As I awaken from my fifteen-year rocketry slumber and take a look around the market for altimeters is very different1. All the vendors I was familiar with–BlackSky, AED, Transolve, Olsen, G-Wiz, Adept–are gone. Cheap, powerful microcontrollers, and even whole computers, can be easily fit into rockets thanks to Arduino, Raspberry Pi, Beagleboard, and others. Why not grab an accelerometer breakout board from Sparkfun (like this one, rated to 200g!) wire it up, and gather all the data your heart desires?
I might take this approach for electronic payloads in the future but I do not want to trust the safe recovery of my rocket to my embedded programming skills. It would be one thing if all I wanted to know is how high the flight was2, but, as it is, I expect a lot more from my avionics.
Here’s what I want:
- Small: the altimeters of yore required a 38mm mounting tube, we can do better than that.
- Multi-sensor: lots of altimeters only have barometric sensors. This works great while measuring the descent of a rocket, but it is not particularly well-suited for measuring and recording the ascent of a rocket3. During powered flight, I want a sensitive accelerometer gathering data at a high polling rate.
- Recording: I don’t just want to what the maximum altitude was, I want to be able to inspect the entire flight profile, export the data, and be able to analyze it later.
- Pyro-channels: I want to be able to at least fire drogue and main recovery charges for dual deployment flights. Safe4 staging and air-start controls are a bonus.
- Li-Po friendly: Lithium polymer batteries are light, can deliver high current, and are rechargeable. It is better battery technology than alkaline 9 volts or the NiCads of old.
- Phone-first: I don’t want to have to lug a laptop around with me to configure or retrieve data from my altimeter. I want to use the computer I have in my pocket–my (Android) phone.
The players (in alphabetical order):
- Adrel Electronics: the ATL-BMP (~$70) is a tiny competition altimeter. It is just a 1g data logger; the maximum altitude and flight profile are calculated after the fact using Windows desktop software.
- Altus Metrum: the EasyMega (~$350) is a fairly-compact (fits in a 38mm coupler), multi-sensor recording altimeter designed to work with LiPo batteries. It is also available with a 2m HAM-band radio downlink as the TeleMega (~$460). The EasyMini (~$93) is a dual-deploy recording altimeter that is much smaller (fits in a 24mm coupler) and cheaper but is barometric-only. All three of these altimeters, even the smaller one, have built-in USB ports for downloading flight data to a computer.
- ENTACore Electronics: AIM USB (~$115) fairly-compact, LiPo-friendly, recording altimeter with built-in USB connectivity–barometric only, AIM XTRA also has GPS telemetry and is thus expensive (~$420).
- Eggtimer Rocketry: the Proton (~$70) has all the features I want but the package size is too large and you have to solder it together yourself–that’s why it’s so inexpensive. My soldering and electronics debugging skills are not up to that level.
- Featherweight Altimiters: the Raven 4 (~$160) has a spec sheet that reads like a dream and you can get a 24mm telemetry bay for it! Still expects a USB/PC interface though.
- FlightSketch: only offers the Mini (~$40) right now and that isn’t what I’m looking for. However, the design philosophy is spot-on so I have high hopes for the rest of their product line. The Sport looks just about perfect and the SST (GPS tracker) also looks promising.
- Jolly Logic: the Altimeter 3 (~$100) is a recording altimeter that works with your phone but there are no pyro channels. The Chute Release is super cool though. It makes other modes of dual-deployment possible in such a way that I may need to rethink my criteria for avionics.
- Marsa Systems: the Marsa33LHD (~$200) is a very sophisticated flight computer–you can even get it ruggedized for $20 more–but it’s really big (54mm diameter mounting tube) and is built for a USB/PC interface. MarsaNet is a wireless (!) expansion module system (~$70/RX–up to 2, ~$50/TX) that adds more pyro channels–this is kind of crazy but a cool idea.
- Missile Works: the RRC3 (~$70-$80) and RRC2+ (~$45) are barometric-only with an “old-school” vibe. The RRC3 is a recording altimeter and it can also be used with their telemetry packages to transmit live data to a ground station.
- PerfectFlite: the StratoLoggerCF (~$70) is a small, barometric-only, dual-deploy, recording altimeter. It requires a separate USB adapter (~$30) to download the data to a PC.
Great. No products on the market–at least that I could find–meet all of my criteria. Given the field, my recommendations are:
- Featherweight Altimeters Raven 4 ~$160
- Altus Metrum EasyMini (available from Apogee Components) ~$93
- PerfectFlite StratoLoggerCF with USB adapter ~$100 total
HONORABLE MENTION: Marasa Systems Marsa33LHD (~$200) is super cool–it’s got that serious flight computer look and the specs to back it up. Because of its size, it is only suitable for HPR. Marasa Systems feels like the spiritual successor to AED.
I’m holding out hope that the FlightSketch Sport will be released soon because it is the only product I’ve found that ticks all the boxes.
I know because I am in the market for an altimeter. Technically, I have one already: an AED R-DAS mini that has been sitting in a cardboard box for the better part of a decade. Have the capacitors degraded? Does it need to be calibrated? I don’t know. Also, AED went out of business last year and the only copy I have of the control software is sitting on a 3.5” floppy. It probably still works fine but I’ll have to use a USB-serial adapter and hope Windows Compatibility mode can sort everything out (assuming I can find a floppy drive somewhere). ↩
If you join NAR they send you a copy of the “NAR Member Guidebook” that has all kinds of helpful information. “How High Does It Go? Electronic Altimeters” by Bernard Cawley has a detailed breakdown of 8 model-rocket-sized maximum altitude altimeters. They are all competition certified. ↩
Reasons for this include low polling rates, low resolution, and worse yet if the rocket breaks the sound barrier there will be a pressure wave that makes the readings unstable during the mach transition. (This used to cause all kinds of problems with old altimeters–like parachutes deploying when the rocket is traveling at maximum speed because of a sharp decrease in pressure registered as an apogee event. Then “mach” timers were included to prevent deployment charges from firing for a set number of seconds. Now, I hear this is all avoided with better software but I’m still wary.) ↩
By “safe” I mean: “Don’t start the second stage if unless it is flying straight up or at a safe angle of attack.” This is why you need sophisticated embedded programming at the heart of your avionics. ↩