Improving Rocket Engines

A lot is spoken about future propulsion systems and how they are going to be much more efficient than our current rocket engines but I havent seen much about improving the existing technology. Rather than thinking about off-the-wall ideas that may take decades, if at all, to bear fruit why not see what could be done with todays engines?

Currently the most efficient rocket engines use H2/O2 and have Isp ratings of anything up to 470 (eg RL-10B-X), how much further could this technology be pushed?

Are Isps of 550 or even 600 likely or even possible? What is the theoretical limit to the technology?

Would spacecraft designs that had been previously disregarded as being to heavy be built with modern methods and materials?

The Russians made significant gains using the closed-cycle technology on the RD-180 engine, could similar or different technology be applied to other engine types to increase efficiency?

Interesting question. I have seen documents with widely differing “theoretical maximum” ISP for various chemical combinations. It makes me think that the theory is not perfect.
That being said, it seems like engineering practice has gotten about as much as it will out of chemical rocket engines.
I still like the old idea of a two stage vehicle with a hypersonic White Knight type first stage taking a Space Ship One type second stage very high and fast before release. This was the original space shuttle proposal that never got built. There are obvious engineering problems, but nothing impossible I think. The MIG-25 goes almost as fast as Space Ship One using only air breathing engines. That should be fast enough to start up onboard scramjet engines and accelerate to mach 10. The orbiter could be nearly out of the atmosphere and going 1/3 of orbital speed before it even had to start it’s engine. And the Bristol Spaceplanes ideas look good too. Why don’t we do these things? At the end of a test, a flight instructor once asked me, “What keeps an airplane flying?” I answered, “lift” and other such technical answers. He said, “Money”. How true!

I'm not sure that is true, which is why I asked the original question. SpaceX for example managed to build a better rocket engine more cheaply, what's to stop others doing the same?

Now if there was an upper known limit that prevented any improvements beyond a certain point and we had nealy reached it, I'd say lets go and concentrate on something else, thermo-nuclear propulsion seems like it might be a good idea for instance. But if there is still a way to go before the theoretical ceiling is hit then why are advances not being made at a faster rate? If a better engine allows you to undercut the opposition or place a bigger payload in orbit then it must be worth doing.

Engine development work needn't cost to much money either, companies like Boeing or Lockheed have all the facilities and staff they need plus what does an engine designer do when he is not developing engines. Small scale test engines could be made rapidly to test new ideas at low cost before moving on to bigger things. I'm assuming this isnt being done because we do not see vast improvements being made or is it that we have gone as far as we can? Surely there could be improvements made by increasing chamber pressures, trying different catalysts or better turbo-pumps.

Space shuttle main engines have ISP=452.
http://www.boeing.com/defense-space/spa ... /SSME.html
But this page:
http://woodmansee.com/science/rocket/r- ... fuels.html
says the maximum theoretical ISP for H2-O2 is only 391. What is going on? Has the theory changed? Is that second link just wrong? Are the “theoretical maximums” based on unstated assumptions that have changed in modern engines? How does one determine theoretical ISP anyway?

Sorry, I have posed more questions without answering any of yours. It just seems that with all the big and small aerospace firms working on engine development that we are probably getting improved performance as fast as we can already.

(EDIT) http://www.blacklightpower.com/pdf/tech ... .30.02.pdf
and other sources say about 500 is the maximum ISP for H2/O2. That 391 number may be in the atmosphere at sea level. 500 is probably in a vacuum. If your 470 is also in a vacuum, then we are at 94% maximum theoretical now for H2/O2. Which agrees with my gut feeling. Engine design for existing chemical fuels is pretty well optimized.

Yes, 470 was in a vacuum.

Like I said time to try something else then.

As I said ealier my favorite for the next step would be Nuclear rocket motors, NASA developed and even built some that worked fine with Isps up to 900. I am amazed that they decided not to pursue this research and build a complete rocket. A single stage to orbit vehicle would probably be possible with this technology.

Here's a link for a nuclear rocket motor (NERVA) built in in the early 70s

http://www.astronautix.com/engines/nerva2.htm

The states appear to suggest that it was better than what we have today what went wrong? With the advances over the last 30 years what could we build today?

Well, the space race has gotten more dysfunctional. People have put up all sorts of barriers around themselves to what they can do. Environmentalists would probably scream for NASA's head if they did anything nuclear.

Would a private business be able to develop a nuclear rocket engine? Probably not, not unless everyone involved wants to spend time in jail. So that whole thing is now out of the question, and we're stuck using much less convenient, much less lasting, and much more expensive technology. If the United States government and other western governments do not repeal a lot of their burdensome regulations, then they will be bypassed by countries like China and India.

It's sorta like how the Chinese in the 16th century banned ships with more than two sails.

An advanced NERVA type rocket could go single stage to the Moon! It was very promising and just thrown away, like so many other good ideas.
http://grin.hq.nasa.gov/ABSTRACTS/GPN-2002-000143.html
Protesters tried VERY hard to get the Cassini launch cancelled due to it’s RTGs, just imagine the protests surrounding a NIRVA launch!

From what I've read Nerva didn't produce anywhere near the amount of radiation that a bomb would because it is not trying to extract all the energy at once but spread the energy over a long period of time, typically years.

I would guess that once in space any radiation is not much more than that produced by the Sun, the tricky part would be how much is produced on the way there.

I don't know how much radiation would be released into the atmosphere during normal operation, but I imagine it could be near zero with proper design. The main problem would be during an accident. Here is a test they did for that very reason:
http://grin.hq.nasa.gov/ABSTRACTS/GPN-2002-000145.html
Can you imagine such a test even being considered today?
Here are some other NERVA links:
http://grin.hq.nasa.gov/ABSTRACTS/GPN-2002-000141.html
http://grin.hq.nasa.gov/ABSTRACTS/GPN-2002-000142.html
http://grin.hq.nasa.gov/ABSTRACTS/GPN-2002-000144.html

(EDIT) Actually releasing radiation into the atmosphere is nonsense. I meant radioactive material.

I wonder if NASA still has these things lying around or whether all the test equipment and engines was disposed of when they cancelled the project?

They could restart the program with the goal of producing an engine using a radioactive source with a short half life, something like a couple of months so that any radiation produced would be short lived.

This would obviously reduce the reusability of the engine but might placate the anti-nuclear protesters and allow the work to progress. Once the technology was deemed safe or robust enough to remain intact in any accident longer lived isotopes could be used.

Failing that they could always mine the moon for Helium3 which is not supposed to produce radiation as a byproduct (as if).

Interesting. Still, NASA actually doing that seems unlikely. In fact, wouldn't private businesses taking up testing NERVA-like concepts be better? That way, we wouldn't have only one group doing it.

Problem is, what are the laws regarding launching something like NERVA? Going to another country to launch would be better.

Check this out:

http://news.ft.com/cms/s/cf6dc8d2-7937- ... 511c8.html

Could this be implemented?

Theoretical ISP's are based on the fuel or rather the by products. And H/LoX engine throws water out the back which is the lightest chemical by product known of. Using Ionic hydrogen and oxygen (pre-seperated) for maximum efficency I think 500 is possible but you can't really store those in their Ionic forms.

"Nuclear" Rockets heat and eject hydrogen or helium, both are much smaller than water and thus can reach higher vlocities or something... I recall their being a maximum speed of a chemical rocket as being twice the speed that the particle could be ejected....? So fuzzy.

Beyond that its pretty difficult to get engines up to ideal ratios etc. because the get to hot. Usually excess fuel is inject along the sides of the engine to cool it.

Big room for improvement exists in the Engine Bell. These things currently kick engineers in the butt. The Aerospike is a neat way of removing 50-70% of the engine bell weight while also removing the non-optimisation penalty.

Air breathing rockets would be cool... less need for oxidizer in below space.

Rocket engines are incredibly fincky to fiddle with, because if you touch the shape of the ideal flow to add something like coolant or air bypass you might cause flow seperation, backpressure, uneven thrust or any number of othe gremlins. Worst case is you build the eninge and test fire it only to discover its got a great big standing wave in it causing it to vibrate and explode. Up until recently nobody even knew how to predict it. Fixing it was even harder... change the shape, size? In solid rockets they used to insert rods to break up the wave.

All of these things prevent efficeny being reached. But working one out and adding just 5 ISP is a pretty huge piece of engineering.

To recall two points: NASA is thinking about use of nuclear based propulsion outside the atmosphere - not inside her - and this might be allowed to private enterprises too. To return to conventional chemical propulsion: it is possible to improve them by laser. There is a NIAC study with the result that the efficiency of hydrogen could be increased sufficiently if it is heated up before the reaction with oxygen etc. . The heating can be done by a battery of lasers. This is possible by ground based lasers as well as by lasers installed in space.

I will look for the link to the study and EDIT it here.



Dipl.-Volkswirt (bdvb) Augustin (Political Economist)

EDIT: The link is www.niac.usra.edu/files/studies/final_r ... 97Kare.pdf .

Theory says that any rocket will accelerate to N times it’s exhaust velocity by expelling 1-1/e^N of it’s initial mass as exhaust. It does not matter if the exhaust is produced by chemical, nuclear, electric or even mechanical methods. All that matters is exhaust velocity and mass of exhaust ejected. The reason light weight molecules make better exhaust is that it is easier to expel them at high speed. So a chemical rocket that is 99.995% propellant at launch could accelerate to 10 times it’s exhaust velocity. The ratio of empty weight to fueled weight is called the mass fraction. The total speed change of the rocket is called DeltaV. All rocket scientists speak in terms of exhaust velocity, mass fraction and DeltaV. Everything else is engineering detail.