[ERPS] tube liftoff

[John Carmack]

It is so nice when things make analytical sense...

We did tube hover tests today. It was cold, but no longer humid, so once the engines warmed up they were clear, which was a relief. What was frustrating was that we did the first test with 250 psi tank pressure, and even though I wound up throttling all the way up, all the tube did was scoot sideways a little bit. This did not make sense, because the test stand results indicated it should have over 500 pounds of lift with a straight shot to the engine. We had modified our procedure to do the engine warmup pulses while the pressurization hose was still regulating the vehicle tank, so there was no drop in pressure, and it had plenty of time to get the initial pressurization cool-down pressure drop compensated for.

We loaded up again, and went to 300 psi, the highest I want to take this tank. It did the same little scooting thing. We were fairly mystified, so we pulled it back inside and very carefully balanced out two scales to weigh the vehicle (280 pounds without the parachute and draw tower), then used one of those scales with a hydraulic jack to calibrate the test stand load cell. Everything checked out. At 300 psi, it should have been pulling almost a full G of acceleration.

I checked the motor valve calibration to see if it might have only been opening halfway, but that was right on.

It finally hit me:

Krushnik Effect!

The base flare is only held about three inches off the ground by the wire rope isolator feet, so the rush of gas from the main engine has to accelerate a fair amount to escape radially. The 4' diameter flare base has 1800 square inches of area, so even a small pressure drop would overcome the 250 or so pounds of net engine thrust.

We cut some more polyethylene blocks to stand the entire thing another 3.5" off the ground, and tried again. It was a good thing I was ready for it, because during throttle up, the vehicle basically "popped" off the ground at a pretty good clip. A caught it just before it came up on the tether, and got it back down. When it hit the ground it knocked off most of our foot extensions, so it was sitting at a crooked angle, but everything came through nicely. I just blew all the peroxide out the attitude engines, rather than try and lift off from a 20 degree tilt, but the test was a great success. It flies nice and straight while accelerating under guidance. It should be a pretty flight when we go to altitude.

If the weather cooperates, we will do parachute tests on Saturday, then we will be ready to fly (probably in Oklahoma, it looks like).

John Carmack

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