Research Council Report Questions NASA Nuclear Propulsion Program
Aerospace Daily & Defense Report 08/31/2005
A new report from the National Academies' National Research Council questions NASA's ongoing effort to develop spacecraft nuclear propulsion systems, recommending the agency study the technologies in more depth to ensure they will be worth the investment. "The committee is concerned that NASA's current nuclear propulsion research and development activities may be too narrowly focused on a single technology - nuclear-electric propulsion - and believes that NASA's efforts might benefit from a broader consideration of other technological approaches," the report says.
Aerospace Daily & Defense Report Research Council Report Questions NASA Nuclear Propulsion Program 08/31/2005 AVIATION WEEK By Jefferson Morris
A new report from the National Academies' National Research Council questions NASA's ongoing effort to develop spacecraft nuclear propulsion systems, recommending the agency study the technologies in more depth to ensure they will be worth the investment.
"The committee is concerned that NASA's current nuclear propulsion research and development activities may be too narrowly focused on a single technology - nuclear-electric propulsion - and believes that NASA's efforts might benefit from a broader consideration of other technological approaches," the report says.
Nuclear propulsion technologies should be investigated more thoroughly to determine if they really can provide fast, affordable access to the outer solar system, as NASA hopes, and move large payloads in the inner solar system in a cost-effective way, the committee said.
The committee also fears that the introduction of "super-flagship-class" nuclear-enabled spacecraft may crowd out other smaller missions, causing the agency to lose its current "diverse and healthy mix" of space science missions.
NASA asked the NRC to identify high-priority space science missions that could be "uniquely enabled or greatly enhanced" by nuclear power and propulsion systems. "Particularly promising" missions include nuclear-powered probes deployed to the inner heliosphere to study space weather, a long-lived Venus lander, a probe to study Neptune and Triton, and an interstellar probe.
"Spacecraft using nuclear propulsion systems, irrespective of the exact technologies employed, will be very large, very heavy, very complex, and, almost certainly, very expensive," the report says. "But it is difficult to imagine that space science goals for the period beyond 2015 will still be addressed with the power and propulsion technologies of the Mariners, Pioneers and Voyagers."
NASA has been developing space nuclear power and propulsion technologies under its Prometheus program. The agency's first proposed flagship mission to take advantage of Prometheus technology was the Jupiter Icy Moons Orbiter, which would have launched around 2015 and orbited three Jovian moons in succession. NASA later backed away from JIMO and its rumored $20 billion price tag, and restructured Prometheus to focus first on surface power generation (DAILY, May 16.).
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To me, this looks like they would rather be moving from nuclear electric to nuclear thermal.
I don't know the technology in details, but I have always felt intuitively that nuclear thermal engines would be less complex and more robust than nuclear elctric, and still effective enough.
There is a delicate tradeoff in performance when evaluating nuclear thermal and nuclear electric propulsion.
Nuclear thermal propulsion is much higher thrust (typically between 30,000 lbf and 150,000 lbf) and much lower specific impulse, Isp is between 700-1000 seconds of specific impulse. Isp is a measure of the amount of thrust generated by a unit of propellant flow. An Isp of 700 seconds means literally that a thrust of 700 lb of force can be sustained by the expulsion of 1 lb of propellant each second. The higher the Isp, the more efficient a rocket is. The Space Shuttle Main Engine typically generates between 390 seconds of Isp at sea level and 425-450 seconds in vacuum--which is very good for a chemical rocket engine. Nuclear thermal typically starts at about 700 seconds for a modest performance engine, and can achieve a theoretical maximum of about 1100 seconds using hydrogen propellant and standard solid state materials--but this is really pushing it. Typically 900-1000 seconds is a good goal to achieve which balances performance with realistic power and operating termperature curves. Reactor power for typical NTR applications are anywhere from 100 Megawatts to 3000 Megawatts for the Phoebe 2 explored during Project Rover and Project NERVA days.
Nuclear electric is a different animal altogether. It uses typically a much smaller reactor (anythwere from 100 KWe to 100 MWe) and uses the heat from the reactor to heat a working fluid which is then sent through some kind of power conversion system to convert heat into electricity. In this sense, NE resembles a more typical ground based power plant in operation. The propulsion component is an entirely seperate system which processes the electricity to accelerate some kind of working fluid. In typical NE applications, such as is envisioned for Prometheus and JIMO, Liquid Xenon is vaporized and then ionized in an ion thruster. Electrons are stripped from the xenon and then the resultant ions are accelerated using strong electric fields created by using a perforated electrode called a cathode. The cathode electrostatically pulls the ions out of the thruster. A seperate electron accelerator squirts out the electrons originally stripped from the xenon so that the vehicle does not develop a strong negative charge, which would eventually bring back the ion exhaust (and of course this would negate any propulsive effects that we want.) Anyways, typically ion thrusters generate an Isp of between 10,000 and 100,000 seconds of specific impulse, with about 15,000 being about the operational standard at this point. However, with this high performance comes a price: extremely low thrust. A 'large' xenon ion thruster able to process a hundred kilowatts of power might be nearly a meter in diameter, and yet generate only a single Newton of force. This would be a little less than the amount of force that a stack of Pringles Potato Chips exerts on the bottom of the inside of a full can! So it's not much force at all.
There have been various proposals to mix the two different methods to take advantage of both but this drastically increases the complexity of such a combined system. The Pratt and Whitney "Triton" NTR engine is one such bimodal engine which can also generate several hundred kilowatts of auxiliary power--needed power if an auxiliary electric propulsion system is needed.
The NTR engines with their higher thrust and power can be used to break a vehicle out of orbit for a relatively fast transfer to another body, and then can be used again for a relatively quick planetary capture, especially if aeroassisst is used. This is more compatible with the notion of a crewed vehicle intent on decreasing exposure for human crews to the effects of solar and Van Allen radiation while in transit. Nuclear electric is more suitable for a long, slow journey in which the costs of a fast transit are not needed, or where the tonnage of cargo is such that it requires greater propellant efficiency, or where higher ultimate delta-v are needed such as in an outer solar system exploration.
Incidently, to get an idea of the magnitude of the exhaust velocity for any propulsion system, multiply the specific impulse in seconds with the acceleration due to gravity on Earth. For instance, to get the overall exhaust velocity of a space shuttle main engine, take 400 seconds times 9.80665 m/s^2=3900 meters per second. For a typical NTR with Isp of 900 seconds: V=900*9.80665=8800 meters per second. For a typical ion thruster where Isp is 15,000 seconds:15000*9.80665=147,100 m/s!
The utlimate rocket efficiency possible with an exhaust of pure light (photon rocket) with exhaust velocity of 299,792,458 m/s (speed of light) results in an ultimate Isp of c/9.80665=30,570,000 seconds of specific impulse. This is the theoretical maximum exhaust speed that a rocket engine could attain.
