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Time Until El Mirage

Tom


Bill Hoddinott: Tom, it was in 2000 that you were timed at 450 mph at the end of the Long Course and experienced COMPLETE FAILURE of your parachute system! So you had a wild ride out into the mud far off the course. What happened?

Tom Burkland: Bill, the first thing to tell you is that parachute science is firm and well known UP to 300 mph at Bonneville. ABOVE that you're on your own. Nobody alive can just tell you what to put on your car. There is an awful lot of what works in these high speed parachute systems that is pure trial and error. The problem is that the errors can be rather dangerous if you do not control the risks through operational decisions and appropriate developmental testing.

We first ran our streamliner in 1996 using two of the proven high speed chutes from the Datsun. To this we added a larger low speed chute that had been on the Studebaker thinking we had a nice redundant system. Turned out it wasn't.

Up to 2000 we had made six runs in the car up to 390 mph and the chutes had worked okay, although the shock on the driver and car was pretty tough at 7.3 -7.5 Gs on initial deployment. We were deploying #1 and 2 with conventional spring pilot chutes
which started the canopy inflation as soon as they came out of the car. The riser line then had to catch the inflated canopy with tremendous shock loading as the car drove away from it.

We had a 2" wide military webbing riser line on the chutes which was rated at 25,000 lbs tensile strength.

Okay, on this run, we went out the Back Door - end of the Long Course - at 450 because we had a pretty good course at the time and could put some power to the ground without too much wheelspin. The run was intended to give us a good mid-throttle fuel system check to confirm the port nozzle distribution and bypass settings. The throttle position at the timer exit was 64% and all exhaust temperatures and engine parameters were looking good. Our data logger showed that at 415 mph #1 chute deployed and instantly snapped its riser line in TWO places - we spent a lot of time figuring out how THAT was even possible! But you understand that the load on the chute equipment increases as the SQUARE of the speed so what seemed like a fairly small 25 mph step in speed was really a 20% increase in applied loading. The backup #2 chute was deployed very quickly after I realized that #1 had either not deployed or failed. There was not even a blip on the acceleratometer when the first chute failed and I had no feel of any tug. The #2 riser line also failed about 3/4 mile farther down the track after giving a perceptible pull on the car.

I felt the deceleration drop off and instantly knew the #2 chute WAS GONE!

I'm slowing from the drag of the air brake doors only now, and at 360 mph saw the pressure ridges at the end of the prepared track surface coming from about half a mile away. Knowing I would need the directional stability of a chute I pulled the #3 low speed chute. This low speed chute was really intended for deployment below 250 mph but there was no time or distance to wait for that limit to happen. But the worst thing that could happen would be a canopy failure of this cross-form chute, and even then it would stream behind the car and help keep it pointed nose-forward in the rough overrun area. The low speed canopy was torn open across the center and streamed out behind the car.

NOW I'M OUT OF OPTIONS. So at 320 mph off the prepared course I go and onto the six-inch-high salt pressure ridges which pound the car HARD! But the situation is not TOO bad because heading north there's nothing solid to hit and if I can just hang on long enough, I can stop the car.

Off the salt, and into the eight-inch bumps on the mud flat area we go. Finally the car slows to 150 mph and I can use the brakes to thankfully bring it to a stop. Can't use the brakes at higher speeds because they would either burn up from the heat or stop the tires and blow them out - that's why we use parachutes. IT TOOK FOUR MILES PAST THE END OF THE PREPARED RACE COURSE TO FINALLY GET THE CAR STOPPED... AND THIS ALL HAPPENED IN LESS TIME THAN IT TAKES TO READ THIS SENTENCE!

Fortunately, it was not damaged too severely despite all the pounding it took. The body around the lower wheel wells took most of the damage from the soft mud being pushed up inside. There were some very long stretches of the overrun that did not have tire tracks at all from the launch ramp effect of the pressure ridges.

The final stopping point was only about 1/4 mile from Floating Mountain, so we can honestly tell you that it really does touch the ground at its base.

Bill: Wow, Tom, that must have been a moment of terror! Good thing it didn't happen on an FIA record attempt, heading south toward the Interstate!

Tom: Right, that scenario is what cost my friend Nolan White his life a couple years later.

Anyway, when we did the forensics we realized the big solid canopies of the Datsun chutes were too much for the riser lines. We did some testing later of the line and discovered that despite its rating, its true tensile strength was only 15,200 pounds. But even this figure shows you the enormous forces set up by 400 mph speeds!

The dual failure of the #1 riser line was determined to be a prolapsed-type loop that had tangled during deployment and strangled the webbing. Both of these problems would be addressed through an all-new deployment system using deployment bags to play out the required long riser lines one loop at a time, stretch the lines using aerodynamic forces, and then release the canopy from the bag to inflate relatively softly on the outstretched riser lines..

During this development Bob Stroud helped us analyze the failures and supplied suitable replacement parachutes. Bob's extensive military and high performance safety equipment background makes him the only real choice for streamliner parachutes. He is willing to work with each specific vehicle feature and supply the custom built solutions necessary to safely stop one of these high speed machines. There are plenty of 300 mph drag-racing chutes out there, but as you have seen, these are not good enough on high speed streamliners.

Here's what we ended up with, and it has been 100% reliable:

Bob makes our riser lines of eight pieces of 1" nylon webbing, rated at about 4800 pounds per strand, to provide over 38,000 pounds of tension load capacity.

He makes spherical ribbon canopies of Kevlar, which is four or five times more abrasion resistant as well as significantly stronger in tension to handle the high applied air pressures. We have eleven runs on them now, and they would be good for 30+ runs before wear would become a problem. The canopy diameters are sized to the vehicle weight and expected deployment speeds. Sequential deployment of progressively larger canopies as the vehicle speed decreases shortens the stopping distances. Riser line lengths are selected to position these canopies far enough aft and apart from each other to keep them all inflated and pulling on the vehicle for deceleration.

We have two complete sets of chute equipment to swap them during a FIA one-hour turnaround situation without having to repack any of them.

The drill for deployment now is: open the tail doors which act as air brakes and give a 3 G braking force. And if I had it to do again, I would make these doors even bigger to provide more of the braking forces. Around 350 mph #1 chute is automatically ejected in its special deployment bag. The bag acts as a pilot chute and pulls the 110' riser line out fully before the bag comes off and the ribbon canopy opens. At 325 mph the driver releases #2 chute manually with the steering wheel mounted switch, and it has a shorter riser line, with a slightly larger diameter ribbon canopy. These haul the car down hard. #3 chute is used as a redundant backup in case of the failure of 1 or 2, but thankfully has never been deployed. Finally, when the speed drops to 250 mph, #4 chute, the big one, is deployed manually; and at 150 the wheel brakes can come into play.

The car can be stopped in two miles from 450 mph, but usually I take it easier and use three miles. Accurate cockpit speed readings are important to optimum deployment of these parachutes and we use an air speed indicator from an F-106 aircraft recalibrated to display miles per hour on the dial. Deployment controls are all located directly on the steering wheel for hands-on operation.. At these speeds even the short reach across the small cockpit can cost you a half mile of stopping distance that may be critical to safely stopping the car.

Bill: Do the chutes jerk the car around when deployed?

Tom: #1 makes the nose of the car swing through five or six oscillations about four feet wide at a rate of two cycles/second. Adding the #2 chute results in wider oscillation at lower frequency. The applied parachute aerodynamic loads are so large during this portion of the operation that steering inputs only result in scrubbing the tire tread surfaces and further oscillation. All this adds another dimension to the vibrations and stresses applied to the driver between hard tires and G forces. So there's a lot going on and it's quite a ride!

One of the photos shows the #1 chute rising up into the air as the car is slowing way down. It doesn't do that at high speed... There's no issue about jerking the back of the car up or anything like that.
And evidently the turbulence and upward air flow from the tail flap push the low speed canopy on its short riser line up as the tension load decreases at lower speeds.

Bill: Okay, for the next part let's explore that crash you had in 2001 and its aftermath. There was a question at the time if your team should repair the car and continue with your quest for the world record, or not...

Copyright 2009 Bill Hoddinott


Back to Part 9 ____________________ On to Part 11


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Introduction

Episode 1) Overview Video

Episode 2) The Transmission

Episode 3) The Engine

Episode 4) The Drive Train

Episode 5) Body and Paint

Episode 6) Dyno Run