back to article LOHAN starts to feel the barometric pressure

As the design of our Low Orbit Helium Assisted Navigator (LOHAN) Vulture 2 spaceplane continues apace, we've been considering just how to fire the aircraft's solid rocket motor at a predetermined altitude. Click here for a bigger version of the LOHAN graphic We haven't yet revealed exactly which mighty unit will eventually …


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  1. Tom 7

    Which one are you going for?

    Altitude or pressure?

    If its just altitude I would imagine it should be possible to mod an android phone app to give you the proper altitude - I believe paranoia prevents most sat navigation jobies working at altitude but the signals should still be there.

    As for fuel most of the solid fuels for the rockets I've played with should ignite happily at those low temperatures. Should be fun testing though!

    1. Reginald Marshall

      Re: Which one are you going for?

      If its just altitude I would imagine it should be possible to mod an android phone app to give you the proper altitude - I believe paranoia prevents most sat navigation jobies working at altitude but the signals should still be there.

      Satnav is limited on a lower level: ITAR (the same export regulations that gave us 40-bit encryption 15 years ago) limits the receivers' working range to less than 60000 ft / 1000 kt (~18 km / 1800 km/h) at the same time -- the idea being that you can't easily piggyback your ballistic missile guidance system on GPS. This is usually done in firmware. Theoretically, LOHAN should be OK, since it would reach a greater altitude, but at a lower speed. However, barometric sensors are simpler and accurate enough; there is no reason to complicate the design.

  2. Geoff May

    A very long piece of string

    No text.

    1. Captain DaFt

      Re: A very long piece of string

      You beat me to it... except I was going to say "fuse" instead of "string".

  3. Richard 72

    Have you tried Ardupilot ?

    Just asking...

  4. Neil Barnes Silver badge

    Pressure sensors

    The problem you're going to have is that at 100,000 feet the pressure is only just over 1kPa (as you've noted) but it's getting thinner up there very slowly - at 70,000 feet it's still only 5kPa.

    So you're really going to need something with a range of no more than about 10kPa to have any hope of being within even a few thousand feet of your selected altitude. MPX10 works in that range, though it will require temperature compensation - it may be simpler just to arrange a small circuit that will trigger once the pressure drops to a predetermined level.

    The problem is, though, that they burst at about 100kPa - i.e. ground level - so it's going to be really fragile if you launch on a high pressure day, or even in storage, with a gauge type. A two-port differential type doesn't really help you either.

    It may be that you have to consider some mechanical approach, as with Paris.

    1. Arrrggghh-otron

      Re: Pressure sensors

      How about a small balloon (or part thereof) partially inflated at sea level that presses on a micro switch when the balloon inflates?

      Sounds too simple, I must be missing something...

      1. Celeron

        Re: Pressure sensors

        I agree a mechanical solution would be simple and should be reliable. As an alternative to the baloon concept what about a syringe. The air in the cylinder will exapnd at a predictable rate and the plunger will press a sensitive switch. It would be repeatable and testable without affecting the results, unlike a baloon who's rubber/latex may degrade and change each use.

  5. Faye Berdache


    Any GPS system should do altitude as well as position (it's what it was designed for). Should be accurate enough. Though I'm not sure if any phones will do the job, as most are Assisted GPS, which relies on land based transmitter triangulation and the algorithms are only concerned with calculating map positions, not heights.

    1. phuzz Silver badge

      Re: GPS?

      GPS is designed more to give position rather than altitude. It can be used to find your altitude, but it's not as accurate vertically as it is horizontally, also, the hight precision will go down as your receiver rises.

      That said, it should prove accurate enough for LOHAN.

  6. Yesnomaybe

    Fire at point of bursting

    Ideally, the rocket should fire at exactly the point where the baloon bursts. Otherwise there is a very good chance that the baloon will reach a greater height than the rocket-plane. Rather embarassing, your plane would go higher if you DIDN'T fire the motors. So perhaps some sort of strain-gague/switch combo on the string holding the mighty truss.

    1. Magnus_Pym

      Re: Fire at point of bursting

      I asked about how much altitude the rocket is expected to add to the total flight but no one has come up with an answer yet. It could easily be less that the margin-for-error launch altitude takes away.

      I was wondering about using two balloons. The launch is triggered by the loss of one balloon. I can't see a reliable way of measuring this though. I think they can be given a large enough separation to prevent the explosion of one bursting the other by having one on a much longer string than the other. The main problem I see is the turbulent lower altitude phase interfering with any reliable measure.

      What about this? Two balloons one high above the other. Each balloon's string is attached to it's own tether on the top of a small switch block. A single string then passes down from the bottom of the switch block to the payload. Each tether is attached to a switch mechanism. A spring pulls the switch towards the 'closed' position and the 'lift' from the balloon acts against it. When either balloon bursts 'lift' is lost and it's switch will be closed. A timer is set to allow enough time for the craft to have passed the turbulent levels of atmosphere before the system is armed. A simple circuit can then fire the rocket if the timer has elapsed and either one of the switches is closed.

    2. Poor Coco

      Re: Fire at point of bursting

      It’s even easier than that: an accelerometer will tell you, since very shortly after the burst the gravitational forse sensed will have gone from ~1g to ~0. The problem with the idea is that you have to design the launch pad to provide directional control when it is itself in free-fall.

      1. Gordon 10

        Re: Fire at point of bursting

        Isnt it even easier? Just put a strain gauge or similar on the balloon string. Strain drops to zero > 1 second - fire.

        1. Magnus_Pym

          Re: Fire at point of bursting

          @Poor_Coco: I suggested launching from a dropping platform on the LOHAN discussion forum. It was soundly beaten down due to the inability to control the direction of launch with the platform in free fall.

          @Gordon 10: It is a strain Gauge isn't it? delaying for one second and resetting after each false alarm adds to the complexity of the circuit.

          After one balloon burst the payload will loose altitude but only slowly and in a controlled manner. The timer is required to prevent a premature due to buffeting by strong winds in some parts of the atmosphere.

          Also it would tend to fail safe because in the event of a balloon bursting early the rocket will be fired immediately on time-out. I think it would be possible to put the switch block in a flexible airtight enclosure (with some silica gel or something) to prevent damp and cold from interfering with the operation.

          1. Poor Coco

            Re: Fire at point of bursting

            Hi Magnus. You should stop by the forum again — Lester read the riot act to the trolls and now they seem to be (a) behaving and (b) reading each other’s posts. Shocking, no? But don’t worry about the naysayers. I suspect these are people who never studied the principle of dynamic equilibrium.

            About the freefall launch platform: it certainly IS possible to get a stable launch from a freefalling platform, and I think it is mandatory that we build that capability into the LOHAN launcher so our flight is not completely stuffed by a premature balloon rupture.

            How would it be possible to stabilize a freefalling platform? Gyros of course. That would maintain the overall orientation of the platform. Next, you would have to make the launch happen REALLY FAST, which means a piston-launcher of some sort (probably detuned, as composite motors deliver lots of thrust promptly) which will apply a large impulse force to the launcher and the aircraft simultaneously to kick them apart.

            So, with (a) small gyros to maintain orientation, (b) a design that places the launch forces through the CG of the aircraft AND the launcher, and (c) a detuned piston launcher, you can get reliable performance when it’s hanging by a thread OR in freefall.

    3. Mike Manes

      Re: Fire at point of bursting

      Balloon burst is a very chaotic event. Not only is there a mess of latex flying every which way,

      there's an abrupt change in line tension, and an abrupt change in G's on the payload which

      would likely alter your carefully tweaked launch lug orientation. So it's best to launch the rocket while the balloon is still ascending. EOSS has flown 175 flights up to the 10 mbar region, and

      one nasty event which we have yet to reliably overcome is PBC (Post Burst Chaos), which

      rapidly evolves in just a few seconds.

      We have flown a number of solid state barometric sensors, and they're OK below about 50K',

      but I'd recommend simply carrying a GPS engine and feeding the NMEA serial output to your payload uP; use the $GPGGA sentence to read MSL elevation in meters.. Our experience is that GPS is good to +/- 500 ft or better over an entire flight. But you MUST get a GPS engine that is designed to report above 63K' - many don't. We use old Rockwell Jupiters (newest firmware) and there's a list of others that are proven to operate well above 100K' on

      Good luck!

      Mike W5VSI, CTO EOSS

  7. binsamp

    Go the other way. Put a small amount of air in a balloon.

    It will expand with altitude.

    When it reaches the desired altitude, close a microswitch.

    Mike Monett

  8. Bubba Von Braun
    Thumb Up

    Low pressure ignition

    As someone with experience in non-professional rocketry, I can tell you that at the sort of release altitudes you are planning ignition will be a small problem.

    The low pressure will be your biggest factor you will need to keep the igniter in the motor until you get sufficient heat to ignite the propellant (550 degrees F is required). You must achieve this without blocking the nozzle as once ignited the pressure will continue to build.

    A note of caution APCP (that’s the propellant in use in your selected motor) and from memory the Aerotech propellant formula is their Black Jack variety, a Smokey propellant :-) will continue to increase the pressure until Lohan has a CATO so use caution in how strong you seal the chamber.

    Once the propellant reaches 550f then the reaction will be self-sustaining as the APCP will provide the needed oxygen and heat to continue the process until burn-out of your main engine. However should your motor chamber be exposed to the near vacuum before the engine is up to full pressure it may chuff and not burn successfully.

    Unfortunately due to the small motor selected my favourite igniter technology copper/magnesium-thermite is unavailable to you.

    So the lesson will be, keep the igniter sealed in the motor but not so tightly that engine and Lohan are recovered in flaming pieces.

    Onward and Upward, want more information visit (A Tripoli members recent project.. and a little traffic Lohan needs to watchout for)

    Bubba Von Braun

    1. Lester Haines (Written by Reg staff) Gold badge

      Re: Low pressure ignition

      Thanks for the advice. We'll see how our REHAB test goes. Assuming we're using a bigger motor, what's your opinion of Magnelite igniters?

  9. Bubba Von Braun

    RDAS and Altitude

    No commercial Barometric sensor will give you an accurate reading. You will need to to use GPS. RDAS has a uBlock based GPS that should work fine, although last time I checked it wasnt integrated in the firing chain, thats a Baro/Accelerometer based event on the RDAS.

    Oh and thoughts on waiting until it bursts maybe unwise, you will need a few seconds before the motor comes up to pressure so I would think you would want a stable platform for release.

    Your other option is to link it to a mobile phone.. cellphone coverage would be awesome at those altitudes. ;-)

    1. Poor Coco

      Re: RDAS and Altitude

      No, cellphone coverage would be nonexistent at those altitudes; it’s pretty much finished by 10,000 feet AGL.

      1. imanidiot Silver badge

        Re: RDAS and Altitude

        That statement is definitely false Coco. Reception with a standard cellphone antenna might be bad above 10.000 feet, but a bit of modding with an improved antenna would get you cell reception well above that altitude. (My standard nokia 3310 still has very good reception at over 7000 feet. (Thats as high as I've ever been with my phone turned on anyway)

  10. Stephen 24


    How about sound activated when the balloon pops?. Or doesn't it sound as loud in the thin air? Might make launching it a bit riskier...

  11. Mostly_Harmless Silver badge


    Given that gravity is a constant, can we also say that rate of lift will be constant (or, at least, calculable)? If it is, then potentially you could just use a timer

    1. druck Silver badge
      Thumb Down

      Re: Timer?

      Gravity isn't a constant with increasing altitude, although that is only a small factor. A far bigger one is an extremely non constant and turbulent atmosphere the balloon is rising through, which will make timing of the rise highly variable.

  12. Robert Heffernan

    Sparkfun order

    I have assembled (in prototype form) a rocket ignition and telemetery system from parts ordered from Sparkfun, basically it was an accelerometer, barometer, gps, sd-card reader for logging, arduino, Bluetooth module for pre launch comms, and then some FET based output for switching igniters, etc.

    With open source driver code and plenty of pre existing libraries for the hardware, writing firmware to do the job is simple.

    That way you don't need to hack on a commercial unit to do what you want and you can release the schematic and source as part of the project. I am sure there would be plenty of people out there willing to send you a firmware package, schematic and bill-of-materials based on the requirements of the project.

  13. mamsey

    Very long blue touch paper?

  14. Vordicae

    if you do decide to strap-on a Droid phone to LOHAN as she gently ascends to the point at which her cherry pops...

    I look forward to her oxygen deprived tweets ~! might make for happier reading than the ... other ... Lohan ! .. or maybe make more sense at least.

    is the rate of ascent a calculable figure ? if you can't get the right baromentric trigger.. then a combined barometric that lights a fuse (or a timer I suppose if you prefer new fangled gadgets) that should coincide with the correct altitude?

    or even a ground based trigger ... (Bagsy volounteering to press BRB (Big Red Button)) when a remote sensor reads the correct altitude ?

  15. lampbus

    Use both.

    As others have already said, you don't want to fire before balloon burst cos that would be silly. And not optimum mission.

    Detecting burst is simple - tension on the support string will almost instantly drop to zero. Depending on rate of climb, G will decay to zero - she has mass and will continue to ascend after loss of baloon lift. This decay will be quite rapid too.

    Potential problems : Swinging about could cause a false trigger - having seen a couple of other project videos with alarming swinging going on. This is where the barometric is used - to inhibit firing below a chosen altitude. THis will aid ground handling and launch.

    Stability after burst is a big problem, but a pre-deployed drogue 'chute just below the balloon would help there. This would not cause significant drag on ascent and may also help dampen out swinging etc on ascent - the baloon is a large spring in itsself.

  16. Paul 185

    launching on balloon pop sounds like the better option to me. chute below the balloon sounds like a fine idea as well; balloon pops, G drops, chute opens and stops it short, wait for the platform to stabilise then rocket fires. It's the most simple option, with a built in fail safe and guaranteed optimum height. Why settle for anything else?

    1. Poor Coco

      A parachute at 80,000 feet would NOT “stop it short”, it would provide a modicum of directional stability as it fell in almost 0g. This may work, but I suspect a few small fast gyros on the platform will do the trick quite nicely.

  17. StampedChipmunk

    Edge 705?

    How about a Garmin Edge 705. It has a GPS, a bariometric altimeter and is sealed in a waterproof & dustproof case.

    Has a two way USB port on the back and has a set of open APIs available so sites can download and make use of the device's data (though I don't know any more than that). It also is an ANT+ device which the latest generation of Smartphone can communicate over, so that's another possible avenue to explore.

    I can vouch for its toughness and reliability as well as its accuracy and long battery life. It also doesn't weight very much and could be easily mounted to the truss...

  18. Francis Vaughan


    The temperature varies with altitude quite nicely, and it actually starts to increase again above about 80,000 feet. Around here you get about 1 C every thousand feet, which isn't difficult to track with a simple semiconductor thermometer.

    Clearly there are variations, but this altitude is above pretty much all of the weather, and you should see a stable relationship with temperature to altitude as good as you see with pressure.

    What you might want to look at is the NRL empirical model that can give you a prediction of the temperature versus altitude taking into account current conditions and location. Wikepedia here: and in particular check out the links at the bottom of the page.

    1. Poor Coco

      Re: Temperature

      Good idea! I wonder if it’s possible to get more refined data for a given location at a given time of year?

      Part of the problem is the slope of the change, which as you say is positive above 20km but with a much more gradual slope than the –1°C/1000' near ground level.

  19. sebacoustic

    superfine loose wire 'round balloon

    Use the "target diameter" of the balloon.. you don't want to go all the way to "POP" but pretty close. You could loosely sellotape a single strand from a high-flex copper wire all the way around the balloon, and some kind of relay that closes when the wire snaps. That way you can actually get really close to the maximum height your balloon allows.

    This is delightfully "shed-engineering", and reminds me of using similar strands wound round fireworks' fuses, as electric ignites, during my misspent childhood.

    1. Poor Coco

      Re: superfine loose wire 'round balloon

      I don’t think so; even a really thin copper wire will be tougher than the extremely thin stretched latex of the balloon at altitude. Worse, the tape on the balloon would induce major stresses and probably cause a premature burst.

  20. VeganVegan

    Sensing balloon burst

    Assuming that you want to sense the bursting of the balloon to use as the ignition trigger, the most direct way is to sense the pressure drop in the balloon itself:

    Inflate the balloon, and then seal a pressure sensor in the inflation tube. The pressure should be positive (one would hope). When the balloon bursts, the pressure goes down, the feed from the sensor is then used to trigger ignition.

  21. Dyspeptic Curmudgeon

    GPS Altitude

    It appears that a GPS derived altitude is the only method which would give any sort of precision in regard to exactly what altitude the launch takes place. Here is a very small GPS unit which interfaces through an SD card slot:

    Very cool little thing, and quite possibly exactly what you want. A quick troll through the SPB/LOHAN archive reveals no information about the nature and capabilities of the "Avionics Package". Maybe another article with specs will cloudsource some amendments to the present design?

  22. PC Paul

    Another off the wall suggestion..

    Could anything be done with optical sensors? Model autopilots often use two simple opto sensors to detect the horizon, the difference between above and below is easy to detect. Ideas:

    - Monitor the curve of the earth with a row of sensors to judge altitude. Probably doesn't change quickly enough to give good resolution.

    - put a sensor inside the balloon to spot the burst point

    - watch the balloon from the gondola to spot the burst point

    I notice on many balloon launches there is a fair old spin going on by the launch point. Can this be stopped, maybe with the same gyros that keep it stable after the pop?

  23. chris 143

    balloon or similar?

    The simplest and probably most reliable solution is to have a balloon that expands as the pressure drops pressing a switch. I'd probably suggest that you'd want two of these in case one fails (better for it to launch early than not at all). It'd be simple, relatively easy to test and lightweight.

    A 'launch when the string breaks' system is probably unnecessarily complex. If the objective was just to get as high as possible, you'd be better to abandon all the rocketry and simply build a system as light as possible and let the balloon do the entire lift.

  24. SirDigalot

    can't you..

    train the playmonaut for this? what do you pay him for? right now all he has to do is be inside LOHAN...

  25. chris 143

    now if you were willing to use two balloons

    you could launch when the first balloon burst, the second ballon should reduce the rate of fall/keep the assembly pointing up long enough to launch.

    Also you could use a similar system as you are using to ignite the rocket motor to burn though the cord holding the remaining balloon

  26. Tempest8008

    We have two scenarios.

    1. The launch occurs at a predetermined altitude.

    2. The launch occurs at balloon burst.

    I know the SPB does not want to rely on a launch-at-balloon-burst scenario. Too much chance of an unstable launch platform.

    But what happens if there is only the altitude sensor, but the balloon bursts before the pre-set altitude is reached?

    I'm thinking two switches might be in order, or at least some logic in the altitude sensing system that will put some willy up LOHAN if a rapid accelerating descent is sensed.

    If two switches, then have one inside the balloon that is a pressure sensor, and when a rapid drop in pressure is sensed it will attempt to fire the rocket; the other sensor would be the altitude sensor with the pre-set ready to fire when the desired height is reached.

    Some effort to effect a launch if the pre-set altitude is NOT reached should be investigated.

  27. Anonymous Coward
    Anonymous Coward

    Boiling points

    Liquids boil at a certain pressure / temperature point. Keeping a capsule of liquid at a set temperature, say 20C is relatively easy. So, is there anything that will boil at 20C when the pressure goes down to the appropriate level? Boiling will give a very large volume change what could be detected with relatively simple sensors. You could even detect the absence of the liquid using some float based gauge.

  28. Anonymous Coward
    Anonymous Coward

    Consistent to the end.

    So, you're using the MPXS4100?

    The Motorola MPXS4100 Barometric Absolute Pressure or BAP sensor?

    Perhaps you should be using two of them to enable sanity checking that way when LOHAN goes into REHAB there'd be a nice pair of BAPs for you to look at.

  29. Anonymous Coward
    Anonymous Coward

    Don't see a reason why, electrically, you couldn't fit almost any pressure sensor with a suitable high level output, it's then a case of tinkering with the calibration of the R-DAS, either on board or in Excel afterwards, you just need to pick a few data points to calibrate the span as these sensors are pretty well behaved.

    Shame the R-DAS isn't open source, had you considered using an ArduinoMini or an MBed instead perhaps?

  30. Acadene

    PARIS - LOHAN twosome turn on


    Use the KISS principle. Electronics introduce complexity and cost. More complexity = more to go wrong. There is an item that you have used that already delivers at 90,000 ft....PARIS' release mechanism! Adapt it to close a circuit and fire up the rocket . It's pretty bulky I know, but you're going to need a larger meteo balloon anyway with all that tackle blowing in the breeze. Even so, you could miniaturise the principle.

  31. roomey
    Paris Hilton

    Post post burst launch

    I think the decision has been made,

    But what if there is a post post burst launch, ie. ballon bursts, as the assembly drops, through very little atmosphere (so ideally little turbulence), a drogue stabilizes the launch platform and allows a launch.

    1. Magnus_Pym

      Re: Post post burst launch

      Due to the very little atmosphere a drogue will have very little effect. You would need an enormous canopy and that would be very difficult to deploy. Hence the massive groundswell of support for the two balloon system.

      1. M Gale

        Re: Difficulties of an enormous canopy...

        ...there aren't any! Well, asides it being enormouse. Use a round parachute and attach the balloon to the centre of it, like "proper" meteorological set-ups use. The parachute is effectively already deployed, and is just waiting for the balloon to burst before the drop fills it up with air.

        I can't see why the parachute can't be whatever size you like.

  32. Anonymous Coward
    Anonymous Coward

    Two balloons

    Use two identical balloons.

    The first balloon provides primary lift and is located above the second balloon. The first balloon is tethered to launch platform with a spring loaded switch/tension gauge.

    The second balloon is located below the first balloon and is inflated to 75-50% of the first balloons pressure. The second balloon must provide enough lift for greater than neutral buoyancy for the launch platform. The launch platform is tethered to the second balloon by at least two tethers attached to opposite sides of the balloon. These tethers can be used to provide a maximum launch angle for the launch platform.

    When the first balloon pops, the second balloon and launch platform continue to ascend at a decreased rate and the spring loaded switch/tension gauge connected to the first balloon's tether triggers the launch sequence.

    1. Magnus_Pym

      Re: Two balloons

      As far as I am aware you can't attach things to the side of the balloon. It's just a very thing latex skin. However the two balloon system is easily the best. Two identical balloons are unlikely to burst at exactly the same moment and the difference in lift can be used to fire the rocket while the remaining balloon provides stability to the platform.

      1. imanidiot Silver badge
        Thumb Down

        Re: Two balloons

        Since the balloons are so damn fragile at altitude, the first balloon bursting is actually rather likely to destroy the second one as well.

        1. Lester Haines (Written by Reg staff) Gold badge

          Re: Re: Two balloons

          Yes, that's my concern about a two-globe set-up.

  33. Schultz

    Get a vacuum gauge

    Wouldn't something like the Freescale MPXV6115V fit the bill? You might want to insulate and mildly heat the sensor (Styrofoam is your friend, and a little heat goes a long way to keep things from icing up).

    Just the first thing that popped up on Digikey.

  34. Anonymous Coward
    Anonymous Coward

    Stabilizing shirt

    Place a loose umbrella-like skirting around LOHAN which hung downwards due to gravity and downward air passing it during take-off. Once the helium balloon bursts, the rocket begins to fall and the umbrella turns inside-out, triggering launch. A correctly designed skirt could stabilize the launch platform and slow the rate of descent. Most importantly, you get to mess about testing designs.

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