Test Validity Question

I had a (nother) strange thought this morning. It is my opinion that I have figured out how to do inexpensive compensating nozzles. I don’t have an engine to test the idea. Has anyone ever seen any nozzle testing that used a blank pistol cartridge? My thought is that I could turn a nozzle and firing chamber with pressure gauge for a 22 blank and record the results with a high speed camera. If the exhaust behaves as I predict at high to low pressures, then it might  be worth discussing with an engine builder to test on a thrust chamber.

Has anyone heard of such a thing?

Would the results be valid enough to convince real rocket men?

Do any of the real players need a compensating nozzle enough to spend money building one even if it is a valid test concept and is successful?


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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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11 Responses to Test Validity Question

  1. Pingback: Transterrestrial Musings - Testing Compensating Nozzles

  2. Peterh says:

    You’re thinking pump down the chamber and run the engine with a short enough pulse to still have a good enough vacuum afterwards?

    A rifle blank cartridge isn’t going to power more than a very tiny engine, and that for only a very short shot. If vacuum capacity isn’t a problem, a model rocket engine cartridge might be a better gas generator for your purposes.

    I understand the big boys test vacuum operation of rockets by rigging the engine in an ejector pump setup, where the exhaust stream from the engine passing through a nozzle pumps out the operating chamber. Would a model rocket cartridge run long enough to pump out your test chamber and give you the data you want? If you ran the nozzle under test with cold gas from an air compressor somewhat extended runs wouldn’t be a problem.

    Also, the free software CFD package OpenFOAM is very good, if you can figure how to setup the model. I know better than to completely trust computer models in these matters, but they are still useful in finding candidates for physical testing.

  3. DanielC says:

    Is your idea for a first-stage or higher stage nozzle? What sort of range of expansion ratios are you looking to accommodate?

    possible deal-killers:
    1 – unsteady flow in your test rig vs. quasi-steady flow in full-scale nozzle
    2 – scaling differences (Reynolds no. etc)

    Given the nature of your design, are these fatal or can you work around them?

  4. johnhare johnhare says:

    There is no need to pump down to vacuum or other complicated stuff if the concept will work at all. A compensating nozzle compensates for PC/PA. At 35,000 or so feet altitude, PA is about 1/4 of that at sea level so it would take 4 times the pressure at sea level to match the pressure ratios to see what would happen. A 3,000 psi pulse at sea level would simulate the pressure ratio of a 750 psi engine at 35,000 feet or a 190 psi engine at ~70,000 feet. The idea is that watching the plume behavior on the high speed would give an idea of its’ behavior at various pressure ratios with very little other test equipment.

    First stage use, especially for VTVL that really need to operate at low pressures during the landing phase. Higher stages are better off with standard high expansion ratio nozzles. I see test range expansion ratios of 2 to 50. At high pressures we see how the 50 works and how well adjustment happens as the pressure drops. Unsteady flow and scaling problems would just be annoyances if the concept could be proven. The idea is not for me to use definitive data to use on an engine but rather to get just enough evidence to convince some serious player to risk one of their engines and some time and propellant to move on down the field.

  5. e.p. says:

    Cartridges vary, if you really wanted to try this I’d strongly suggest getting someone who reloads cartridges (and does it well) to load you up a bunch with the fastest burning and smallest grained powder available to try and be as consistent as possible, but even at that depending on the grain, how caked it is, etc. I’d be surprised if you get reliable results from run to run. -e.p. (rocket guy and reloader)

  6. e.p. says:

    Cartridges vary, if you really wanted to try this I’d strongly suggest getting someone who reloads cartridges (and does it well) to load you up a bunch with the fastest burning and finest grain powder available (fine grain and fast burning to reduce transients, and hopefully hit a consistent pressure), but even at that depending on the grain, how caked it is, etc. I’d be surprised if you get reliable results from run to run. -e.p. (rocket guy and reloader)

  7. johnhare johnhare says:

    I know a few reloaders and will use your suggestion if I can get anything like useful information from the cheap .22s. I really don’t expect high quality data in either case. Just enough information for as little green as possible to prove or disprove the concepts.

    On paper, a compensating nozzle could boost GLOW by 5% or more and payload by slightly more than that for similar vehicle costs. That is a 1,000 pound payload increasing to 1,050+ pounds, not 100,000 pound vehicle increasing payload by 5,000 pounds. The 100,000 pound vehicle could take off at 105,000 pounds with slightly higher Isp through most of the flight.

  8. John B says:

    as usual, sounds like an interesting concept. A few questions:

    Why is a transient pressure pulse required for testing your compensating nozzle concept?

    What is the pressure ratio (Pc/Pamb) range that you think you’ll want to cover?

    If you want to discuss this off-line (and or need an NDA), you know how to reach me.

    Happy New Year!

  9. john hare says:

    John B,
    The transient pulse is only because I don’t have access to an engine at all, not that it is required or even as good. I did a few nozzles with aircraft style yarn tufting and an air compressor to 125 psi that make me believe I have something. I need some kind of hot flow with more pressure to convince someone with an engine to try the concept.

    I would like to see effects from 200 Pc/Pamb down to as low as possible. At 30 km altitude, 200 ratio is a 2 bar engine which would require 200 bar pressure at sea level to simulate with reasonable accuracy.

  10. DanielC says:

    I continue to be intrigued by this idea (and would love to correspond/brainstorm if you’re interested John).

    The extreme difference in scale between the test article and the full-size engine, and the very short pressure pulse may very well render the whole test meaningless. Then again, if it is observed to show promise at this micro-scale it might be convincing enough to try it at a larger scale.

    A .22 blank is quite a small gas generator, and won’t necessarily be very repeatable from shot to shot. And the amount of mass it has is *so* tiny, the duration of the flow at the sort of mass flux rates you need is going to be really really short. I have a suggestion for a larger version of the same idea that might still be practical cheap and quick (it would require that you conduct your testing at a rifle range, or somewhere it is safe and practical to fire a rifle):
    – use the gas tap port of a semiautomatic rifle. Straightforward high pressure plumbing job; no permanent mods to the rifle would be necessary.
    – this will generate a pressure pulse starting at 10,000-15,000psi.
    – gas tap ports are typically on the order of one millimetre in diameter; you could probably make a test nozzle with a throat as large as half a millimetre and sill be assured of choked flow. To me this is “scary-small-scale”, dunno if it scares you as much; will you be able to make a nozzle of that scale to suitable size and finish?
    – the pressure pulse will last about a quarter millisecond
    – by the time the bullet passes the gas tap port, the great majority of the powder will be burnt; you will have much less solids than you might have with using a blank as your gas generator
    – your gas will be coming from a “reservoir” of relatively large volume compared to your nozzle (say 1.5X to 2.0X the cartridge’s internal volume).

    Your entire flow event is going to be over in about half a millisecond, maybe a full millisecond. I don’t know what sort of high speed observing equipment you have available – can you capture several frames from a single shot, or would you capture a single frame from each shot, varying the trigger point?

    Detecting a reference time for the flow start is very straightforward and cheap. You can cement a strain gage to the outside of the barrel, and use a signal from that to trigger your timebase when the bullet approaches.

  11. johnhare johnhare says:


    That would certainly be a better option than what I had in mind. This test concept is shelved now though because I have been offered some quality testing help at a price point and terms I can afford. If the nozzle concept checks out valid, I would expect to see it in flight use within a couple of years.

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