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THUNDERBIRD Mk1 SAM FLIGHT
The flights of the RAF Basset prototype were widely popular, but it was always a bit of a disappointment to me that the ignition of the air-started boosters were barely visible from the ground. I was much happier when I flew it without air-starting the boosters. So I decided early that the Thunderbird cluster would ignite all five motors on the pad. This has proven to be both more reliable and more popular.
There is a bit of an art involved in timing the delays so that the boosters would peel off upon firing of their ejection charges so that the sustainer will continue on in flight without them. I spend a lot of time studying burn times and delay times in preparing for the flight of this rocket. One of these days I'll get it right.
The prep time for this rocket is extraordinary. Five motors to load, five chutes to pack, motor retention time five, etc., etc. It isn't all that bad if you choose to load the boosters with single use motors, but reloadable motors require a lot more prep. Look at all the pieces needed that five reloadable motors need for a single flight. It's almost enough to drive a guy back to modrocs.
Flight One
For the maiden flight of the Thunderbird, the sustainer was loaded with an
I161-6 and the boosters each held an E15-5. Tungsten Fire-in-the-Hole igniters were used to ensure the ignition of all
five motors. There was an anxious moment when the four boosters came up to
pressure first and the rocket started slowly up the launch rail - but the
sustainer quickly kicked in and it shot out of the smoke cloud raised by the
four E's. The rocket headed slightly upwind parallel to the flight line,
giving spectators a great view of the boosters individually peeling off the
sustainer and deploying their parachutes. The sustainer parachute deployed
moments later and the overall effect was quite spectacular.
Click Here for video.
Flight Two
For the Thunderbird's second flight, the sustainer was once again loaded with an
I161-6 but the boosters now each held an F30-6. Tungsten Fire-in-the-Hole igniters were also used again to ensure the ignition of all five motors. This time, however, the leads for the four booster motors were much shorter - this is a significant lesson learned.
The I161 ignited immediately this time, and the Thunderbird roared off the pad - but with only two of the four boosters lit. Number three spouted flame just a few feet off the rail. The rocket veered downrange despite fact that the booster nozzles were vectored toward the Center of Gravity. (Without this design feature, I'm confident that the trajectory change would have been much more severe. The breeze was also quite gusty that day, so it is impossible to discern if the wind was also responsible).
Click HERE to view a freeze-frame chronography of the Thunderbird Mk1 leaving the pad.
Although this was not the flight path I had hoped for, it was actually pretty cool to watch - very true-to-life. The Thunderbird traveled almost horizontal to the ground to an altitude of about 800 feet, and was practically horizontal when it reached apogee. Soon it was headed straight down, and I heard cries of "CORE SAMPLE!" coming from the crowd behind me. Fortunately the ejection charges of the four burning motors popped almost simultaneously and the parachutes deployed. The unlit booster separated from the sustainer when the main chute deployed at high speed, but it tumbled harmlessly to the ground with no damage.
Click Here for video.
Flight Three
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The third flight of the Thunderbird Mk1 occurred at the NAR 2002 National Sport Launch hosted by the Superstition Spacmodeling Society. It was a hometown launch, but with national attention, so I really wanted this flight to go well. As it turned out, both Extreme Rocketry and Sport Rocketry magazines had a brief description of the flight -- and I can call it a "flight" only by applying the very loosest definition of the term.
As usual, the RSO built up the launch of the TBird by describing the intricacies of the flight: five motors, five chutes, etc., etc., Heads Up! All eyes on Pad 2!. 3-2-1 -- and only three booster motors ignited. The rocket crept slowly up and off the rail, pitched painfully downrange, and did an awkward bellyflop onto the desert floor. Three ejection charges fired, and when the smoke cleared the Thunderbird lay in pieces about 20 feet from the pad.
Three of the five booster cones were broken - the wood putty castings, while easy to make, were too brittle (as suspected). One booster tube was crushed, and the sustainer tube had a crimp in it.
I saved the fin can assembly of the damaged booster and bought a new 54mm tube to replace the crushed one -- this was very easy to fix. I glued a coupler inside the 4" sustainer tube to strenghten the airframe. All four booster cones were replaced with new hollow castings I made using the latex mold described on the Construction page. This time the casting material was epoxy resin fortified with fiberglass microfibers. It cleaned up to look as good as new!
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Flight Four
Special thanks to Dan Russell for manning the video camera. These pictures, and the sequence photos, were lifted from the video taken by Dan.
Click [HERE] for a frame-by-frame replay of the liftoff of the Thunderbird Mk1 at GHS 2002!
The TBird motor cluster with too-short igniter leads
Lessons Learned
After flight three, I found the spent ignitors next to the pad. All five had burned. I suspect two of the ignitors popped late after it had already been pulled from the booster by one of the other motors as it roared to life. Longer leads would have kept this from happening. For the fourth flight all the leads were about three feet long, and they were carefully kept away from each other when connecting to separate gator clips.
Shortening the delays of the F24's to about 4 seconds was intended to ensure that sustainer would have a reasonably long flight without boosters prior to deployment of the main parachute. I learned that it needs a lot more velocity. I'm toying with the idea of trying a J350 in the sustainer on a future flight. (For a relatively small rocket, this is a VERY expensive model to fly - I can stuff a K motor in a big bird for the price of one Tbird flight!)
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