[BenLurkin]

I imagine the reason NASA calls this a "new" concept is that it hopes nobody remembers that the idea using of nuclear rockets has already been rejected in the past.

[cripplecreek]

It was even successfully tested using conventional explosives here on earth.

I’d still like to hear more from DARPA on their fusion research. I generally write people off when they say fusion but when DARPA says it I listen.

[skinkinthegrass]

in before Star Trek reference....

[Vince Ferrer]

The first thing we need to do before this is build the next generation of telescopes. These will be so powerful that we can decide where to go before we leave.

[nomad]

So I guess they`ve forgotten about the Challenger, what happens when one of these blows up during launch?

[Bloody Sam Roberts]

To my thinking, propulsion is a secondary consideration when planning any kind of extended missions for humans.

The primary consideration should be to develop an electro-magnetic type of shielding to protect the ship and its human crew from the effects of cosmic radiation from our Sun and other sources. An emulation of what protects the Earth would be a good start.

Without that, the type of propulsion is immaterial if the crew is killed or severely effected by the hostile environment they travel through for extended periods.

[thackney]

Flying on Nuclear, The American Effort to Built a Nuclear Powered Bomber
http://www.aviation-history.com/articles/nuke-american.htm

After conversion, the engines were removed and a new configuration was incorporated. The NB-36 now had four GE J47 nuclear converted piston engines generating 3,800 hp augmented by four 23.13 kn turbojets generating 5,200 lbs of thrust. Each of the engines utilized the Direct-Cycle Configuration for power conversion. The NB-36 was designed from the beginning, to be propelled to the air with a conventional chemical mixture, and then the crew would switch on the reactor after achieving the necessary heat requirements on its core. On landing approaches, the aircraft would switch back to chemical mixture. This procedure was implemented in order to minimize the possibility of a major radiation leak in case of a crash landing.

The NB-36 made 47 recorded flights between the summer of 1955 and the fall of 1957. All these tests were made operating the NB-36 with conventional chemical power. The R-1 reactor was turned-on on many of these flights, not to actually power the aircraft, but to test and collect data on the feasibility of a sustained nuclear reaction on a moving platform.

Much more info at link

[AZLiberty]

This all seems to depend on mass ejection. Why hydrogen instead of some more dense gas? How much such mass do you need to bring along to make a quick trip to Alpha Centauri?

Don’t forget to save at least half of it for decelerating at the other end!

[JoeFromSidney]

There's a lot of confusion in this article.

Unless you're intending to add some form of electrical acceleration to the exhaust gas, there is no point in converting it to plasma. I think the author is confusing things like the VASIMIR concept with other forms of propulsion.

Note that we do not have any spacecraft using nuclear reactors. We do have Radioisotope Thermal Generators in space. Again confusion by the author. These depend on radioactive decay, not on a fission reaction.

Back in 1968, a friend of mine did his dissertation on an open-cycle nuclear rocket. The idea was to have uranium particles mixed in with the exhaust gas, in sufficient density that the thing achieved criticality. The reaction would then heat the gas. The problem, which he worked on, was how to separate out the uranium before ejecting the exhaust gas. Note that this is an attempt to mimic a chemically powered rocket, in which the heat-generating reaction takes place inside the engine.

The fundamental problem with nuclear rockets is wall temperature. In a chemically powered rocket, the hottest part of the exhaust system is the exhaust gas itself. The walls absorb heat from the gas. That heat must be removed somehow, to keep the walls from melting. The point is, the gas is hotter than the walls. In almost every proposed form of nuclear rocket, the walls (or plumbing, or whatever) are hotter than the exhaust gas. The heat is generated outside the engine, and must be transferred to the gas inside the engine. The exhaust gas is therefore cooler than the engine walls. For any given wall temperature, then, the specific impulse of the nuclear rocket will be less than that of a chemical rocket with the same wall temperature.

A nuclear rocket makes sense only if the combined weight of the reactor, engine and fuel are less than the combined weight of engine and fuel for a chemical rocket of the same total impulse. That is, lesser specific impulse must be offset by additional operating time or total thrust.