NASA returning to NERVA?

[KitemanSA]

There are "Greens", and then there are "FauxGreens". Watermelons are FauxGreens but then so are the paid shills on the petro-carbon industries.

Ah. but the petro-carbon boys deliver actual wealth. The Watermelons deliver negative wealth.

True, but the PC boys also PREVENT wealth in the form of supressed nuclear power. Coal and petro-methane should have been out of the electrical grid by now. Grid power should have been cheaper and carbon free.

[93143]

Also want to point out that subsequent projects to NERVA demonstrated much higher T/W than 1, high enough to SSTO.

Wasn't this contingent on ISPs of 1600 or more?

I don't know what you're referring to, but I do know that (a) solid-core NTRs can't do 1600 s, and (b) Dumbo was not subsequent to NERVA; it was concurrent and was deselected for "non-technical reasons". Despite a significant amount of study and even manufacturability tests, it seems to have acquired a reputation for being unfeasible; as far as I can tell this reputation is baseless. A fairly recent MSFC study suggested a very similar idea, though not by name.

The project report I read showed one notional design with a core T/W of nearly 130, but this was only the reactor assembly; the rest of the rocket engine assembly, and (I think) the required shielding, would have been extra. Also, that was with molybdenum; the best design shown using tungsten didn't quite reach 80, although it should be noted that these were point designs and were not optimized. The cladding on the fuel elements might have been thin enough to shed fission products, but at one point the report notes that increasing the fuel element thickness a bit seemed to improve the predicted T/W...

The "Super Dumbo" could potentially have exceeded 1000 s, but only by running at low pressure (leading to low T/W) in order to enhance high-temperature dissociation. Modern design and manufacturing techniques might improve this picture a bit (I won't say modern materials, because tungsten-clad tungsten-uranium oxide cermet was actually in the study).

The LANTR approach might be useful for SSTO.

[krenshala]

Isn't the decision between high thrust-to-weight and low ISP versus low thrust-to-weight and high ISP the normal design choice you have to make with any kind of rocket engine?

[Tom Ligon]

For deep space, sure. For launch from Earth's surface, thrust must exceed weight at liftoff. (Or pretty close ... IIRC the Saturn V needed to burn for 3 seconds before it could lift off).

From reviewing Dr. Bussard's fusion-powered rocket designs, some of the principles managed to get thru my thick skull. One set of designs is ARC, or All Regeneratively Cooled. That means he used reaction mass flow to carry off waste heat from the power production system. If you dropped below the mass flow needed to do this the system would overheat. Thus, you did not have the option of leaning an ARC system out for very high Isp options. For those you needed heat radiators.

Fission designs will have a similar problem. If you are producing thrust by transferring heat from the reactor fuel to a reaction mass gas, that's essentially an ARC system in which the waste heat itself produces the thrust. There will be a minimum reaction mass flow below which the reactor will overheat. But I think there is also a basic power loading needed to get optimum performance. Generally these engines are designed to operate at high power for a relatively short time. Backing off power and temperature would tend to make for an inefficient engine. If you can score one of the Bussard and DeLauer books on nuclear rocket propulsion, you will find all this worked out.

Alas, while Dr. Bussard's copy of one of these books had a price tag of under $5 from his university's bookstore (fun to be able to peek at his bookshelves), the used copies have gotten rather dear on Amazon.

[Skipjack]

Well high thrust and high Isp are not mutually exclusive. It depends on the total amount of energy. The more the expelled mass is accelerted, the higher the Isp for the same amount of thrust (or more thrust for the same Isp, depending on how you handle it). For fission rockets that means they need to get hotter.

[Tom Ligon]

Well high thrust and high Isp are not mutually exclusive. It depends on the total amount of energy. The more the expelled mass is accelerted, the higher the Isp for the same amount of thrust (or more thrust for the same Isp, depending on how you handle it). For fission rockets that means they need to get hotter.

And run the lowest molecular weight of reaction mass possible. Hydrogen is a pest but it gets the exhaust velocity up better than anything else, unless you can break it down to atomic hydrogen.

There is a problem with hot-running fission reactors. Normal fission reactors run on thermal neutrons, and they lose reactivity as they get hotter. So making a reactor with sufficient reactivity to run hotter than normal means it is a touchy mutha when cold. Most modern fission reactors are pretty safe and it is difficult to make them explode, but fission rocket engines are more dangerous.

Rover story, told to the best of my recollection from what RWB said. Some blamed fool didn't understand the thermocouple color codes (red is usually negative). The first four thermocouples were hooked up correctly, but then all the rest, hundreds of them, were wired backwards. They found this out after they ran a short power blip on it. This was a problem. The reactor was now hotter than a two dollar pistol and they could not fix all those backward TCs. But they were under the gun because there was a rain storm coming and these test reactors had the exhaust nozzle pointed up, a big funnel to collect rain water in a prompt-critical reactor that normally ran without water as a moderator. They really did not want it full of water.

Some poor schnook got to run out and reverse the first 4 thermocouples, which could be done quickly enough not to cook him, and they could then get useable reading off them all.

We have not mentioned the VASIMIR option, at least for deep space. If you use the fission reactor to drive an electrical powerplant, you can then make any Isp you want. However, that's a lot of complex hardware, not to mention needing huge heat radiators that probably out-mass the reactor, so this means a worse power to mass ratio than the simpler versions. This is much better with a p-B11 Polywell providing the juice.

[93143]

Isn't the decision between high thrust-to-weight and low ISP versus low thrust-to-weight and high ISP the normal design choice you have to make with any kind of rocket engine?

For chemical rocket engines, generally that's taken care of with the choice of propellant combination. (The analogy with an NTR would be running ammonia vs. hydrogen.) Once that's decided, increasing the chamber pressure boosts both T/W and Isp. Mostly sea level Isp, due to the reduced back pressure penalty, but even vacuum Isp tends to be slightly better. This last is probably because at lower chamber pressures, dissociation sucks up more of the energy available from the propellants, and the exhaust doesn't completely recombine before leaving the nozzle, wasting the remaining dissociation energy.

What they were suggesting with the Super Dumbo is the opposite - lowering the chamber pressure so the hydrogen will dissociate more, absorbing more heat at the same temperature (because NTR). You wouldn't want to use this at sea level - not only would the T/W be bad, but the ambient pressure would absolutely murder the Isp. If you want an NTR-based launch vehicle, go with regular Dumbo, maybe with LOX augmentation for early in the flight.

[choff]

Apparently NASA will officially adopt VASMIR

http://www.dailymail.co.uk/sciencetech/ ... ngine.html

[Diogenes]

Apparently NASA will officially adopt VASMIR

http://www.dailymail.co.uk/sciencetech/ ... ngine.html

I always thought that was a no brainer. I have long advocated this approach. I know of no better engine design.

[GIThruster]

It's not actually a very promising approach:

"Existing ion thrusters routinely achieve 70 percent efficiency and have operated successfully both on the test stand and in space for thousands of hours. In contrast, after 30 years of research, the VASIMR has only obtained about 50 percent efficiency in test stand burns of a few seconds’ duration, and that is only at high specific impulse. When the specific impulse is reduced, the efficiency drops in direct proportion. This means that the VASIMR’s much chanted (but always doubtful) claim that it could offer significant mission benefit by trading specific impulse for thrust is simply false."

http://www.marssociety.org/home/press/t ... vasimrhoax

The problem is really the power requirement to create the plasma. This could be offset if one had a way to generate plasma much more efficiently. For instance, if the BlackLightPower scheme were to work, that plasma could be had on the cheap, and so you'd see very different figures here, but as is, this requires a fangdangus reactor and if you're going to build that, a nuclear thermal rocket could be much more efficient.

http://phys.org/news/2011-07-zubrin-vasimr-hoax.html

I've seen some worse criticisms as well.

[93143]

I would take Zubrin's opinion on this with a grain of salt. He seems to have something against VASIMR (probably because it conflicts with his own Mars scheme), and his arguments are overblown.

Handwaving based on a tiny Russian reactor from the '70s is disingenuous, especially when studies of potential future high-power designs do show system power-to-weight ratios in the required range (for nukes, this parameter scales strongly with size) - not to mention that it may well be possible for solar to get there, at least in the inner system (though it doesn't scale as favourably). And his claim of ~50% efficiency "only at high specific impulse" is wrong - that number was obtained from a test unit running around 3000 s, with a propellant (argon) expected to be good to 5000-6000 s. End-to-end system efficiency is typically calculated to be at least 60% at nominal conditions. He also claims thrust efficiency is linear with Isp (that's what "in direct proportion" means), which it isn't. It's close at low Isp on that test thruster, but it levels off considerably in the upper range.

Now, apparently VASIMR doesn't look as dominating* next to stuff like advanced Hall-effect thrusters and the MSNW ELF, both of which NASA is also funding. And it may have been oversold a bit. But that's a far cry from it being a "hoax"...

*Gridded ion and older-generation Hall-effect thrusters have low system power-to-weight ratios, suffer from erosion issues and have trouble scaling up, so VASIMR crushes them for manned missions.

[GIThruster]

I think the "hoax" portion is a fair reaction to the hype, which is that VASIMR is some sort of game changer, when it is not as efficient as Ion or Hall, both of which already have 40+ years work, Ion at NASA and hall in private industry and Russia. The only game changer is it can dump lots of propellant for higher thrusts than the other electric systems, and no one going to Mars would do that. Why haul more of it when you can use it at higher Isp? And I don't think Ad Astra has duration figures even beginning to approach the Hall thrusters we see on so many satellites. This is a big deal when going to Mars. He did do an analysis a couple years ago that was much more thorough. I should look for it. It seemed pretty persuasive at the time and not really tied to his own ideas so much as annoyed with the hype.

[GIThruster]

"Handwaving based on a tiny Russian reactor from the '70s is disingenuous, especially when studies of potential future high-power designs do show system power-to-weight ratios in the required range (for nukes, this parameter scales strongly with size) - not to mention that it may well be possible for solar to get there, at least in the inner system (though it doesn't scale as favourably)."

To the best of my knowledge, the state of the art for in space fission was the SP100 and Prometheus follow-on that was planned for JIMO, and that was cancelled several years ago with JIMO. US Navy took over all the Prometheus stuff, but there's been no work on it available to the public since that time, and I think the power figures were much closer to Zubrin's than Diaz's. And the scaling is not very much past linear since you have the overwhelming portion of the system is radiators, which are about the same for any power level. Now if they were to go to higher temperature radiators, they could reduce this but we're now talking about hundreds of millions or perhaps even billions of dollars for all new systems. Why not just build TRITON?

In any case, more than 5,000T for a Mars trip is way past what is necessary for chemical. This just doesn't make much sense. You know I think the thing to do is develop better propulsion, but I think Zubrin makes a lot of excellent points here and I think we need this kind of critical thinking if we're going to spend billions on a system that won't be ready for decades and one hopes will be abandoned as obsolete before that time.

I'm just listening to this now and though dated, I'm unaware of any serious corrections to these numbers:

https://www.youtube.com/watch?v=myYs4DCCZts

[hanelyp]

When running a power or energy limited system, expending more propellant mass for higher thrust can work to your advantage. Using more propellant you can get your desired impulse with less power plant. Most electric thrusters actually have far higher exhaust velocity than you can make good use of with the near term spacecraft power plants.

[KitemanSA]

For deep space, sure. For launch from Earth's surface, thrust must exceed weight at liftoff. (Or pretty close ... IIRC the Saturn V needed to burn for 3 seconds before it could lift off).

No, It just has to be greater than the drag + weight vector. No one ever MANDATED a vertical launch. I pretty much guarantee the jets on a 747 don't have a T/W(craft) ratio greater than 1.