Flight Testing: HAPP-T2

At last, here we are at the first airborne mission, HAPP-T2.

The goal of this mission was to shake down the balloon system and perform a tethered flight, thereby demonstrating all the functionality of a full mission while constraining the HAPP below a certain fixed altitude, in this case about 250 meters. Everything about this mission was live: Ground crew operations and staging, balloon inflation procedures, HAPP assembly and startup procedures, launch, ascent, communications, video, cutdown from the balloon, parachute deployment, and recovery.

This was my first time to inflate a large balloon, and fortunately I had an outstanding group of ground crew volunteers helping to wrangle the thing. Thanks guys!

I'll let the mission video speak for itself:

OK so what just happened? 

Problem 1:

Well, obviously we didn't fly very high, and therefore we didn't get to test the Earth Landing System, a.k.a the parachutes. Everything worked great up through launch. The HAPP started to ascend, and then at an altitude of about 5 meters there was a gunshot sound - the cutdown pyros severed the connection between the balloon and the HAPP. The Flight Controller computer decided to abort the mission!

I had to get home and check the computer log to understand what happened. It turns out that the FC performed correctly. The software has a geofencing feature that terminates the mission if the HAPP drifts out of a pre-defined geographical zone. I forgot to check the software settings, and the latitude geofence was set to 42.398 degrees north, which is a bit north of Kalamazoo, Michigan - a value chosen earlier in development for no particular reason other than it looked a bit farther north than my home town of Ann Arbor. Who knew that our launch site for HAPP-T2 at a park near Ann Arbor was at 42.399 degrees north? As soon as the HAPP became airborne, the FC detected a geofence violation and aborted the flight. 

This issue won't reoccur as I have now modified the FC sotfware to perform a geofence check during prelaunch procedures. If we're outside the geofence, the FC will go into an error mode and won't let us launch.

Although we were unable to witness the glory of the HAPP descending under its three huge parachutes à la Apollo, we were able to confirm almost everything else (Note: As an homage to NASA, the HAPP chutes utilize the Apollo red and white color scheme). In particular, the calculations were spot on for the amount of helium required to achieve the desired lift. The balloon harness and tether system also performed well.

One thing I could not confirm were flight dynamics. The mission software was configured to let the HAPP fall from 250 meters altitude for a few seconds before blowing the chutes. Inertial data from the HAPP's IMU could then be used after the flight to calculate acceleration, which in turn enables calculation of the aerodynamic drag coefficient or Cd. Knowing Cd, I can predict various things like the freefall speeds at different altitudes and anticipated shock forces when parachutes open. I had also hoped to confirm aerodynamic stability - see my old issues with the center of gravity. Those issues are solved on paper (well, in the CAD model at least) but I'd like to confirm with a physical test.

Problem 2:

When the FC aborted the mission, it also should have blown the parachutes. The chutes would not have done much good from an altitude of 5 meters - no time to inflate before ground impact - but they should have blown nonetheless.

It turns out that the FC did fire the parachute pyros. However, the steel ram pin that's supposed to puncture the liquid CO2 canister did not fully penetrate the canister. This is the first failure I have had of the ELS, including all of the early prototypes. This system has been highly reliable. I'm not sure what happened but I suspect I loaded the pyros with a short charge of black powder. I'll check carefully in the future. I'll also swap out the fire pin for a new one with a sharper tip. You can see the pin if you zoom in on the center of this photo; it's centered inside the steel spring.

Problem 2 is slightly concerning to me, but given the past reliability of the system, I'm not going to lose sleep over it. Not to mention, I calculate the terminal free fall velocity at low altitude without chutes to be around 40 kph / 25 mph, so the HAPP will likely survive a total chute failure. Heck, in HAPP-T2 the ground impact speed was around 29 kph, and there was almost no damage. By design, the HAPP is essentially a big, hollow, flying wing.

Overall, I judge HAPP-T2 to be an 80% success.

The Good News:

The HAPP structure performed in an exemplary manner during HAPP-T2 and the subsequent ground impact. My original design spec called for crash resistance from a free fall drop height of at least 1 meter onto a hard surface. In T2 the HAPP fell from about 5 meters onto hard packed dirt. The aeroshell flexed as designed and did not suffer any damage. The internal structure and custom-molded crash pad also performed well with only slight damage to the structure in two locations.

Location 1 was the bottom of the structure where the tank deck is bonded to the central strut with specialty epoxy and small L-brackets, all made of carbon fiber (see my previous post for details about the construction). The epoxy bonds fractured before the carbon fiber components - as intended - and in this picture you can see my fingers in the resulting gap between the brackets and the now free-floating tank deck. A little fresh epoxy in the gap and the repair was complete.

Had the tank deck been fixed to the brackets with bolts, one of the carbon fiber components almost surely would have failed, and the repair would have been quite expensive, possibly requiring CNC machining of a new tank deck (see video of machining posted previously). Bonding was a good choice.

Damage location 1

Damage location 2 was the joint between one of the aerodynamic strakes and the ELS deck, as shown below. In the picture I've gone ahead and snapped the remaining good bond between the strake and central strut so I can chip away all the old epoxy and make clean new bonds.

Damage location 2

This repair was also pretty easy, but I had to break out my old assembly jigs to ensure the strake was aligned correctly when bonding. As described in a previous post, I 3D-printed various assembly jigs way back when I first built this version of the structure - glad I kept them around! 

3D-printed jigs held in place with clamps; very useful for alignment when bonding carbon fiber pieces with epoxy

This post was a bit lengthy, but hopefully you enjoyed the walk through mission HAPP-T2. I was hoping to avoid a T3 - see the previous post describing costs for the balloons and helium! - but I'm going to repeat this test mission. I simply don't want to release the HAPP for a full mission without validating the parachutes and flight dynamics.

Stay tuned - we're just waiting on a break from the crappy weather during a typical Michigan winter!