Copyright 2015 Robert Clark
By now all Mars advocates have heard the argument that VASIMR's 39 days to Mars promise is illusory because the needed space nuclear power sources do not exist at the needed lightweight, ca. 1,000 watts per kilo.
This led me to propose using concentrated solar power for VASIMR or Hall effect thrusters instead:
Short travel times to Mars now possible through plasma propulsion.
I was therefore startled to read when looking at the specs of space nuclear engines that the engines themselves actually put out orders (plural) of magnitude higher power than this. See for example the specs on the "bimodal" nuclear rocket here:
Bimodal NTR.
Bimodal NTR.
Engine (Thrust Mode)
Thrust per engine 67,000 N
Total Thrust 200,000 N
T/Wengine 3.06
Exhaust Velocity 9,370 m/s
Specific Impulse 955 s
Propellant
Mass Flow 7.24 kg/s
Full Power
Engine Lifetime 4.5 hours
Reactor Power 335 MWthermal
http://www.projectrho.com/public_html/rocket/realdesigns.php#id--Bimodal_NTR
Thrust per engine 67,000 N
Total Thrust 200,000 N
T/Wengine 3.06
Exhaust Velocity 9,370 m/s
Specific Impulse 955 s
Propellant
Mass Flow 7.24 kg/s
Full Power
Engine Lifetime 4.5 hours
Reactor Power 335 MWthermal
http://www.projectrho.com/public_html/rocket/realdesigns.php#id--Bimodal_NTR
At a thrust of 67,000 N and T/W of 3.06, this means the engine weighs, 21,900 N, or 2,230 kg. So at a 335 MWthermal power this is a 150,000 watts per kg power to weight ratio. And the conversion of this thermal to kinetic energy is over 90% efficient as measured by the engine exhaust velocity.
This means the problem with getting electrical power out of the space nuclear reactors has nothing to do with the nuclear reactors themselves. The problem is with the conversion to electrical power, specifically, the conversion/generation equipment is too heavy.
In that vein note then there are electric motors, i.e, electric-to-mechanical conversion, with the necessary lightweight:
2.1.2 Electric motors/Electromotive generators.
It turns out that electrical-to-mechanical energy conversion and vice versa is very efficient, typically in the 90% range and above. So you would run these electric motors in reverse to generate the electric power. Note then the best in that list is at 10,000 watts per kilo, sufficient for the VASIMR, and other plasma propulsion methods.
It is important to recognize that the low electrical specific power, i.e., electrical power per weight, for space nuclear reactors is not due to the reactors themselves but due to the electrical conversion equipment. Then the focus is put on improving the electrical power generation weight efficiency. But this has importance beyond just space power systems. For instance the defense department wants lightweight electrical systems to power their UAV's. And aircraft manufacturers are investigating electrically powered aircraft for low-noise and zero-pollution aircraft. For instance LaunchPoint has produced high power density motors for UAV's in the 8200 w/kg range, which they say can be scaled up to large aircraft.
Another area of research for high specific power motors and generators is operating them at cryogenic temperatures. According to this report 10 times as much power can be put through the windings of a motor at liquid nitrogen temperatures than at room temperature:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070028414.pdf
Then using the electric motors already getting ca. 10,000 watts/kg, we could conceivably get ca. 100,000 watts/kg by running them at cryogenic temperatures(!) This has great relevance to the space propulsion application since we could use liquid hydrogen or other cryogenics as the fuel that would also serve to keep the electric generator at cryogenic temperatures.
However, in an upcoming blog post I'll show you don't need to do the conversion to electricity and run a plasma engine. You can get the high speeds from the nuclear engines themselves, with some minor modifications.
Bob Clark
However, in an upcoming blog post I'll show you don't need to do the conversion to electricity and run a plasma engine. You can get the high speeds from the nuclear engines themselves, with some minor modifications.
Bob Clark
8 comments:
Your calculations aren't quite right.
Yes, nuclear reactors put out a lot more thermal power than electrical power. But if you want electrical power, you have to pay attention to the electrical power output, not the thermal power. The problem is not entirely the weight of the conversion system; it is also the conversion efficiency.
It is indeed efficient to use a rocket nozzle to convert thermal energy into exhaust energy (aka jet energy). But exhaust energy is not trivially converted to electrical power! An electrical motor doesn't convert jet energy into electrical power-- put the exhaust of a nuclear rocket onto a electric motor, and what you will get is an ablated elecric motor. You can convert jet energy into electrical power using a Brayton converter.
But, that's exactly what nuclear electric power sources do-- they use Brayton converters.
Thanks. Ground-based nuclear plants do the conversion that way. But previously space nuclear reactors used thermoelectric methods which have the advantage of having no moving parts but are quite poor in conversion efficiency and weight efficiency.
Using the Brayton cycle should improve the conversion and weight efficiency for the space nuclear reactors. However, to maximize it I suggest using as a model the SSME turbopumps. These had a remarkable specific power of 150,000 watts per kg, and a conversion efficiency ca. 80%.
It is important to keep the conversion efficiency high to minimize the size of the radiators. The radiators could be quite large and heavy when dispensing with large amounts of heat.
If conversion to electric was only in the range of 5%, which some space nuclear systems had, that would mean you need radiators to dispense with 95% of the power generated as waste heat. That is, we would spend all that trouble producing the power only to throw almost all of it away and at great mass penalty.
Bob Clark
thank,s man
Dear professor Clark:
The issue of conversion efficiency is key to nuclear electric propulsion. The heat output from a nuclear reactor may be as high as you desired, but converting heat into electricity is critical: it can be done via thermodynamic machinery (Brayton cycles (turbines), Stirling engines, Rankine cycles, whatever, but efficiency is only about 30 to 40% at most. So you must reject 70 to 60% of your reactor heat output with a space radiator. Typically, that is heavier than the nuclear reactor besides being very cumbersome.
You mention thermoelectric generators: they are even less efficient: the best ones have efficiencies of order 5-10% at most. And scaling them up (besides being complex) raises total weight much above thermodynamic conversion anyway.
In this discussion I don't understand why mentioning SSME turbopumps, or mechanical efficiency of electric motors...we don't need any electric motors in space propulsion.
This said, I want to acknowledge you for raising the issue of reactor life vs. power. It affects both nuclear thermal and nuclear electric propulsion equally, but has a deeper impact in electric propulsion using low thrust, since mission times become very long. In fact, for interplanetary missions they might be too long for crew safety, due to GCR and Solar radiation.
Hope this is useful. Regards,
Claudio Bruno
The point was that having large expansion nozzles can improve the conversion efficiency from the heat of the combustion chamber to the kinetic energy of the exhaust to the 90% range. Recent research also shows using, for example, supercritical CO2 for the working fluid can reduce the size of the turbines to a fraction of the size as for steam turbines.
I discussed the SSME turbopumps to give an idea of how high specific power you can get converting thermal to mechanical. The discussion of the electric motors was to indicate running them backwards you get a electric generators at similar high specific power and efficiency.
Bob Clark
The electric generators (not motors) are required for ion and plasma type drives for missions beyond Mars. The advantage of such drives over Straight NTR or bi modal NTR is the high Isp produced. Current VASIMR test models have already achieved in excess of 5000 s. Although the thrust is much lower, actual delta v is considerably higher, and only one fifth the propellant mass is required.
Thanks for the post.
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