Redneck Rocket Test

I finally did a kludgy  test Saturday with one of my engine concepts. Several years ago I posted some ideas here about pump impellers that were also regenerative cooled turbines running a full flow staged combustion cycle. I kinda sorta did that. I used a fire extinguisher bottle combustion chamber with nitrous from a 15 pound bottle from the local speed shop. I ran a gas oil mix (demolition saw mix) through the impeller/turbine direct into the precombustion chamber so that the burn would all take place before spinning the turbine/impeller. What would be the main chamber in a normal rocket was just large enough to neck down from the two inch  diameter turbine discharge to the half inch throat. L* about four as best I can figure. The nozzle was a two/notch/twenty cone as I couldn’t get a bell to shape right on the lathe.

Test set up would be funny to those of you doing real testing. A four foot long plank hinged on one end and hanging from a hundred pound fish scale on the other with the engine up side down in the middle. Any thrust would be double the fish scale reading. The fire extinguisher preburner hung below the board with the nozzle above and facing up. The nitrous fed in through the bottom of the preburner that would be the top of a functioning rocket in proper orientation. The oil mix in the gas was to lubricate the turbine/impeller bearing. The gas because I couldn’t get diesel to ignite the weekend before. Ignition was an Estes C motor. The only cooling was the film from the fuel on the preburner walls. The chamber above the throat and the nozzle was fiberglass over regular steel for ablative cooling.

The nitrous read 825 psi on the gauge that came  from a mechanic friend of mine. In a perfect world I could get about 750 psi in the preburner and 500 or so in the regular chamber at the start before pressure dropped except that I didn’t have gauges in either place. The gas mix was at 10 psi to start with compressed air in that tank. This was all in a pit dug with the excavator out in the woods. The property owner thinks it’s all a big joke, probably right.

Henry turned the gas mix petcock with a long stick through the dirt bank just after I hit the button for the Estes rocket and the switch for the nitrous valve. There was a loud pop and a kind of weird roar and whistle that must have been the turbine spinning up. My guess is about a second before a steady roar too loud for the earplugs at fifteen feet and at least six mach diamonds going about fifteen feet straight up for a few seconds before the burn just stopped. There was still a scream that must have been the turbine driven by the nitrous that was still flowing before I realized I had the nitrous switch. I don’t think there was any combustion instability at all.

James read 70 pounds on the fish scale during the early burn which should mean 140 pounds of thrust. If that is right, then we had something like 600 psi at the throat after driving the turbine. The whole contraption weighs just under five pounds so the T/W would be just over twenty eight which would beat the V2 engine if this were flight ready. The pump must have produced at least as much pressure as the nitrous with a weight of just three ounces for the rotating parts and four more for the housing. The nozzle seemed to keep the flow attached to the far wall for the whole burn.

The fiberglass didn’t protect the steel in the chamber and nozzle with a piece of the steel looking like a cutting torch had hit it for a second or so. The turbine/impeller steel had minor blacking on it that rubbed off on the finger. The extinguisher preburner looks strange like it got hot near the nitrous port but not in the rest of it. The burn apparently stopped when the pressure in the gas tank got too low to keep the pump primed, we are just lucky that nitrous didn’t get into the gas tank. Note to self, check valve might appease Murphy as next time we might not be so lucky. The pump was the risky part and it was the only thing that worked exactly like it was supposed to.

None of us know how to rig data collection to a test rig. We also didn’t set up a camera as we didn’t really expect it to work as it did, so I don’t have any pictures or hard data to post. We are sure enough of the results that we are ready now to pay somebody to do that for us.

The turbopump thingy can easily incorporate a nitrous pump, so it seems possible that we can have a full flow pumped system by this summer. We need to build a regenerative lower section as we seem to be running too much heat flux there. The upper (lower in this test) preburner needs a much smaller volume so that there are no hot oxidizer sections without the fuel film cooling like in the extinguisher bottle.

We should be able to get the weight down under two pounds with material selection more for performance than cheap or free off our shop floor. That would give us a T/W of 70 pumping both propellants. The pump seems like it should be able to get at least double the pressure that we got on this test, which would give us a T/W of 140 in a pumped engine in a perfect world. Fingers are crossed. The whole layout would be much easier if we could afford to build it bigger.

Yesterday I spent Easter with the grand kids. I tried bragging about the rocket but they wanted to see it fly and couldn’t get interested in something that just sat there and I wouldn’t have let them get close to the test anyway. My son had what I would call tolerant amusement at my “hobby”. I’ll bring pictures of the rig and parts to Space Access as this time I have something to show.

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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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8 Responses to Redneck Rocket Test

  1. Andrew Swallow says:

    I suspect that Robert Goddard could swap testing stories with you.

  2. john hare says:

    Did he do April Fools testing also? I thought I would get at least one or two people pointing out the stupidity of a test like I described from a safety POV.

  3. Andrew S. Mooney says:

    Me, Little Enis and my brother Scooter are going to be grindin’ and mixin’ some zinc and sulfur in a coffee can tonight. Screw Estes. They’re overpriced.

    The Bobbsey twins are coming over later, having stopped by Willy”s Rib Pit and gotten some takeaway. The grease is good for eatin, but it cooks off nicely with Peroxide so we’re strainin the bits out and saving it up.

    Does Henry have a safety helmet? Mine is a Stahlhelm that my granpappy stole off a dead guy in the Alsace on the last day of hostilities. Big Enis, my uncle, he always wanted a Luger, but this was what he we got and it is now a life saving heirloom. I’ve always wondered about what granpappy was doing there, ever since the Discovery Channel said that Easy Company and the rest of the cast of Band Of Brothers were in the Eagle’s Nest at that point, but I digress.

    I read about that Wern, Whner, that Von Braun guy, and he used stones soaked in manganese as his catalytic bed. That is my kinda thinking. You shouldn’t be so quick to condemn these guys for only having the technology that they did. Needs must as the devil drives. Anyways. I’ve got to find some Jews to solder up a guidance system. I’ve got an abandoned railway tunnel for them to work in and everything. Anyone got any alternatives? They don’t work for food anymore.

  4. George Turner says:

    I’ve been doodling on a LH2/LOX powered pump where a massive amount of cryogenic propellant is dumped radially into a pipe mounted at the end of a fully expanded thruster nozzle. As an example calculation, conservation of momentum says 1 kg/sec of 4,000 m/sec exhaust blasting 40 kg/sec of 0 m/sec fuel eventually results in 41 kg/sec of propellant & ice traveling at 196 m/sec, which can be converted back into pressure in several simple ways.

    Something tells me a method that simple and efficient must not work or we’d already be doing it, and as I can’t figure out where the screw-up is I might be compelled to try some simple water/compressed air experiments with PVC pipe.

  5. john hare says:

    I did a tribrid post on August 12 2009 that might have some bearing on your idea.

    Yew jest ketch them little monkeys sneekin throo the RIa Grund down sooth an feeed im beens.

  6. Andrew S. Mooney says:

    Tried it. They can’t soldier. That Rutan. He must be using Laotians.

  7. George Turner says:

    The tribrid post was very interesting, and definitely related yet somewhat different from what I’ve been thinking of.

    What I’m doodling doesn’t use venturis. Instead it uses a fully expanded, low pressure, high velocity supersonic exhaust stream traveling down a tube, into which you inject relatively low pressure LOX or fuel from the outer wall just as you would for a scramjet.

    A key difference from your tribrid is that the fuel and oxidizer aren’t mixed in the pipe. Each is injected into a seperate tube for downstream pressure recovery, and the mixture ratio is heavily shifted toward the liquid propellants (40:1 or more, depending on the desired final propellant velocity, and thus pressure). At injection, assuming pretty ordinary exhaust conditions for a LOX/LH2 thruster (acting as the preburner), the 40:1 fuel/exhaust ratio by weight would something like a 1:27 ratio by volume, so the propellant would be like a very heavy rain in a hurricane.

    Conservation of momentum will eventually get the propellant up to speed (doing the job of the impeller of a centrifugal pump), and the thermodynamics should see a moderate rise in fuel temperature (perhaps 30K for LOX) while the exhaust stream hopefully freezes to microscopic ice crystals so it can be injected into the main combustion chamber along with the propellant.

    Seperation of liquid and residual gas could be done with a bend in the pipe (perhaps with slots and blades on the outside curve for pressure recovery), or having the stream directed toward the inside lip of a spinning cylinder whose bottom exit is a conventional turbopump volute, like spraying a garden hose into spinning gallon paint can stuck on a nail.

    The supersonic injection doesn’t seem to be an issue, which is one thing that worried me. SS injection papers

    I went into a bit more detail on it in a thread at TTM last week, hoping somebody can tell me why it won’t work so I can quit doodling on it. ^_^

    Perhaps a garden hose, an Estes rocket engine, and some PVC would be a good experiment.

  8. john hare says:

    Just for the record, I think the relative difficulty of pumps is exaggerated to a considerable extent for large systems. I also think that most current design approaches are unimaginative.

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