Full scale prototype and system test

System testing has begun on the HAPP full scale prototype!

This flight-worthy proto is basically what will go on the main missions, although the final version will be cosmetically enhanced and will also contain a few fixes for issues discovered during testing. We're going to beat up this proto pretty good...

During the next several weeks I'll be running a series of tests with ever-increasing difficulty, culminating in low-altitude tethered flights.

First up is deployment of the Earth Landing System parachutes. You saw the ELS in action previously. This time, however, the ELS has been integrated into the main structure and we need to verify mechanical integrity - in other words, does the structure survive the recoil from the parachute mortars? In previous trials on a test bench, the recoil bent some steel mounting brackets. The HAPP doesn't contain steel; the entire structure is carbon fiber composite. Will it survive?

The production quality of this video may not be quite up to the standards of the previous one. Sorry! This one was strictly for engineering analysis. As a little bonus, this new video contains footage from the new HAPP chase plane, a.k.a. my DJI Mavic Pro drone.

Enjoy

Technical notes:

• Delay between first and last chute deployment was 50 milliseconds. Pretty good, considering that opening of the three mortar caps relies on the shearing of nylon pins. See the old ELS post for details.
• There were a few spots where the structure experienced minor cracking, all of it along epoxy-bonded joints. I will reinforce these joints with carbon fiber lapping material.
• None of the structural components received any damage. Good to go!

Structure: Crash pad

Here's a quick update on progress with the HAPP's structure.

From previous posts you might recall the HAPP has an outer aeroshell that I custom-fabricated from carbon fiber in an extremely labor-intensive process spanning multiple months. For a refresher, see my old posts about the Plug, Mold, Composites, and Part. The central structure, which includes the main strut and Earth Landing System (a.k.a. parachute mortars), drops into the top of the aeroshell and cinches into place with Velcro straps.

Between the bottom deck of the structure and the lower aeroshell is a small gap that needs to be filled with some type of impact-absorbing material. This material will help absorb kinetic energy as the HAPP touches down at landing. Which material should I use?

I played around with materials of various densities and elasticity, such as styrofoam (inelastic) and neoprene (too dense and heavy). I also experimented with expanding polyurethane foams, both the rigid and flexible varieties. I finally found the right material: FlexFoam-IT! X from Smooth-On. It's a flexible PU foam, but one of the stiffer varieties. The foam expands and hardens in a few minutes after mixing the two components together. The resulting material is spongy and fairly lightweight.

For those who care about such details, the two chemical components are a polyol (Part A) and an isocyanate (Part B). Together these react and form an open-cell polyurethane. Although Smooth-On doesn't divulge the details, I suspect one of the components contains water. The presence of H2O, after some intermediate steps, eventually results in the production of carbon dioxide gas. The CO2 acts as a blowing agent and foams the polyurethane like a mousse. FYI the giveaway for the water is the visible presence of an amber-colored polyurea "hard phase" in the material under certain conditions...one of the chemical consequences of adding H2O.

A pinch of this, a dash of that...

Next I needed a mold to contain the foam as it expands and deliver the proper shape. For the bottom side of the mold, I simply used the inside surface of the lower aeroshell. This ensured a perfectly-curved pad that fits the aeroshell precisely.

I made the walls of the mold from an old plastic bucket turned upside down. I made the the top of the mold from a piece of generic foamcore board cut to fit. Fortunately, the diameter of the bucket almost exactly matched the diameter of the main structure's lower deck, which gave a properly sized pad. I secured the foamcore board in the correct location with metal angle brackets... and the location was easy to determine now that the HAPP is fully modeled in 3D CAD.

Inserting upper side of mold into bucket.
Metal brackets are covered with foam tape.
This whole assembly will be turned
upside down during molding.

Foamcore board is a little chewed up because
I took this pic after de-molding 2 pads.

After waxing the makeshift "mold" with release agent, it was time to mix the expanding PU foam and pour it into the mold. I poured a blob right into the center of the aeroshell and quickly set the inverted bucket on top, holding it down with a heavy weight to contain the foam's expansion.

After the foam cured, I peeled it out of the mold and trimmed off any excess flash. Voilà - a perfectly sized, custom-molded crash pad of just the right density. Pad weight is 248 grams. That's a fairly substantial weight penalty to pay for good impact protection, but probably worth it. We'll see how things hold up in flight testing. It might be possible to cut some chunks out of the pad to economize on weight, perhaps leaving a thick ring with a central donut hole.

Clockwise from top left:
(1) Holding mold in pace while foam expands;
(2) Removing mold from aeroshell;
(3) De-molding the foam pad;
(4) Finished pad after trimming flash.

Below you can see how the pad is situated in the HAPP. Before inserting the structure and ELS assembly, the pad is positioned in the location where it was molded. The structure is dropped into place and the lower deck compresses the pad slightly.

Note that the aeroshell shown in the photo is the "B" version I'm using for early flight testing. I never bothered to clean up the exterior after vacuum molding the carbon fiber shell, and I've hacked it up a bit during the final stages of development. I'm saving the cosmetically perfect "A" version for a future unveiling. After all, I have to maintain some kind of dramatic suspense...

Left: Structure with ELS & 360 VR
camera, sitting next to aeroshell.
Right: Inserting structure into aeroshell.
Foam pad placement is visible.

Recovery: Cutdown pyros

What goes up must be made to come down.

On mission day the HAPP will be carried to an altitude of 30 Km / 100,000 ft by a weather balloon. Once it reaches apogee, we need to cut the umbilical cord between the HAPP and the balloon. There are two possible scenarios.

First, the balloon may burst as it expands in the near-vacuum conditions, in which case we need to cut the cord and release the floppy balloon to ensure a stable descent and a clean deployment of the parachutes. This is the most likely scenario, and in fact is our Plan A for returning the HAPP to Earth.

If the balloon does not burst, we need to cut the cord to ensure the HAPP comes down and doesn't turn into a perpetual "floater" that proceeds to circumnavigate the globe until ultraviolet radiation finally degrades the latex skin of the balloon. In fact, FAA regulations require a redundant Plan B for unmanned balloons for exactly this reason (Federal Code of Regulations, Title 14, Part 101, Subpart D, 101.33).

Most folks I've seen on the internet use a NiChrome cord cutter. This type of system takes a small piece of NiChrome wire, spirals it around the cord (usually made of Nylon), and at the moment of truth sends a relatively high current through the wire, thereby heating it up and melting the cord. Here is one commercially available system, and here is a video of someone's hobby version in action.

As the 9 kg HAPP is heavier than most hobby balloon payloads, we need to use a fairly robust umbilical cord: Kevlar instead of Nylon. However, Kevlar has a melting temperature of about 500C/930F, whereas Nylon 6 melts at 220C/428F. Thus, we'd need to get the NiChrome almost twice as hot to melt the Kevlar, which means more current, bigger batteries, more weight, and lower reliability.

Here is where we have another opportunity to use pyros and blow stuff up (and those are always welcome opportunities :-)

After exploring a few alternatives for home-made, explosively actuated guillotines and the like, I decided to keep it simple and use the lovely little Cable Cutter from Prairie Twister Rocketry. This device is designed to cut plastic zip ties, but I figured it might work for Kevlar string as well.

You can glean the operating principle from the picture below. Moving left to right, we have 500-lb yellow Kevlar braided string threaded through two small radial holes in a blue anodized aluminum cylinder. A small steel fire pin goes into the cylinder. Next comes about 0.1 cc of black powder (not shown, but it goes where my finger is located in the picture). After that we insert a J-Tek electronic igniter which has been threaded through a black steel end cap. To complete the assembly we simply tighten the endcap onto the cylinder. The entire assembly weighs only 10 grams. The igniter is triggered simply by hitting it with 5V from the flight control system.

You might wonder whether the powder charge will burn effectively in a nearly zero-oxygen environment at flight apogee... it's a reasonable question. The answer is that the potassium nitrate (KNO3) in the black powder provides the oxygen needed for combustion, so the powder is essentially self-oxygenating. Guns can be fired underwater for the same reason.

Cable Cutter from Prairie Twister Rocketry

So does it successfully cut the Kevlar cord? Check out the video below.

Looks like a solid solution. We'll use two of them on flight day for redundancy.

FYI, a rough analysis of the video shows the cable cutter jetting away at about 90 kph / 55  mph  as the hot combustion gas shoots out of the endcap...

Onward!