I finally did a kludgyÂ 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 inchÂ 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 cameÂ 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.