Thursday, October 8, 2009

38mm madness!

There are two crazy 38mm projects being discussed right now in the Rocketry Planet Forums, both of them expected to be (to have been) flown at Balls 18:

One is a 38mm rocket built by "Binder Design" (user name, also a well known rocketry company) to fly on an L5000. Yes thats right... an L. A 68 inch long custom L motor with a full diameter smoke plug at the end. Simmed to about 30,000 feet and mach 3, perhaps a bit more. Remember that, generally speaking, an L3000 in a 54mm rocket is pretty extreme. It still is really, but this project is just seemingly impossible. Indeed most of the thread is a debate back and forth about if this motor really is possible, and if it is really a full and efficient L motor to begin with. The Length to Diameter ratio in a 38mm x 68 inch motor is almost unheard of. Also, each APCP grain is 10 inches long!

The rocket has an aluminum fin can that was constructed using Durafix. The airframe is basically the motor case (which is the hallmark of professional rockets.) It doesn't look like this project will fly this year, but hopefully some day soon.
38mm L5000

The other project is by "Adrian A" who owns Featherweight Altimeters. These are a set of very small, and very capable altimeters. They are perfect for small rockets, high altitude flights, and anyone who wants the many smart features including high G (250) reporting in some models. Despite his modesty, I think Adrian does some of the best small carbon fiber rockets in the hobby. He makes tiny, short, ultra-light kits that are all asking to break altitude records. Here, he plans for a 3 stage 38mm rocket. Now I find this so exciting because 3 stage high power rockets are so rare in the hobby, and they seldom have successful launches when they are put forward. The only way to change this record is to build and fly more 3 stage rockets. And an electronics expert is the right person for the job, as you can see in the thread. Planning out the staging and recovery sequence is a very complex task for such a project.

The estimated altitude in a J to a J to a J configuration is a bit over 50,000 feet (to as high as 60,000 feet) and a bit over mach 3! That is INSANE performance for a 38mm hobby rocket, and also insane performance for K impulse. It would be a no-limits K record (possibly even doubling the previous record).

As of a few days ago, here is the last update:

"Yep, a launch report is in work. Stay tuned..."

3 stage 38mm

Click this simulation to view the expected performance of this three stage flight. Note the top speed just hits above mach 3. You can also clearly see the drag, accelerations, and altitudes. A big part of the discussion in the forum is timing, electronics, recovery, ignition of stages... and also interestingly the temperature concerns. What happens in the upper stage around 50,000 feet when it comes time to recover? In larger rockets, one can be confident that things wont cool down that quickly. But how about in a 38mm rocket with a thin airframe? One great idea, mentioned only quickly in passing, is to thermally link the motor to the payload bay. I instantly got an image of heatpipes coming from the motor to the payload bay to keep things warm. But probably a few rods of copper would be good enough to serve this purpose. And also there was the mention of the surgical tubing for the ejection charge... it looks like this will be more and more common as hobby rocketry gets higher.


Anonymous said...

I was wondering if heating of the airframe due to atmospheric drag might be a concern at mach 2-3.5 ? Additionally couldn't this heat (if substantial) be transferred by design etc to where its needed? Greg

R2K said...

(Months later, I reply...)

This amount of heating can pose a problem, usually in rockets of this size it results in burning paint and other drag causing changes. The smaller rockets spend very little time above mach 2, let alone mach 3 (just a few seconds) so much heat damage winds up being superficial. There is a small risk that critical epoxy structures would burn (or char, evolve gas, and sublimate) will fail but that isn't too likely because this guy builds rockets really well! When possible, the metal structures are obviously superior for all-out performance.

There has always been talk of using copper or other metal structures to move heat around. In flights above 50,000 feet, it would be nice to heat up electronics this way.

High Power Rocketry said...

Most small composite rockets are fine at mach 2. Above mach 3 is a real challenge for any composites, because of delamination and damage to normal epoxy. For mach 4,5,6 you really should use all metal instead. Even aluminum will be at risk above mach 5 in the troposphere.

For very high performance rockets, space shots etc, you are much better off going with something like the first rocket above - motor case as airframe, and metal fins and nosecone.

I am currently working on the design for a ca 4 inch rocket that is basically a motor, 12 inches of payload, a nosecone, and fins that weld (or high-temp epoxy) to that motor case. The idea is to add no more than a few kg to a motor case, so that you can get the maximum performance from it. Will post if it ever gets beyond the design stage.