Wednesday, February 27, 2008

Smash Lab - Runaway Trailer - SPOILER

Retro rockets to stop a truck and trailer? Toasty!

Egads, their rocket expert is the infamous Jerry Irvine of US Rockets! When grabbing the link, I noticed he acknowledges his role and asks for feedback. He starts by demoing what looked like a Polecat Thumper. I didn't check if USR has a similarly named rocket of similar proportions. I also missed what motor was used. Maybe I should have taped the show? The flight was nice and the Smash Labs folks were happy.

Next came a test to determine how much force would be necessary to stop a truck and trailer. This ended with a spectacular pile of crushed and twisted metal. They see that the trailer fishtailing is going to be a problem, leading to a decision to place the retro rockets on the trailer. The computed they'd need 6000 lb over 5 seconds to stop a truck and trailer.

They then tried installing rockets on a 3-wheel cart. They thought of several alternatives: producing a down-force on a friction pads, rockets on the wheels to slow them down, and simple retro rockets. Simple is good. They built a test for all three scenarios. The motors look like CTI Pro-38 motors but might be USR considering Jerry's involvement. The break pad idea failed with only 3 of 4 motors igniting. The rockets-on-wheels actually ignited (I'd like to see how they rigged them) and the trike remained on a straight trajectory despite there being rockets on only one wheel. The trike stopped in 180 ft, half the distance of the friction pad. However, they wisely decided this was a dangerous, stupid idea. The retro rocket test worked but took 210 ft to stop , more than the rockets-on-wheels method. It was still chosen.

They stepped the testing up by putting rockets an a small RC controlled cart and trailer. This was basically a disaster. Ignition problems, but one rocket 'spontaneously' fires. "Rockets are not reliable," says one Smash labber, "We have to solve the detonation problems." ACK! Another 'rocket expert' provides a redundant igniter set-up.

They returned to Irvine's Rocket Ranch to get big motors - 3 K-lb of force and $21,000 each. With a 5 sec. burn thats about 15 K-lb total impulse - a 'P' motor. These looked custom to me, not surprising at that impulse range. They mount them at a 15-degree angle to minimize the chances of frying the test bed or adjacent motorists. I wouldn't trust that either would be safe. Off to Mojave.

Wow, the motors go off before the test starts and people are running for the hills. The burn lasted for 20 sec. Hardly what I assumed above, so I'm not sure what size motors they used. Luckily, nobody was damaged. Some scary footage there.

Post-mortem: the remote RC controller fired the igniters when is was switched-on. They decided rockets are too unsafe and unpredictable for this application. The unpredictable nature was largely operator error and poor design IMO. But we have our mis-fires too. Too unsafe for the application? I could have told them that before they started!

BTW, stunts like this are NOT sport rocketry!!! I wish these folks would have done more homework. The idea was bad to start with but the implementation was terrible.

I can't wait to see what's said on this Rocketry Planet thread! Jerry is contributing so said thread, so this should be interesting!

Updates:
  1. The motors were in fact 9" 'Q's.
  2. I agree with others that this show is a Mythbusters wannabe. I doubt if I'll watch another episode. Despite its shortcomings, this episode was worth a look (did I ever say I love rockets?). I may wish it didn't play out like it did, but it is what it is.
  3. Elsewhere, it's mentioned that these were all are USR motors. Jerry implies the same.
  4. Other than Jerry, the 'experts' were FX pyro types, not rocketeers.