Wednesday, May 24, 2017

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.


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!

Saturday, May 20, 2017

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 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. VoilĂ  - 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...


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!

Thursday, March 2, 2017

Structure: Carbon fiber cutting and bonding methods

Ever since I finally got the internal structure re-configured for optimal balance and aerodynamic stability using some serious 3D modeling, I have been working frantically like Doc Brown jacked on Red Bull to get the structural components built and assembled.

Soon I'll post a walk-through of the entire structure and the various components, but for now I thought I'd show you what the parts look like and discuss how I processed them. This picture shows the whole suite of structural components, which are all fabricated from carbon fiber.

Need more fiber in your diet? Carbon fiber, that is.

The closed sections (round and square tubes) were purchased from Rockwest Composites and cut to length. The round tubes are mortars for the parachutes, and in the photo you can see one of the 60-inch parachutes peeking out of its mortar tube - stay tuned for excellent videos of explosive parachute deployment! The square tube is the main strut that runs vertically down the center of the HAPP.

The flat sections, also from Rockwest, were a bit more tricky. These are, variously, the internal decks (round), three aerodynamic strakes (triangular), and holders for the parachute mortars (various small pieces). I purchased rectangular plate stock of various thickness from Rockwest and had to machine the parts using a CNC mill. Thickness ranges from 0.7 to 2.9 mm, depending on the part. The largest single plate was 24x24 inches and 2.2 mm thick (SKU # 403-22). Not cheap!

Although I've received my checkout to run the large CNC flatbed router at Maker Works in Ann Arbor, Michigan, in the interest of time I had the awesome Jason Davis at Pioneer Cuts do most of the work. This is what the process looks like:

To save weight and maximize flight altitude, I had to abandon the aluminum mounting system described in a previous post. Now I'm bonding the structure together with specialty adhesive - in particular, this 2K epoxy from 3M. This approach means the craft will not be easy to repair if any parts are damaged in flight testing, but such are the engineering tradeoffs in aerospace!

It also means the assembly process is a bit more complex. To hold the parts together while the epoxy does its bonding magic, I decided to 3D-print some assembly jigs using high-accuracy SLA. Here's one of the jigs. This one holds the end caps onto the parachute mortar tubes. I made a total of 5 different jigs. Thanks as always to Steve and the great 3D printing team at ThingSmiths in Ann Arbor.

3D printed assembly jig on
end of parachute mortar tube

All together, what you see laid out on the table in the first photo is the entire structure of the HAPP. It weighs in at 1671 grams - not bad for a craft that's 1 meter in diameter, almost as tall, and will support a lot of serious hardware, including twin pressurized gas tanks, pneumatic valves, cameras, a custom-molded carbon/kevlar aeroshell, and a few other important bits (like the pyrotechnic-deployed parachutes!). All of which will be exceeding Mach 1 on a plunging descent through the atmosphere after flying to the edge of space...

Monday, January 30, 2017

Passing 35,000

Back in May 2016, I wrote a post describing the geographical composition of my audience as I crossed 10,000 readers. I had every intention of refreshing the data periodically, perhaps as I hit 25,000. Well, I should have checked the stats sooner, as not only did I blow by 25,000, but I should hit 40,000 in a few weeks!

The total today stands at 37,419. As I did back in May, allow me to give some color, followed by some awards to you, my dear readers...

For starters, here are some basic stats:

And here is the geographical breakdown:

Back in May I pointed out the curious (to me, anyway) fact that the highest source of readers outside the USA was Pakistan. Mystery solved: In the description of the blog for Google's search engine, I had included the keyword "bang bang." This is a reference to the particular type of control algorithm I embedded into the flight computer's autopilot function, explained here. It was an innocent but fateful mistake, as folks in Pakistan apparently search for popular Bangladeshi celebrities using the word "bang" followed by a name, and for whatever twisted reason harbored by its nefarious AI, Google was returning a link to the HAPP Blog amidst the search results. I know this to be true because Google shows me the search terms used by those who clicked through to the HAPP blog website!

For those curious, the #1 most searched-for Bangladeshi celebrity by those who also visited the HAPP blog is this lovely person:

Apu Biswas, Bangladeshi actress

Alas, with the "bang" reference deleted, the Pakistanis have fallen out of the Top 5 (although someone apparently has enough spare time to continue visiting the blog). The crown now belongs to France! Here's a special tribute to mark the occasion:

Vive la France!

Finally, I simply must give an honorable mention to a recent newcomer on the list. I have ZERO idea how folks in Oman found this blog, but my dear Omani readers - I salute you!

Monday, January 23, 2017

Cap'n, we need more power!

Now that I've started testing the field of view and placement of cameras - including the primary camera, the stonkin' Nikon KeyMission 360 - I've run right smack into an unfortunate reality: My old PC is useless for editing 4K video. It literally cannot run the editing software without immediately crashing. As the primary goal of the HAPP project is to get 360-degree 4K video and give my dear readers a virtual reality ride to space, this is a pretty big problem.

In addition, I'm having difficulties manipulating the full 3D model of the HAPP in AutoCAD Fusion 360. It crashes frequently and I've lost hours of work. For that matter, the flight control simulations in MATLAB/Simulink are painfully slow.

As I have lots of work remaining in all those areas, it's time to bite the bullet. I'm retiring the PC I built nearly 7 years ago (it was a banger back then!) and building a new rig this week that can handle these tasks.

For those curious about such things, here are the parts I just ordered today:

An 8-core CPU, a screaming graphics card, 64 GB of RAM, 12 TB of storage, and lots of liquid cooling ought to hold me for a while :-)

It sucks to deviate from the parachute-testing plans I had for the coming weekend, but this is a necessary step...

Saturday, January 21, 2017

Tether and camera mount: Initial design

In the last post I described how I learned a hard lesson about taking time to do a thorough engineering analysis when necessary. In this post I'll apply that lesson to a small but critical part of the HAPP: The tether ring, which is where the HAPP is attached to the balloon that will lift it over 30Km into the sky. It turns out that this part is one of the most highly engineered components in the entire project.

From the start of the project I knew I'd need a robust place to attach the balloon. Initially I assumed this would be a simple metal ring of some sort. However, after completing the full 3D model of the HAPP, I decided to integrate two additional functions into the tether.

First, the tether will also serve as the attachment point for the parachutes that will deploy during descent. There's no sense adding additional mass for a completely separate structural element to achieve this function.

Second, the tether will also contain the mount for the primary camera, which is a 360-degree 4K ultra HD camera (specifically, the Nikon KeyMission 360). The only place where the camera will have a mostly unobstructed view of the world is from the top of the HAPP. If I attach the camera to, say, the side of the main strut, then a large portion of the view will be occluded. The camera therefore needs to go at the apex, but it can't interfere with the tether or the parachutes.

After several iterations using 3D printing to verify the geometry (thank you Owen and Steve at ThingSmiths!), I arrived at the following design:

In this rendering you can see a few important features. First, note that the base of the part will insert directly in to the square-section carbon fiber tube I'm using for the main body strut. The rendering shows four of the M3.5 mounting bolts that will go through the strut wall. The bolts are mainly to ensure position while I bond the metal body of the tether to the carbon fiber strut using some specialty adhesive.

One side of the camera - and one of its lenses - is visible in the rendering. This camera is symmetrical and has a second 180-degree fisheye lens on the other side. The camera automatically stitches together the two 180-degree videos to form a 360-degree virtual reality video in all its glory.

You can also see two of the three parachute attachment rings at the ends of short arms that jut out horizontally. There is the main ring enclosing the camera, and finally there is the actual balloon tether ring sitting on top. It's not obvious from the picture, but the parachute rings and camera enclosure are designed to be almost invisible to the camera's field of view. Here's a quick demonstration. In this 360 video, note the kevlar/carbon fiber lower aero shield that I spent the entire summer of 2016 learning to fabricate...

360 camera field of view confirmation.
If this doesn't play on your screen in 360,
try viewing with YouTube here.

What you can't see is how the camera is attached. There's a 1/4"-20 UNC bolt going up into the camera's standard mounting socket. However, as the part will be bonded to the main strut, the bolt must be started and tightened using a long tool inserted from the bottom of the 836 millimeter-long main strut. To facilitate this blind insertion, I added a "docking cone" to the bottom of the part, which is visible below:

Bottom view of tether / camera assembly
showing blind insertion cone for 1/4" bolt.

With the basic geometry defined, it was time to optimize the part for light weight while insuring it can withstand the forces resulting from opening shock of the parachutes. A skinny part is better as it minimizes total weight and maximizes ultimate flight altitude, but more importantly, this part is at the apex of the HAPP and will strongly contribute to a higher center of gravity. This is bad - see my struggles with CG in the last post. Too skinny, however, and the part will fail when the chutes deploy. How to find the right balance?

For this we need some real engineering, and the right tool is finite element analysis. The 3D modeling package I'm using, AutoCAD Fusion 360, happens to have a nice little FEA component that's extremely intuitive and easy to use. To run the analysis, however, we need to know what maximum force we can expect from the parachute opening shock.

I performed the shock analysis by hand using some references from NASA back in the 1960s (the technical gift that keeps on giving!), the Naval Ordnance Lab, and the Naval Surface Weapons Center. I used numerical integration to scale time/force curves given known parameters, yielding a conservative maximum force estimate of 1150 newtons.

If you want to play around with such things and don't feel like rolling your own, I recommend you check out a little piece of software called OSCALC. It gave a worst-case force of 1050 newtons, thereby validating the accuracy and conservative nature of my approach. It also comes with a user manual that walks through some of the math.

The analysis assumes we are using three of these lovely parachutes from the folks at Fruity Chutes. The number and size of chutes was selected to yield a landing speed of 2 meters per second, which is equivalent to being dropped from a height of 0.2 meters. If one chute fails to open, the landing speed will be only 3 m/s, which is still pretty cushy.

With the forces defined, it was time to optimize the design using FEA. I started with a design based on manual calculations using cantilever beam equations. From there, I went through 7 cycles of fine-tuning the design and the choice of materials. Finally I settled on aluminum 7075-T6 and the design shown in the renderings above.

Here's a little video showing the final design after optimization. The amount of deformation is greatly exaggerated - in real life you'd not be able to see it with your eyes. The blue color denotes a high safety factor, and yellow, orange, and red colors denote progressively lower ones. A safety factor of zero means the part is right at the yield strength for the material. I optimized the design so the region with the lowest safety factor is at 0.5 - appropriate given the conservative analysis of parachute forces.

Analysis of safety factors using finite element analysis.
The short video show how the arms deform as
the parachutes deploy.

Same analysis, just a snapshot instead of the video

To give you an idea of how the design evolved with the FEA analysis, here's a comparison of the parachute attachment arms from version 1 and the final version 7. The arms got shorter, they changed from a round profile to a variable elliptical profile, and the chute attachment rings got substantially smaller. All of that extra metal was not necessary. Besides changes to the arms, the base portion that inserts into the main body strut got longer. This was mainly to accommodate the cone-shaped feature that assists with blind insertion of the camera mounting bolt.

Evolution of parachute arm
from v1 to v7 using FEA

Final part weight is 139 grams. Currently, the part is being CNC milled out of a single piece of 7075-T6 aluminum by the outstanding team at ProtoLabs.

OK kiddies, this concludes our case study in mechanical engineering for the day. See you next post for some live testing of parachutes!

Sunday, January 8, 2017

Structure: Re-design for center of gravity

Happy New Year, and Happy 1st Anniversary of the HAPP project! It's been a whole year since I decided to undertake this project. It might also seem to have be a year since my last blog update...

There is a reason for my lack of blogging. Well, two reasons actually. The first reason is that I got a new day job in mid-July, and I've been pretty focused on the new gig. Gotta pay the bills for the HAPP project!

The second and more crucial reason is that I hit a roadblock as I started to assembly all the flight hardware, especially the structure. Turns out I had balance issues. Specifically, I couldn't figure out how to package the guts of the HAPP such that the center of gravity was sufficiently low. The CG must be lower than the aerodynamic center of pressure if the HAPP is to remain aerodynamically stable during the free-fall portion of its descent from balloon burst down to the altitude for parachute deployment. If the CG is too high, the HAPP gets tippy.

Think of a shuttlecock - most of the weight is at one end, the nose, and the center of pressure (where the wind catches it) is at the other end, the feathers. This make it stable as it flies. We're trying to achieve something similar with the HAPP, but we want it to be stable with the lower shield facing downward - you may recall that the HAPP is essentially a scale replica of an Apollo Command Module.

The "final" design I've been using was based on some internal packaging that evolved from multiple prototypes, and it relied on some rough hand calculations for the center of gravity. I thought I could roughly calculate the CG accurately enough that some final adjustments after building the main structure would be sufficient to fine-tune the CG. Not so! After weeks of staring at a lab table full of parts, I just couldn't make it work.

To find the solution, I had two options. Option 1 was to keep buying lots of parts and try, try again until I found the answer. This is OK when dealing with plywood and cardboard prototypes. This is not OK when dealing with flight hardware that's primarily carbon fiber - the stuff is expensive!

Option 2 was to model every single part in 3D software and play with the design until I figured it out. This option is cheaper, but requires time to model all the bits and pieces. Every screw. Every panel. Every piece of electronics. Every pneumatic component. All of it.

Having already spent too much money on this project, I went with Option 2. I used AutoCAD Fusion 360, which is an excellent free program. It took some weeks, but I finished the modeling, and I found my solution to the CG issue. It entailed a serious re-arrangement of the HAPP's guts. As a bonus, I was able to cut almost 1 kilogram of weight out of the design. That means higher potential maximum altitudes on flight day.

With the design completed, I have started ordering parts and fabricating the final structure (no really, it's final this time!). In the coming weeks I'll post some photos and videos of the build process. In the meantime, here's a rendering of the section between the lowest deck (the tank deck) and the second deck (the propulsion deck, which supports the pneumatic valves). I omitted the pneumatic hoses and some wiring for clarity. Two of the three canisters holding the main parachutes are just visible at the top of the image.

With the CG roadblock cleared, the project is back on track, and with a hard lesson learned. Sometimes there is no substitute for thorough, quantitative design work before you build!

Valves and low-pressure regulator hanging
down from the Propulsion Deck.
Tanks visible at bottom.

Bonus pic: 3D-printed thruster pack and
quick-connects for air hoses, mounted
on the end of the jet arm

Sunday, September 18, 2016

Lower aeroshell finale: Part

If you've stuck with me this far, you're familiar with the overall strategy for fabricating the HAPP's outer aeroshell: Make the plug, then the mold, and finally the part. Well, after perhaps 250 hours of study, experimentation, failures, and troubleshooting, here we are at last at the part-making stage. Let's get busy.

First, let's start with the end in mind. Below is a picture of the part we're making - the lower aero shell. Recall that the HAPP is essentially a scale model of the Apollo Command Module. The lower aero shell is analogous to the lower heat shield on the Command Module. Of course, the HAPP will not experience the blow-torch heat that the CM felt upon reentry at Mach 32, but we desire aerodynamic stability at supersonic and trans-sonic speeds, and the Apollo design achieves this. (Interesting technical footnote: Blunt shapes experience less heating upon reentry compared with pointy shapes. This was a military secret until NACA, the predecessor of NASA, finally published it in 1958. Read it here.)

If you just came here for a sexy picture of the finished product then you're done. See you next post. If you care to see how the sausage is made then proceed!

Finished lower aeroshell.
Mold parting line is visible and will
be sanded out and polished.
This part is 1 meter in diameter.

Lookin' like a BOSS with that Kevlar/carbon
fiber weave on the A-surface

OK so here we go. Below are two pics from my first attempt at the full-scale finished part. The left frame shows the beginning of resin infusion - recall that we're using the vacuum infusion process I described previously. The portion that's been wetted by resin is darker than the surrounding material.

The right frame shows the "finished" part - a total disaster! The resin did not thoroughly infuse around the part, and there was a lot of raw fiber that I had to cut away. You can still see loose fiber around the edges. In addition, the layup consisted of only two layers of fabric, and it did not provide sufficient rigidity. The part is quite floppy and feels a bit like leather. Very expensive and useless carbon fiber leather, that is...

Trial #1: Total failure!

So onward to trial #2 with lessons learned.

The first process improvement involved the PVA. Recall that this is a non-stick coating sprayed onto the mold surface so the layup and resin don't adhere to the mold. In trial #1 the PVA pooled in the bottom of the mold. In trial #2 I drilled a drain hole and let the excess PVA flow out. This resulted in a highly uniform coating inside the mold.

PVA drip hole and uniformly-coated mold.
Mold legs were amputated to facilitate
covering the entire mold with the vacuum bag.
Easy come, easy go.

Next came the layup. The main improvement over trial #1 was to add a third layer of fiber and an intermediate layer of Lantor Soric foam core. This improved layup was developed in the previous post.

Clockwise from top left:
Kevlar A-surface layer;
3K carbon twill;
Lantor Soric foam core;
3K carbon plain weave B-surface.

Here's a view of the first three layers. I trimmed the excess overhanging material before bagging and infusion.

Working on the layup

For the outlet port I wrapped a spiral plastic tube with permeable nylon peel ply, ran it around the lip of the mold, and connected it to the vacuum pump. This provided an even vacuum draw all around the part.

Nylon-wrapped spiral tubing (green) on mold lip.

Next I placed the entire mold in a vacuum bag and affixed the inlet ports. For the bagging material I used a very elastic sheeting called Stretchlon 200. And below you can see the final major improvement versus trial #1 - the addition of more inlet ports. This allowed for faster resin flow and more even coverage around the part.

4 resin inlet ports

At last it was time to infuse. After activating the vacuum pump I opened the resin valves and established an even flow. Below is a short video that gives you a sense of the flow speed. This is 3/4" outer diameter plastic tubing. In the future I will eliminate the bubbles by de-gassing the resin in a vacuum tank prior to infusion. For this first aeroshield, which will be sacrificed in flight testing, the bubbles won't affect the final part significantly.

With the additional resin inlet ports, infusion progressed evenly and quickly.

Infusion progress

I also discovered that the infusion ports needed constant support. With the help of a friend (thanks John!) we quickly improvised the support "lattice" shown below. In future builds we'll use an actual lattice.

Resin feed tubes clamped in place

Infusion: Done. Resin cure: Done. Now it was time to decant the mold from the vacuum bag. It was a little tedious and some helping hands made for fast work.

Decanting from the vacuum bag.
Chunks of red flow media are visible.

Below is the decanted part and a close up of the B-surface. Remember that the B-surface is not normally visible and its appearance is not critical. Nevertheless, the surface doesn't look too bad. You can clearly see the imprint of the red flow media that sat on top of the layup.

B-surface detail

Finally, after a couple of hundred hours of learning and failures, it was time for the coup de grace: De-molding of the finished part. That's me in the left frame below with a big smile on my face because the part looks good. John is holding the part in the center frame with half of the mold removed. The right frame shows the part and A-surface in all its Kevlar glory.

De-molding the part

This picture says it all. Success!

Thanks for helping, John!

Here's a view of the part being rotated so you can get a better sense of the shape and dimensions. There are a few tasks for later - Trim the edges, fill in a few minor defects, and polish the outer A-surface.

And here's some of the A-surface detail. Kevlar, beautiful Kevlar... the mold parting line is clearly visible in this picture. I will sand it out and polish later.

A-surface detail

Here's the lower shield in the approximate position it will maintain on the HAPP. I'll do a post about the mounting system later. There is an upper aeroshell as well - pics to follow soon.

The lower shell goes here

And once again, here's the finished part. I essentially spent the entire summer learning to fabricate an aerospace-quality composite aeroshield. It was far more work than I expected, but it was a fantastic learning experience and the part turned out better than I expected.

Aeroshell... beast mode

During this final trial (#2) I noted many process refinements for future production runs. My friend John provided many of the more useful observations and ideas. There are also a few small quality issues with the part that need to be ironed out but the process refinements should fix them.

Undoubtedly I will make a few more parts in the future as we test & destroy before the first mission flight. With some luck, the process will only get easier and the parts will be even higher quality.