Monday, March 27, 2017

Recovery: ELS / Parachutes

First the goodies. Check out this video to see the first system-level tests of the HAPP's parachute deployment system. Video analysis shows those chutes firing out at about 120 kph (75 mph). The video is a bit silly - gotta have fun while you work - but read on down below for the serious technical details.

The project is moving right along toward a summer mission flight. Flight controls are done. Structure is fully designed, parts have been machined, and final assembly is underway (details to follow soon). The next major system to validate is what the Apollo Program folks called the Earth Landing System or ELS, a.k.a. the parachutes.

From the rendering below you can get an idea of how this is supposed to work. Protruding from the upper deck of the HAPP are three mortar tubes which contain the parachutes. They're nestled between the vertical strakes or fins. During the mission, the HAPP will blow the chutes at a low altitude after free-falling from the edge of space. Most "space balloonists" use a chute that's always deployed. While that approach is simpler and less risky, those crafts can drift a large distance away from the launch site as they descend slowly through 30 Km of windy atmosphere while trailing a parachute.

Overall HAPP rendering

ELS shown in isolation

We're going for rapid descent. While the risks may seem high, consider the math. The HAPP weights only 9 kg and has an aeroshell that's 1 meter in diameter. In essence, the HAPP is a round wing. I calculate terminal velocity without chutes to be around 47 kph, or 30 mph for the Americans. If all three chutes fail, the craft will suffer damage on impact, but the data payload (4K, 360-degree virtual reality video on a micro SD card!) will most likely survive, and the HAPP is not going to do major damage to buildings, cars, or people.

If at least one of the chutes deploys from its mortar, the ground impact speed will be even lower. It doesn't even have to inflate - a tangled chute makes a fairly effective streamer. If all three chutes inflate successfully, the impact speed will be the same as dropping an object from about 30 cm in height (about 1 foot). Pretty cushy.

The chutes I'm using are Iris 60" Ultralight chutes from Gene Engelgau and the good folks at Fruity Chutes.

As for the deployment mechanism, I spent several weeks toying around with various concepts. I tried springs, elastic bands, and even a simple method that relied on the rushing wind surrounding the HAPP to pull the chutes out of their containers, assisted by small drogue chutes. None of those methods worked well. They were either too heavy, or did not eject the chutes with enough force, or both. The chutes need to be thrown forcefully clear of the HAPP so the canopies have time to inflate before the wind carries them up and tangles all the lines together.

In the end I decided to get with the times and do a proper mashup of various technologies. For the deployment force, I adopted the approach of Cameron Tinder over at Tinder Rocketry. He has a system called the Raptor that was designed for model rockets. It uses redundant electronic igniters to light a black powder charge which in turn drives a sharp metal spike into a canister of compressed CO2. Once ruptured, the canister vents the CO2 into the parachute compartment and eventually forces out the parachute.

For the packaging, I went with the approach of the drone recovery system available here from Skycat. Skycat uses carbon fiber tubes to hold the parachutes. Chutes are deployed using a proprietary piston system, which is interesting, but the applicable model from Skycat is listed at 0.32 kg without a parachute. To get redundant systems and equip them with chutes would result in a heavier system than my approach. Plus, where's the fun in buying a finished system when you can waste weeks building your own?!

However, unlike the Raptor's single chamber, I need to direct the gas into three separate mortars, so I designed a new manifold for the Raptor and had the awesome Jason Davis at Pioneer Cuts machine the manifold on his CNC mill. In the photo below, the manifold is the silver aluminum part into which the CO2 canister screws. It directs the gas to the mortars using high-pressure red macro lines and push-connects. My manifold mates to the charge and fire pin portion of a standard Raptor. You can see these standard parts in the photo below: The red anodized aluminum part, the spring, and the small cylindrical steel parts in between.

A tech mashup: Part Raptor, part Skycat,
part home-brew, and 100% fun.

Let's walk through the details.

When it's time to blow the chutes, the flight computer energizes the igniters. The igniters set off the black powder, which in turn drives a steel fire pin into the CO2 canister. Gas vents out into the manifold and gets routed to the mortar tubes by the high pressure macro lines.

As the mortars pressurize with gas, the caps are pushed outward with significant force. Each of the caps are secured with four #2-56 nylon screws. These plastic screws are tiny, and eventually they shear off as the pressure accumulates. This allows the caps to fly off and the parachutes follow behind them. Getting all the math right was a fun exercise - what size of CO2 canister to use, what size of nylon pins, diameter and volume of the mortar tubes, etc.

For those who care, the theoretical maximum force required to shear four nylon pins is 603 N, which is equivalent to a 61 kg / 135 lb weight pressing on each of those three little mortar caps. The 45-gram CO2 cartridge I'm using, a Leland 87202, has enough gas to generate 3382 N on each cap should the pins prove stubborn or the caps get jammed, perhaps if the sealing tape around the rims freezes up at high altitudes. That's a safety factor of 5.6. I was shooting for 5, but cartridges only come in certain standard sizes.

I also put some one-way check valves in the macro lines. They're the black, blue, and silver cylinders located midway down the red macro lines. These valves allow gas into the mortar tubes but they don't let it flow out. This is critical, as the nylon pins are each a bit different (ah, the beauty of polymer chemistry), and they will shear at different times: In the video you can clearly see delays up to approximately 10 milliseconds. If one mortar opens first and there are no check valves, gas will leave the unopened mortars, flow back through the macro lines, and rush out of the open mortar. The unopened mortars will fail to deploy.

The mortar tubes are 9" sections of Size 18 INFINITubeV from Rockwest Composites. The caps utilize 1" sections of Size 17 tubes. The end plugs are machined circular sections of 2.2mm thickness carbon fiber plating. Plugs are bonded to the tube sections with this specialty 2K epoxy.

All in all, the total system weight including parachutes came in at 1006 grams. That's a little over 2 pounds for 5.4 square meters / 59 sq ft of total parachute canopy, explosively discharged, with redundancy for the chutes, the mortars, and the black powder igniters. Potential single points of failure are the CO2 canister and Raptor fire pin, but that's a risk I'm going to take.

Here's a picture of the chutes ready for folding and stowing in the mortar tubes.

Foosball table is optional for
successful parachute deployment

So that's about it for the Earth Landing System. Onward!