LANDS END AEROSPACE

Punching Holes in the Sky Since 2010

Choosing a Tracking System

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Aside from the balloon and the helium, the tracking system is arguably the most important part of a high-altitude balloon system. Unless you’re planning on a one way trip to points unknown, a reliable tracking system is necessary to get your payload back safely. There are a number of different solutions that teams use for tracking their HAB projects.

  • A lot of teams use the SPOT satellite tracking system, which was originally designed as an emergency Personal Locator Beacon for sailors and backcountry travelers. The SPOT is relatively inexpensive, simple to use and reliable pretty much any place on earth.
  • Other teams use smartphone-based systems which have onboard GPS tracking apps that essentially call home once they’ve landed and report their final position. These seem to be a bit more finicky and dependent on cellular network coverage – not something we want to rely on.
  • The third common solution is an APRS beacon, which uses a GPS receiver to determine position and broadcasts that position (along with a callsign) on a fixed frequency in the 2 meter amateur radio spectrum. That broadcast is picked up by internet-connected gateways that again relay the location to APRS services like aprs.fi and others. It does requires a ham radio license to use, which is probably a deterrent for some people. Fortunately, I’ve had a technician class license since I was a freshman in high school (made me extra popular with the ladies). From the altitudes we’ll be flying, a relatively low power unit can broadcast hundreds of miles, so we should have a pretty reliable system working for us.

Originally, I planned to use a Tracksoar APRS beacon as our primary tracking system. When they fell short of their Kickstarter funding target, I started checking out other options and quickly settled on a Beeline 2M APRS beacon from BigRedBee. I have another one of their 70cm (non-GPS) tracking beacons (beep, beep, beep) that I’ve used for a few rocket launches. It arrived back in January and I’ve been playing around with it since.

The BRB system has a couple of drawbacks from my point of view. First, It’s not an extensible, open source system. I would really like to latch some additional sensors onto the board and have their data recorded alongside the GPS data so they are synchronized. Not really an option with BRB. The second drawback is the programming software; it’s only available as a Windows application. We have four computers in our house…all Mac. There are ways around this, but none are convenient. This is the type of issue that is easily solved with an open source project. At the same time, I can appreciate why someone would choose to keep their system closed and protect their IP. Despite those limitations, the strength of the Beeline is that it’s flight proven in both rocketry and balloon applications, with hundreds, if not thousands, of launches. The Beeline unit is also much more powerful, with 1W and 5W transmit modes, depending on the input voltage.

Testing & Modding the Beeline

After I got the Beeline unit in hand, I did a couple of tracking tests. I fired up the unit, dropped it in my backpack and started walking. I used aprs.fi (both the website and the iOS app) to see how well it was tracking my location. It did a nice job keeping a bead on me as I wandered around the waterfront near my office. The unit was hitting an APRS iGate about 12 miles away, on the other side of the bay (San Pablo). When I got in and around taller buildings downtown, the systems struggled, which was no surprise. This obviously won’t be an issue when we’re using it in a balloon payload.

Beeline Test

The beeline came with a customized pelican case and battery pack that held six AA batteries. It’s a nice, sturdy package, but a bit pudgy for something being carried aloft by a surplus of He2. I weighed out every component of the system:

  • Beeline transmitter: 31.9 g
  • Empty battery holder: 50.9 g
  • Empty pelican case: 167.9 g
  • Energizer lithium AA batteries (6): 87 g
  • Whip antenna: 11.2 g

During the flight, the Beeline unit will be encased in a foam cocoon, so the pelican case is redundant and unnecessary. The other area to save weight is power system. The included six-cell AA system is heavy and bulky. The Beeline needs a minimum of 4V to transmit at 1 watt, which should be more than enough for our purposes. I picked up a 2,000 mAh LiPo battery from Sparkfun. LiPo cells output 3.7V, so we’re running it through an Adafruit PowerBoost to bring it up to 5.2V. Initial tests look good, though we need to run some extended tests to see if 2,000 mAh will have enough capacity for a two- to three-hour flight. If not, we’ll push it up to a larger capacity cell.

Beeline Mods

I also swapped out the whip antenna for a high-quality 2m rubber duck antenna. It’s a bit heavier, but more compact. The updated power system is a quarter pound lighter, and the total system is down by over half a pound:

  • Beeline transmitter: 31.9 g
  • Duck antenna: 36.9 g
  • LiPo battery (2,000 mAh): 37.5 g
  • PowerBoost: 5 g

Next up are some battery capacity tests and then we’ll begin dialing-in the payload bay itself. I’m still trying to decide if we should have a backup system (like a SPOT) for redundancy. We can handle the extra weight, but it adds another $200 or so to the cost of the launch – something I’m not eager to do.

 

Questions & Answers

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One of the reasons I stalled on my L2 build was because I had to make some key decisions about the recovery system and was looking for some guidance from a more experienced builder. I thought it would be easiest to corner a friendly L2+ flyer and pester them with questions until they cried “uncle”.

A couple of weeks ago, I organized my questions and posted to the LUNAR Google Group looking for a volunteer. The general response was, “Just post the questions to the list, someone else might benefit from the discussion.” So I did, and the response was awesome.

My questions concerned deployment redundancy, shear pin use in a cardboard/paper airframe and ground testing. You can view the full thread here on the LUNAR Google Group. This is what I took away from the discussion:

  1. Altimeters fail. eMatches fail. Redundancy is a good policy. Just how far you want to go is a decision you need to make for your project. I’m taking that principle and designing my av bay and deployment system with flexibility in mind. It will have the potential to support two separate altimeter systems and two separate deployment charges for each parachute.
  2. Shear pins aren’t strictly necessary for lower power, lower altitude flights as long as you can get a tight friction fit. Drill vent holes to relieve the pressure differential that increases with altitude and can cause airframe separation. If you do use shear pins on a cardboard/paper airframe, treat the holes with CA glue and re-drill. This provides enough strength to prevent too much damage during deployment.
  3. There are eMatch options for folks who don’t want to go though the hassle of holding a LEUP. Bay Area Rocketry carries them.
  4. Ground testing is absolutely essential to confirm the size of your deployment charge and work out any other bugs in your system.
  5. L2 certification does not require redundant electronics. When testing for L2, go for a simple, predictable approach.

I’m sure there’s going to be many more questions as this build progresses and the I approach my first flight. Looking forward to the next step of the build.

O-Ring Failure

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My last launch was almost four years ago. At the time I was newly certified at L1 and excited to get more mid- to high-power flights under my belt. It was July so Snow Ranch was in its off season. I drove down to Maddox Ranch to join the Tripoli Central CA club for a launch.

I was flying my LOC Forte on H power motors. It’s a very simple rocket that uses motor deployment for the recovery system – a glorified model rocket. The first flight of the day was on an CTI H225-11. It screamed off the launch pad and flew to an estimated 3,200 feet. The chute deployed at apogee and it landed nearby with very little drift on a calm day.

Feeling confident, and with another reload ready, I prepped the rocket for a second flight on a CTI 266H125-12. The take-off was clean, but just over a second into the flight the airframe shredded and the LCO jumped on the PA to warn about falling debris. Totally bummed, I collected the bits and pieces of my rocket which were scattered over a couple acres of dairy farm.

Upon initial inspection, I was confused. The airframe had zippered and torn into two pieces. The kevlar shock cord had snapped and the nomex chute protector had ripped straight through. Amazingly, the shock cord mount – nothing more than a simple epoxy joint inside the airframe – held fast.

Airframe Zipper

Recovery Harness Mount

The motor was still mounted in the rocket, so I couldn’t figure out what happened. I drove home trying to think through what went wrong. Later that evening, I unscrewed the motor retainer and was greeted by nothing more than the rear thrust ring and nozzle. The entire reload case was missing…nowhere in sight. After a few minutes scratching my head and examining what was left, it became obvious.

O-Ring Failure

The rear o-ring had failed (see 8:00 on the photo). This blew the seal on the rear enclosure. At that pressure (still under thrust), the motor case separated from the rear enclosure. The motor case, no longer held in place by the thrust ring, flew straight through the body of the rocket, scorching the length of the motor tube, blowing off the nose cone and taking the recovery system with it. So, the parachute deployed at about 300 mph, causing the epic zipper and shock cord failure. I never did find the motor case.

Of course, it begs the question: why did the o-ring fail? Well, maybe it was cracked? But looking at the photo, you can see (at 2:00) a small piece of debris, maybe grass or a small leaf. I might have picked up some foreign object when I was reassembling the case after drilling out the delay. Either way, I take responsibility for the failure. I’m pretty sure that a close examination of the o-ring seal would have prevented this incident.

In the long run, I was able to repair the rocket with a coupler and new section of 4″ airframe tubing. I applied a fiberglass patch over the joint. After a good bit of sanding and paint it looks as good as new. I kept the zippered airframe and hung it above my workbench as a reminder to take my time and be careful with my work.

Fin Alignment Jig

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Here is a quick look at the fin alignment jig I made for my L2 rocket build. I wasn’t keen on the standard approach, where you eyeball down the fin to make sure it aligns with a printed template – that always seemed hokey to me. This was a relatively quick build. I drew it in Illustrator and cut it on the Epilog laser at TechShop. It’s made from 1/4″ birch plywood and some dowels I had lying around the shop. Total cost was about $5.

The jig has some nice features. First and foremost it ensures the fins are aligned in all planes: they are evenly spaced around the airframe, they are attached perpendicular to the airframe (no lean), and parallel to the rocket’s primary axis (no twist). I used this jig to tack them to the motor tube with epoxy. The triangular shape provided a nice way to rotate through each fin placement. There’s a small margin at each attachment point so I didn’t accidentally glue the fin to the jig with squeeze out. After these were dry, I removed the jig and followed up with solid internal fillets. I like this design and will use it again.

Here is a link to the Illustrator file in case anyone would like to modify and use it for their own purposes.

Fin Alignment Jig 1

Fin Alignment Jig 2

Journey to Level 2

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It’s been several years since I attended a launch. Back in July 2011, I experienced my first major launch failure when my first L1 rocket shredded at high speed during ascent (more on that later). It was only my third high-power flight, and I was bummed by the setback. But I picked up the pieces (literally) and resolved to get the rocket back into working order.

At the time, I had already started on my next build: a Binder Excel that I planned to use as my gateway to dual deployment and L2 certification. I had the kit, I had the altimeter and I began running simulations in RockSim. And then everything went on hold…for a bunch of good reasons. I picked it up for brief periods and assembled some components, but it’s been in a partially done state for four years now.

I don’t like leaving things undone, and I’m ready to finish this project and get this bird in the air. I still have a lot of questions and plenty of work to do. I’m going to use this site as a place to document the build. Maybe it will be helpful for someone else, or maybe it will just be a good way to organize my thoughts. Posts related to build will carry the hashtag #journeytoL2 and everything related to this as-of-yet-unnamed rocket will carry the hashtag #LEA-6

Getting High

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Just placed an order via Kickstarter for a Tracksoar – an APRS tracker for high-altitude balloon flights. I remember a few years ago when HAB drifted briefly through the public consciousness – I think I even saw it in a car commercial. I made a mental bookmark, promising to revisit the idea when I had fewer projects mid-flight. Since then, it seems to have faded a bit from the maker zeitgeist. I stumbled across a reference to Tracksoar in a recent issue of MAKE: and got excited again. It seems to lower the barrier to entry (time-wise) to get started in HAB. Delivery is estimate for February 2016, so I have a few months to figure the rest of it out. Looking forward to something new.

http://kck.st/1Lbr30i