Exploring the Universe with Nuclear Power

universetoday.com ^ | on January 30, 2015 | by Matt Williams

[BenLurkin]

Although no nuclear-thermal engines have ever flown, several design concepts have been built and tested over the past few decades, and numerous concepts have been proposed. These have ranged from the traditional solid-core design to more advanced and efficient concepts that rely on either a liquid or a gas core.

In the case of a solid-core design, the only type that has ever been built, a reactor made from materials with a very high melting point houses a collection of solid uranium rods which undergo controlled fission. The hydrogen fuel is contained in a separate tank and then passes through tubes around the reactor, gaining heat and converted into plasma before being channeled through the nozzles to achieve thrust.

...

Many of these problems were addressed with the liquid core design, where nuclear fuel is mixed into the liquid hydrogen and allowing the fission reaction to take place in the liquid mixture itself. This design can operate at temperatures above the melting point of the nuclear fuel thanks to the fact that the container wall is actively cooled by the liquid hydrogen. It is also expected to deliver a specific impulse performance of 1300 to 1500 (1.3 to 1.5 kN·s/kg) seconds.

...

The final classification is the gas-core engine, a modification of the liquid-core design that uses rapid circulation to create a ring-shaped pocket of gaseous uranium fuel in the middle of the reactor that is surrounded by liquid hydrogen. In this case, the hydrogen fuel does not touch the reactor wall, so temperatures can be kept below the melting point of the materials used.

[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

[BenLurkin]

I will forgo all super powers if Jessica Alba...... will marry me.

[BenLurkin]

Interesting!

[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!

[BenLurkin]

Indeed, but if acceleration/deceleration is at a constant one G then you’ll have gravity the whole way, yes?

[JoeFromSidney]

I worked on the guidance system for the proposed nuclear powered aircraft. No matter what design was proposed, the problem of shielding always killed it. Adequate shielding made the vehicle too heavy. Inadequate shielding brought problems, from neutron bombardment of the structure weakening it, to irradiation of the equipment (that was my concern), to irradiation of the crew. To illustrate the contortions the designers went through, one proposed design called for two vehicles: one would be a nuclear powered "tug," which would bring a conventionally powered bomber near the target, release it, and recover it later. The conventionally powered vehicle would be much faster than the "tug," and thus better able to penetrate enemy defenses. No matter how we sliced it, nuclear powered aircraft turned out to be a bad idea.

[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.

[thackney]

Thanks for that

[Bloody Sam Roberts]

I will forgo all super powers if Jessica Alba...will marry me.

In my book getting Jessica Alba to marry you...IS a super power.

[nuke rocketeer]

Nothing except that the reactor falls intact and unpowered into the ocean along with the shards of the carrier rocket. The weight of a fission drive capable of powering a manned ship ensures that you launch the reactor separately from the fuel bundles, which will be inserted in orbit. Then you will have to have several more launches to get the working fluid. whether it’s hydrogen or water and fill the tanks.

The old idea that NASA had of using a NERVA as the third stage of a Saturn 5 type rocket and launching directly from the ground with everything already loaded was not workable. It will take multiple launches. probably on the order of 10 to 15, to get everything in orbit.

[nuke rocketeer]

Go to this site:
http://www.projectrho.com/public_html/rocket/engines.php

It is a gold mine on spaceship propulsion.