5
$\begingroup$

In the 1986 and 1987 papers by Tanatsugu et. al. they first proposed and then built a 150kN closed expander cycle LH2/LOX engine with a heat exchanger within the main combustion chamber. Their motive was to overcome the square-cube drawback of scaling up closed expander cycle engines. Their work seemed promising, but 40 years later there has not been any further mention of this obvious solution to the scaling problem.

My question is, why don't companies implement this relatively straightforward concept and make very light, efficient and simple expander cycle engines that can potentially generate 1000kN+ of thrust? The only problem they encountered in their experiment was that the heat exchanger produced a higher pressure drop than anticipated, but this could easily be remedied by redesigning their prototype with modern computational tools.

I have attached some images of their successful prototype below.

Schematic of the improved expander cycle Drawing of the prototype that they tested in 1987 Performance calculations for various engine cycles

Improve this question
$\endgroup$
2
  • 3
    $\begingroup$ By adding a heat exchanger inside the chamber, the heat you take away is no longer waste heat, but useful heat, then the design is no longer efficient. $\endgroup$
    – user3528438
    Commented Oct 21 at 7:47
  • 1
    $\begingroup$ @user3528438 I see how this may be true from a qualitative analysis without the data, I have added an additional figure to my post from Tanatsugu's 1986 paper showing that the Isp is not significantly reduced by an internal heat exchanger. It has even improved performance over the normal expander design. This theoretical prediction is experimentally validated in the 1987 paper. $\endgroup$
    – nando2002
    Commented Oct 21 at 11:12

1 Answer 1

Reset to default
4
$\begingroup$
  1. "Their motive was to overcome the square-cube drawback of scaling up closed expander cycle engines."

It seems, the days of scaling up any rocket engines are past. The success of F-1 was a miracle. Soviet NK- series engines spelled doom of their N1 project and killed their dream of moon landing. There are countless problems with scaling the engines up, and this doesn't address most of them. It seems if you want more thrust, you add more mid-sized engines, which solves most of the problems and adds benefits of redundancy and economy of scale (mass production of the engines makes them cheaper). So, it's a solution for one bit of a problem that was taken care of in its entirety by a paradigm shift. No-one needs it anymore.

  1. "The only problem they encountered in their experiment was that the heat exchanger produced a higher pressure drop than anticipated"

The pressure drop is an inherent property of this solution. Sure you can fine-tune how much of pressure drop you get, but regardless, it will be a significant amount - the temperature in the combustion chamber directly contributes to pressure of the exhaust gas, thrust and specific impulse, and you're tapping into that heat to vaporize cryofuels. Reduce the temperature - you reduce the engine performance. user3528438 has it right: you're not using waste heat, you're using useful heat. Currently we're in a race for higher chamber pressures. We don't want to reduce the chamber temperature.

  1. Issues of throttling.

The amount of heat for use of this kind of heat exchanger depends directly on how much the engine is throttled - how much fuel and oxidizer is burned, producing heat. And throttling is critical for control of the rocket flight. With regular preburners and separate heat exchangers you can control and fine-tune the performance of the preburner or exchanger independently, as required, to provide optimal mix for required engine throttling level. Here you're tightly coupling two only indirectly dependent parts of the process; the chamber pressure and temperature is strictly subordinate to flight control requirements and we can't tune it willy-nilly to serve regulating performance of the heat exchanger.

Improve this answer
$\endgroup$
7
  • $\begingroup$ Thanks for the clarification. Regarding point 2 and the sapping of useful heat from the combustion chamber - the energy to pump at higher pressures would have to be taken from the fuel in any rocket cycle. I have added another figure from Tanatsugu's 1986 paper in which he asserted that the method of sapping heat from the combustion chamber would still be more efficient than a gas generator (and presumably a staged combustor) of the same dimension. Wouldn't this class of rocket find use in a strap on booster or some other cheap first stage? i.sstatic.net/fWUn316t.png $\endgroup$
    – nando2002
    Commented Oct 21 at 11:02
  • $\begingroup$ @nando2002 Even if the process of drawing the energy is more efficient, the combustion chamber is not where you want to draw energy from. That energy is earmarked for propulsion, and you don't want to divert it to any other purposes. The limiting factor is not the amount of fuel but what temperature and pressure the combustion chamber can take, and it can take the same with the exchanger and without, in the former case providing less for propulsion with some sunk into the exchanger. $\endgroup$
    – SF.
    Commented Oct 21 at 11:15
  • $\begingroup$ @nando2002 Also, energy efficiency is not everything; especially with big engines combustion stability is the most problematic factor, and injecting partially-burnt gases into the chamber, instead of spraying liquid or barely gaseous fuel/oxidizer helps a lot with that. Only Russians use liquid-fuel strap-on boosters, and they aren't innovating much nowadays. (unless you count Falcon Heavy where the design is unlikely to change). It's a neat solution to a part of a problem nobody's interested in dealing with in current rocket engine ecosystem. Even if it would help, who's gonna try it? $\endgroup$
    – SF.
    Commented Oct 21 at 11:22
  • 1
    $\begingroup$ @nando2002 I'm not knowledgeable enough to answer these questions - just a hobbyist, not a rocket scientist. I have some doubts about "for the same engine size/pressure" - I'm fairly certain for the same pressure the combustion chamber would need to be more rugged/durable, unless the 'control' engine's chamber was overengineered and heavier than necessary. For same performance the chamber needs to produce (and get exposed to) more energy. So either you need stronger chamber or you started with a stronger chamber than was needed in the first place. $\endgroup$
    – SF.
    Commented Oct 21 at 17:07
  • 1
    $\begingroup$ RE point #1, IIRC the main reason for scaling up engine size back in the day was the difficulty in getting simultaneous ignition to work right. Synchronizing a handful of massive engines was just simpler. We figured out how to do it, and now SpaceX regularly lights dozens of engines at once with few issues, which would have been daunting if not impossible in the '60s. $\endgroup$
    – Darth Pseudonym
    Commented Oct 23 at 15:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.