Common Nozzle with Multiple Chambers

guest blogger john hare

During the discussions on the TAN concept, it turns out that the original invention requires some seriously difficult injection into the supersonic stream to avoid shock losses and other difficulties. Jon confirmed that there were problems, just not the ones I was thinking of. There does seem to be a simple way of getting results as good or better that is accessible and affordable for newspace companies.

Cluster seven thrust chambers with the individual throats feeding a common expansion nozzle. At sea level, all of the chambers fire so that there is a low expansion ratio in effect. A ratio as low as 10 would allow decent performance from relatively low chamber pressures, though 20 would be better if possible. Even an expansion ratio as low as 5 would give the sustainer chamber a ratio of 35 in vacuum. The center chamber is pump fed from the thin walled core tank which goes all the way to orbit. The outer chambers are pressure fed from the drop tanks surrounding the core. As altitude is gained and ambient pressure drops, perimeter chambers are shut down to increase expansion ration and Isp from the remaining thrust chambers.

The outer drop tanks are pressure feed with cross feed to the sustainer tank. Tanks four and seven act as header tanks feeding the outer thrust chambers and replenishing the core propellants. The remaining four pressure tanks feed tanks four and seven two at a time before being dropped in pairs. This makes the vehicle four stage with just dumb tank drops. Or possibly two stage with the first stage dropped in pieces over a period of time.


With all of the expansion nozzle ‘charged off’ to the center sustainer chamber, the six outer chambers should be fairly high thrust to weight ratio affairs with the pressure feed not requiring pumps. Thrust vectoring could be done with differential throttling until the outer chambers are shut down, after which fluid injection, possibly with combustion, could be through the outer chambers. Roll control could be more challenging.

After the outer chambers are shut down, the center sustainer chamber has a very high expansion ratio for very high Isp vacuum operations. Since the sustainer chamber has to handle all of the cooling fluids for the expansion nozzle regenerative cooling as well as the center chamber, an expander cycle might be possible considering the heat flux to be handled. Expansion ratio is 140 if the initial ratio was 20. 85% of the propellant that was aboard at lift off was in tanks that were dropped before throttling down to the center chamber, so thrust to weight in vacuum should be good enough to avoid serious gravity losses. With high Isp, and a clean vehicle after the tanks are dropped, a quite substantial orbital payload should be possible from a vehicle with just one expansion nozzle.

As a side effect, a fairly high thrust to weight ratio should be available right off the pad, which reduces gravity losses. As velocity builds up aerodynamic losses become a factor. Throttle reduction to postpone transonic drag to a higher altitude is done by shutting down chambers which increases expansion ratio some in the middle altitudes.

Congratulations to Armadillo. You’ve earned it.

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I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.

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12 Responses to Common Nozzle with Multiple Chambers

  1. Eric Collins says:

    Just a thought. If you are going through the trouble of putting a pump on board, why not have it drive all of the thrust chambers? The fuel for all of the engines would come from the central, low-pressure tank, while the drop tanks refill the inner tank using only a slightly higher pressurization than the central tank.

  2. johnhare john hare says:

    I was thinking that one pump with 1/7th of the capacity would be cheaper and easier early in the game than a pump that must throttle to 14%.

    It does seem possible though that several pumps could pump large volume to the various chambers at lift off and change to series as chambers are shut down to increase pressure and performance of the remaining chamber(s). Bit of a plumbing and development challenge though.

  3. jsuros says:

    If each of the drop tanks had a pistonless pump integrated in its volume, then you could arrange to drop increments of tankage and pumping capacity as needed. That has to be simpler that a pump with such a large throttle range.

    In your diagram, you place multiple combustion chambers in a heavy/complicated looking staggered cone. Could you put all chambers in a single plane for design and cost simplicity and get the same result?

  4. Doug says:

    Seems to include some V2 ancestry?

  5. johnhare john hare says:

    The pistonless pumps would be a trade off between performance and cost. It does seem logical that the Flometrics units would improve the breed.

    If you put all the chambers in a single plane it would make development and testing much simpler also. A company could mount several low ratio nozzle engines on a flat plate with large expansion nozzle attached. A few charictoristics burns would answer a whole lot of questions without the complex nozzle construction I was thinking of.

    As an added bonus, the number of thrust chambers becomes arbitrary. Two or nineteen could work if the flat plate works. That would simplify building larger total thrust vehicles with common sizes of well known thrust chambers.

  6. johnhare john hare says:

    The V2/A4 burner cups is a good visualization if as many as 19 chambers are used. The difference being that the V2/A4 burner cups were injector elements to the chamber, while these would be full chambers to the expansion nozzle.

    Or if you mean that the 19 V2/A4 burner cups were lifted straight off the 1 cup A3, then exactly. Clustering something that you know works is frequently a good thing.

  7. Habitat Hermit says:

    Wanted to reply to this earlier as I’ve been fooling around with/thinking about this myself both for use with liquids and solids. A benefit nobody has mentioned yet (perhaps it’s too obvious) is that one gets the same kind of engine-out capability with X chambers 1 nozzle as with X chambers X nozzles.

    For certain strange and unorthodox solid rocket configurations (I’ll save myself the embarrassment, it’s crazy stuff ^_^) a possible benefit could/should be that one can launch and circularize with the same engine/nozzle with a minimal amount of additional equipment and perhaps also achieve smoother flow.

  8. johnhare john hare says:

    I hadn’t thought about the engine out with common nozzle compared to multiple engines. Seems like control authority would be better this way.

    If crazy stuff embarassed me, I’d be in serious trouble. 🙂 As I’ve said before thoigh, I consider this to be serious fun. Just for fun unless we get something worthwhile, then it’s business.

  9. Tim says:

    I’ve been thinking that this might fit well with a dual mode propulsion rocket.
    You could use the centre tank solely for the monopropellant (I’d say peroxide or Nitrous), and the drop tanks solely for the fuel. This would simplify plumbing somewhat, and eliminate bulkheads. For the sustainer engine you could use something similar to what you describe in “Peroxide and Aerospike” (except without the aerospike), since it looks like it would work well enough in monoprop mode.

    Would it be possible to configure the tanks so you could drop them three at a time instead of two at a time? I’m thinking this could possibly add some mission flexibility. Additionally shutting down the engines in threes leaves you with pitch/yaw control from the engine until you shut down to the sustainer, while shutting down in twos means that you lose one of these axes before the last shut down.

    Looking at the “Peroxide and Aerospike” post made me wonder about using concentric annular combustion chambers instead of conventional ones, but that sounds exceedingly complicated.

  10. johnhare john hare says:

    If you are going for single propellant tanks, dropping peroxide tanks sounds good. With mixture ratios of 7 or more, several of them would empty before one fuel tank. The simpler tanks would improve both mass ratio and cost. It might get a bit interesting figuring out how to drop the singleprop tanks in threes.

    With LO2/Kero with singleprop tanks, empty two LO2 and one Kero with just a little off center mass later in the flight. The center tank could still use your monoprop of choice.

    I think the concept is flexible enough that dropping tanks or shutting down engines could be done in threes just as well as twos.

    I have seen a description of the nested annuler chambers. I think it was in Hutzel and Huang, but can’t remember the proper name offhand. I’ve really got to get my references out of storage.

  11. PeterH says:

    Fluid flow from the center chamber across the other chambers strikes me as problematic.

    A configuration I’ve considered for the same objective has a throat something like a needle valve. For full throttle / sea level the throat is unobstructed. As the rocket climbs and throttles down, a conical rod lowers into the throat reducing throat area and keeping chamber pressure up. Biggest difficulty I see is cooling the conical rod.

  12. johnhare john hare says:

    A cooling loop for the rod I would think. It has been tested with solids under the name pintle throat.

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