Altus ARCAS

Motivation

This airframe started as an on-sale 4.5" HV ARCAS kit with 98mm motor mount from Composite Warehouse.

Design Details

As the prototype ARCAS included a boat tail but the kit did not, I decided to modify the kit. I added a a fabricated-in-place epoxy tailcone about 3 inches long, which shifted the fins aft by that amount. This in effect gave me another 3" of length to play with in the fin can part of the airframe, which meant a CTI Pro98 6xl case would fit very nicely.

I also did a thing I'm growing to like where the "electronics bay" is just a stepped bulkhead with stuff hanging behind it, which also serves as the forward end of motor retention using all-thread into the forward closure and a lifting eye ahead of the electronics. This allows for hard attachment of the forward airframe tube in flight, with dual deploy from a single airframe break at the back of the nose cone

The plan was to complete the project in time to fly at Airfest 2022 when my son would be in attendance. A CTI N3301 White motor which was in stock at Chris' Rocket Supplies was purchased for the flight, and a high altitude flight request was filed with and approved by the Kloudbusters. The motor was delivered to us on the flight line during setup on Thursday evening in the Rocket Pasture (to avoid hazmat shipping charges).

Sadly, a glue-bonding disaster occurred that night in our hotel room, as the Gorilla Glue recommended for use in glue-bonding the grains in the liner "kicked off" before we could get the last grain in the liner, leaving us with an utterly un-flyable mess!

To avoid just tossing the motor, my big plan then became to use my huge 1930's era metal lathe to cut the liner off the grains, cleaning them up to be glued in to a replacement liner. Thanks to help from Chris, we managed to procure an exact replacement liner assembly from CTI.

I hoped to get the motor rebuilt in time to fly on the Argonia Cup weekend in 2023, and the Kloudbusters re-approved my high altitude flight request. Unfortunately, my father passed away in March before the motor work could be done, throwing that plan out the window.

Fortunately, I was able to make time to extract the grains from the original liner over the summer of 2023. The only problem is that at one point, the cutting tool on the lathe got loose in the holder and jambed into the end of one grain, forcing me to trim that grain to get a non-damaged end. I lost about 303 grams of propellant in the processing, leaving 98.1% of the original propellant mass. With a third high-altitude flight approval in hand from the Kloudbusters, my business partner Keith and I managed to get the grains bonded in to the replacement liner before leaving for Airfest 2023, and the rest of the motor assembly went smoothly.

The recovery system plan was a 2' Giant Leap TAC drogue to be released at apogee by blowing the nose with 2 2-56 nylon shear pins off using 2.5g primary and 3.0g backup 4F BP charges in CPVC charge cannons, then releasing the main in a deployment bag witn a Tender Descender slightly modified to fit two of the Chinese e-matches I prefer so both flight computers on board could have a chance of getting the laundry out.

The OpenRocket design file is ARCAS_ala_Bdale.ork, and that design file plus all content on this page are released under the Creative Commons Attribution-ShareAlike 4.0 International license.

Electronics

One each TeleMega v3.0 (primary) and TeleMetrum v2.0 (backup).

Build

Photos

Result and Lessons Learned

The airframe was 49 lb 13 oz on the rail, and flew on Saturday of Airfest weekend, reaching Mach 2.2+ on the way to 36555 feet above ground! This was a new personal best for Bdale.

Recovery went well, except that the main parachute was clearly under-sized. The ground hit appears to have been at about 47.5 ft/sec. One fin popped loose as a result of the hard hit on landing, and later inspection revealed that the fin can airframe tube "banana'd"! There is a clearly pronounced bend in the tube starting a bit aft of the front of the motor case. Since the motor mount is fairly short and it's a 98mm case inside a 4.5" inside diameter tube, it sure looks like the tube really was bent on landing. It could not have bent during ascent, and the hard hit plus long moment arm plus very high temperatures on the day and from Mach 2+ flight must have contributed to soften the tube enough to let it bend!?! None of us have ever seen anything quite like it!

I'm still satisfied in considering the first flight of the airframe a success since my mission objectives of a new personal best altitude and proper operation of the recovery deployment were achieved. However, the fact that I clearly will need to rebuild/replace the fin can before I can fly again (on a larger main parachute next time!) is annoying.