of 450-460 seconds. A nuclear thermal rocket will have an ISP of around 900-1000, or roughly twice as "good". "Good" is defined by the amount of impulse/momentum change you get for a given amount of fuel consumed.
A nuclear thermal engine can be built to provide almost any desired thrust level, with 25000 lb thrust engines actually built and tested.
By comparison, a Hall Current or other ion engine will have an ISP of around 1800, but use vast amounts of electrical power for extremely feeble thrust of far less than a pound in typical cases.
> maximum final Delta V from source of circa 58,000 ft/sec
Einstein would like to have a word with you. That word is "relative". Suppose there is a planet traveling away from the earth at at 50,000 ft/sec. An alien on that planet can fire a rocket, which can travel away from that planet at 50,000 ft/s, meaning 100,000 ft/s relative to earth. As it catches up to another planet, it might photograph some other aliens launching their own rocket at 50,000 ft/s, which is 150,000 ft/s relative to earth.
In fact, the SAME rocket could from earth to the first planet, then be launched from that planet, then stop at the next planet and be launched at 50,000.
Come to think of it, stopping at each planet doesn't change anything. It's ALWAYS standing still relative to something, and can launch away from that something to 50,000 ft/s. The gas leaves nozzle at 58,000 RELATIVE TO THE COMBUSTION CHAMBER. In other words, it can always go 58,000 faster, as long as it can fire it's engine. 58,000 is the limit for a BALLISTIC projectile, one that is fired from a gun and doesn't carry a working engine with which to keep accelerating. The limit is 58,000 RELATIVE TO the chamber in which the gas is burned. By carrying the combustion chamber within the craft, it can accelerate until it approaches C.
Peak uranium in 41328, we're running out of time people!
Mod me down, my New Earth Global Warmingist friends!
Open cycle nuclear engines are a bad idea anywhere close to earth orbit. They are essentially an open system that expells nuclear fission byproducts as well as propellant. They are not permitted to operate in earth orbit for a good reason. They would leave significant trails of radioactive material in orbit. This has implications for the sensors on satellites and is still going to fall to earth eventually. So these open cycle reactors may be useful for longer missions but would still need to get a heavy reactor into orbit. They also run essentially unshielded so on a manned mission you'd need lead or water shielding. Nuclear power sources using decay heat are probably better suited due to low levels of gamma an neutron radiation. The idea of collecting propellant along the way is quite attractive too but beyond our current engineering. http://en.m.wikipedia.org/wiki...
Nuclear thermal rockets will be heavy, though, and that detracts from their efficiency.
I wonder if gas core nuclear rockets are so pie-in-the-sky nobody worked on them, or they're pie-in-the-sky because nobody worked on them. In theory you could get crazy ISP and thrust numbers from a gas core rocket.