While developing Rapier and Jetex propellants, I've been static testing and dissecting vintage Rapiers from Dr. Zigmund, taking measurements and notes as would an eager reverse-engineer at Area 51. Case burn-thrus! I've noted burn-thrus with some vintage L-3 motors of unknown date and lot number. Anyone, everyone, please advise me about what model Rapier motors have experienced case burn-thrus, so that I can better compare notes and advance this research.
By the way, there's a *lot* of "secret sauce" in Dr. Zigmund's invention. The motor cases themselves are ingeniously designed, and from what I'm seeing, there's been a long evolution of improvements in the cases alone, not to mention nozzles, end caps, adhesives. Have I said that Jan's Rapier fuse sticks are amazing, as well?
(This week I'm continuing static tests of both Rapier and Jetex propellants, with excellent progress [and only one test stand explosion that sent all parts back to the workshop for redesign]. Today I've started working with carbon-fiber composite thin-wall tubing that's ideal [though costly, though remains reloadable after use] for Rapier-type engines, intending to come up with a Jetex pellet series that's compressed in a very thin carbon-fiber shell, and doesn't need wire gauze to work. Recent test notes are posted here:
I'm an enthusiastic fan of Rapier motors, and while designing a "knock-off" for consumption beyond Europe, I've a lot to learn from seasoned Rapier fliers. As I develop a new series of Rapier-type motors for free-flight, help me better understand details from the flying fields that will make a better product. Jetexeers, what's bad about the Rapier? Rapierists, what's so good about them? Reaction-motor aeromodelers, let a reverse engineer hear Ugly Stories, too.
1. Not sold in USA.
2. "Model Jet Propulsion Units," *not* rocket motors.
I'm answering here Edward's questions about 'desirable features' of any new motor with a bit of history,
When Rapiers were first available, in L-1, L-2, L-4 sizes, they came in neat green boxes with the thrust and burn time specified. The L-2 of that period, 2000 AD was rated at about 100 mN (a third of an ounce), which was less than the Jetex 50 of old. Modellers therefore adapted the old Jetex 50 designs, usually reducing them to 85% or so of the original. They flew very well.
Dr Z then upped his game, producing L-2 motors of 140-150 mN. These we loved. He also produced an 'L-2 HP with a thrust more like a Jetex 50C (250 mN or so). This powered my 'full size' Keil Kraft MiG-15 in a very lively fashion, and ace modellers like Steve Bage and Richard Crossley designed some very sophisticated models, like the MiG 21, Thunderchief and Su7 for these motors. The L-2 LT, with its burn time of up to 30 sec and 85-95 mN was also well-regarded by scale modellers. It was a golden period. .
Problems arose when (we think) Dr Z continued to change the specifications, did away with the boxes, and what you got was not predictable. There were problems of casing supplies, crooked nozzles and swelling on storage with subsequent more than acceptable cases of unpredictable flight patterns (due to overpowering and off-centre thrusts) and, worse, 'burn throughs' and immolation of models.
Essentially, there was, in short, a problem with what could be called 'QA'.
Below an example (2009) of problems with cases, which are barely up to the job they are expected to do:
To cut a long story short, things started to improve in 2010, and the latest motors are very nice (particularly the 'L-2X' motors, which are just perfect for those old Keil Kraft and Skyleada 'Flying Scale' designs) but they are sold loose, and, as we know, cannot be safely posted.
Based on all of the above, any new JPU (Jet propulsion unit) would ideally:
1. Specified as a 'smoke generating unit' and freely distributed and posted.
2. Of fixed and reliable specification - say a thrust of 150 mN for 20-25 sec would be just fine.
3. Adequate manufacture - straight nozzles, less than 2% burn through.
3. Sold in boxes with specification - attractive to the punter!
4. Motors of 65-80 mN and equivalent to the old PAA Loader (say 350-400 mN are also desirable.
There is a range of kits and plans, new and old, out there suitable for these motors.
Above are just a few thoughts from a chap still struggling with a 'flu-like bug and not thinking that clearly.
Dr Ed followed up with some interesting and pertinent questions, viz:
given some undated vintage L-3 motors, of which 2 out of 2 (so far)
have burned through. Any information on L-3s being especially prone to that?
And, at any point in development, did each model--the L-1, L-2, L-2
HP, L-2 LT, L-3, and L-4 motors--each have burn throughs from time to
time, at any point in their history? Or was one motor model *not*
prone to burn throughs?
I get a bit confused about L-3's and L-4's. Our current L-3 motors are very nice - 400 mN and I have not heard of any 'burn thu's'.
However, a few years ago the front plug could come out under pressure, so wise modellers would seal the edges with cyano glue.
Nearly all my experience has been with L-1 and L-2 motors. When discussing burn thru's or, more casing failures due to charing, it is important not to mix up two possible causes.
1. cracking or swelling of cases on storage leading to localised 'hot spots'. These seemed to affect L-2 and L-2 HP motors five years ago.
2. genuine burn thru's from under-specified cases. Dr Z was forced to change the source of his cases and we had many failures and loss of models.
It was at this time started to build profile models.
Fortunately the current red cases are much more robust, and, sticking my neck out think:
1. Burn thru's or near burn thru's are more common with the L-2 LT (low thrust, long burn > 20 sec) motors. Standard L-2's can show some discoloration of the case near the nozzle, and some break when removing them from the model. But so far, I've lost no models.
2. I have not yet had a failure of a red cased L-2 HP. Drakens cavorting about and attacking onlookers due to the unexpectedly high thrust, but no motor burn thru's.
3. The green cased L-1 motors have a low failure rate - I've only had two 'blow outs' in perhaps 100 plus launches.
What to conclude from this I am not sure. I think a heavier case is viable as Rapiers are much lighter than Jetex motors. Also, if you go for a larger diameter motor (but still thinking of 140-150 mN thrust, would a 'shaped propellant' like those on the Space Shuttle be both steadier burning and protect the casing?
Meanwhile, my spies have spotted Dr Ed testing his latest motors in the Mojave Desert:
I hope others share their experiences with Rapiers. Am I wrong about the robustness of L-2 HP's, which can have a very high thrust, but 15 sec burn time? Does a burn time above say 18 sec over stress the red cases?
Jim - any thoughts on the L-3 motors? All the ones I've had worked fine if occasionally lacking grunt.
And - what are the secrets of storing Rapiers. Is silica gel a good idea, or is a bit of moisture good in preventing cracks in the propellant?