™
Go UP young man!
A few weekends ago we went out and did another tethered test. This was the first tethered test with the legs attached and no bungee cords helping. We got a positive result that we definitely have enough thrust to lift the vehicle. However, we also had a little unwanted excitement along the way.
I thought we would probably need all the thrust we could get to lift off so at the start I slammed the throttle fully open. The vehicle lifted off. From the telemetry we later calculated a thrust to weight of 1.15, which should be sufficient for a clean ground liftoff when we get that far. This was with enough peroxide to fly for ten seconds. But we don't have a lot of vertical clearance inside the silo so I wasn't able to find the hovering throttle point in time. I wound up going too low on the throttle and the rocket dropped and bounced on the tethers. After that the rocket went out of control and I shut it down.
It doesn't look too bad in the video, but it was a LOT scarier in person.
After I got home I started digging through the telemetry. The bounce was obvious in the IMU data. Interestingly, it was only 4.5 G's. We are using car towing straps as tethers and they are designed to damp out those kind of shocks. But right after the bounce we lost communication with the IMU. After that there was no hope of flying under control. 4.5G's shouldn't be enough to damage the IMU. I wasn't so sure about the consumer grade electronics elsewhere in the rocket. But I was hoping it was something simple like a loose cable.
This week I got a chance to get back out to the silo and check things out. Sure enough, it was a loose cable connection and everything is working fine. The IMU has a DB9 serial connector, but the small form factor PC I'm using as a flight computer doesn't have any real serial ports, so I'm using a USB to serial dongle. USB connectors don't have any screws or hold down features like serial connectors, and the plug came out. Obviously, USB is not the ideal (or even a suitable) connector for this application, but I'm kind of working with what I've got so I'm going to tape it down and continue testing.
This test was progress, and nothing was broken. The next test will be basically the same except for ramping up the throttle slowly to find the 1.0 thrust to weight point to hover.
I'll just mention in passing that for the last two years SpeedUp has exceeded the web update rate of Armadillo Aerospace. (2010: Armadillo 3 updates, SpeedUp 4 updates, 2011: Armadillo 5 updates, SpeedUp 7 updates) I'm starting to feel rather heady and am considering challenging Mr. Brockert to a blog-off. I expect I will be flying my class 3 amateur rocket to 100,000 feet in only a hundred or so more updates.