back to article LOHAN fans drawn to magnetic coupling

Earlier this week, we published results of some tests on the proposed heater for our Low Orbit Helium Assisted Navigator (LOHAN) rocket motor heater, designed to keep the Vulture 2 spaceplane's powerplant toasty at altitude. Click here for a bigger version of the LOHAN graphic We also invited our tech-savvy ballocket mission …


This topic is closed for new posts.
  1. DJO Silver badge

    Rare earth magnets, not so rare

    A quick shufti online located a budget pack of nickel-plated neodymium micro disk magnets ranging from 1mm diameter, with a pull force of 25g, to 6mm, offering a pull force of 1.3kg.

    Buying magnets! I'm disappointed in you - you should open up some old disc drives for the magnets within, like a proper bodger.

    1. Pookietoo

      Re: you should open up some old disc drives

      Use those magnets and the rocket's going nowhere.

      1. M Gale

        Re: you should open up some old disc drives

        Use those magnets and the rocket's going nowhere.

        Maybe in one piece. Damn those things are powerful.

        Also brittle though. Hint hint.

    2. Michael Thibault

      Re: Rare earth magnets, not so rare

      Re: Buying magnets

      Shocking, simply shocking! Yoofs!

      In some drives, there are one or two small magnets (never more than 4-5mm on any edge, and down to 2-3mm on each edge of a cube) that might serve here. Unfortunately, when you heat a neodymium magnet, at some point it ceases to be a magnet, so using such magnets in circuit will require a bit of a careful thinking. On the flip side, the effect of high heat on the magnetic properties of neodymiums could actually be useful.

  2. Gordon 10

    Magsafe FTW

    I cant believe ElReg havent got a few dead rats Mac PSU's lying around that can be repurposed for free.

    Or even nick Lewis' when he's not looking.

    Call yourselfs journo's!

  3. Pet Peeve

    Yay, I am helping! TeeCee's version sounds really solid.

    Be really wary of pull force on magnet sites. They are magnet-to-magnet or magnet-to-surface, without anything under them. Pull force goes down really fast as you shove stuff under the magnet, so you'll need to pick a few sizes and experiment.

    Or, use my favorite little magnet: These are super-cheap, and you can add an extra one if you want more force. I have a bunch of them on my fridge holding up photos, and they're surprisingly strong considering how tiny they are.

  4. Michael H.F. Wilkinson

    How about a "BIGGER HAMMER" variant of flaming

    Use a strip of magnesium as a conductor. In the path of the rocket exhaust it should burn up readily (brightly).

    There might be a delay, but it does not rely on melting

    And then I just love the extra fireworks display

    Icon, because, ..... well guess

    1. Vulch

      Re: How about a "BIGGER HAMMER" variant of flaming

      Will there be enough oxygen around at altitude for the magnesium to catch though?

      1. Michael H.F. Wilkinson

        Re: How about a "BIGGER HAMMER" variant of flaming

        Good point. Maybe coat it with potassium perchlorate, for that extra oomph.

        I just love things that go BANG

      2. breakfast Silver badge

        Re: How about a "BIGGER HAMMER" variant of flaming

        It would also be in competition from the hydrogen in the balloon, when they finally get around to fixing that.

    2. KA1AXY

      Re: How about a "BIGGER HAMMER" variant of flaming

      The pros at NASA use explosive bolts, I believe.

      You're not chicken, are you?

      Explosives make everything more fun!

      1. FrankAlphaXII

        Re: How about a "BIGGER HAMMER" variant of flaming

        Yeah they do, in some launches depending on where you or the feed camera are you can see the bolts go off before the exhaust from the rocket obscures that part of the pad, also depending on weather.

        But its not a solution in this case. The amount of force that those bolts generate when they explode would more than likely destroy both the launch platform and the rocket.

        1. M Gale

          Re: How about a "BIGGER HAMMER" variant of flaming

          But its not a solution in this case. The amount of force that those bolts generate when they explode would more than likely destroy both the launch platform and the rocket.

          What about two matchsticks glued head-to-head and wrapped up in tinfoil?

          I know, a silly idea. Would be fun to watch though.

  5. horsham_sparky

    Why Re-invent the wheel?

    There are already technologies for quick disconnect, the simplest and most reliable is spring pins, good example here;

    The biggest problem which you've missed with the other "contact" solutions is these connections tend to fail or increase contact resistance with environmental factors (moisture, temperature, dust etc). Vibration also plays a big part, particularly when combined with the harsh environment LOHAN will be subjected to.

    The spring pins are designed for reliable contact under harsh conditions, the special sauce being the hard gold over nickel plating, and the spring which applies a constant precise force to the contacts

    I've got a lot of experience in this area, so feel free to get in touch ;-)

    1. TeeCee Gold badge
      Thumb Up

      Re: Why Re-invent the wheel?

      You may be onto something, but looking at those I'd be wanting to see 'em work at minus god-knows-what after condensation and freezing has had its way with the mechanism and the extreme cold's worked its magic on the springs.

      1. horsham_sparky

        Re: Why Re-invent the wheel?

        We did environmental testing on these down to -10C.. can't remember what the humidity was (don't have access to the test reports anymore). The spring mechanism is pretty well sealed inside the unit, however if you're worried, you could always put a dob of low temp grease on the barrel immediately after soldering (this will drive out the moisture and guarantee the seal)

        my best advice would be to get hold of some, do the above process and do some testing at representative conditions. Remember that putting a multimeter on it to test the resistance is not representative, you need to put the nominal load current through it and monitor that. (the reason being, high current will clean the contact area by arcing and vaporising any microscopic contaminants)

        testing these things can be a project on its own! best to start with something engineered for job than a garden shed solution (sorry fellow boffins!)

        1. horsham_sparky

          Re: Why Re-invent the wheel?

          p.s. meant to say you can buy these from Farnell, about 50p each :-) don't forget to by the mating half, the contact pad is as important as the spring pin.

    2. Chris H

      Re: Why Re-invent the wheel?

      I agree with you on using spring pins having used them before - they're great and easily mounted by drilling a couple of holes in a block of nylon (so long as you don't crush them with a grubscrew or fill the hole in the back with epoxy mounting them!). There are types designed for mating to flat surfaces (PCBs) - you don't always need the mating part.

      The magnets wouldn't have too much of an issue with vibration though - momentary loss of heater current wouldn't be an issue.

      1. horsham_sparky

        Re: Why Re-invent the wheel?

        In this case gold pads on a PCB would be OK. However if you need any significant amount of mating cycles, then the normal gold flash is insufficient as it wears away too quickly. In that case you need electroless gold over nickel plating (same as they use on the finger contacts on PCI cards), which is a much thicker and harder gold plating :-)

        Another idea if the worry over the spring pins freezing is too much, you can also use battery contacts (like they use in smart phones). an example here:

        might also be worth a look, though I've never used them in harsh enivornments, so couldn't vouch for the actual contact quality of these

  6. Neil Barnes Silver badge


    As described, the #6 magnet idea has the same issue as most of the others: dangly wire short-circuit potential from the battery.

    Put the steel strips on the battery pack and the wire/magnets on the heater so the wires go with the rocket. Or even modify the sliding strip approach so that the contact is a rigid steel strip on the rocket mating against a steel strip on the launcher, with the magnet between the two to hold them together and provide the conductive path. Sliding force on a magnet is a lot less than pull release force...

    Provided we don't dunk the thing in salt water that ought to go...

    1. M Gale

      Short Circuit Avoidance

      Tape or glue the magnets onto a balsa backing board?

      Strengthen the balsa with some thin cyanoacrylate first. When LOHAN slides along the slippery pole and the magnets detach, they stay nicely spaced apart.


  7. StampedChipmunk

    Why not use the blast plate for the power connection

    I may be missing something but why not use the blast plate for the power transfer?

    Instead of it being one piece of aluminium, why not have it split electrically in two (perhaps a chunky circuit board with two big pads) and then have two spring loaded copper pads (use biro springs) in the rear of LOHAN that then transfer the power to the heater (with the whole thing being held in the rear of the plane, which makes for easier wiring).

    So long as Lohan remains in contact with the blast plate, the power should be transferred. The only difficulty I can see is if Lohan slides away from the blast plate during her ascent, but this would also affect the performance of the rocket motor so may have already been solved... In fact the contact pads may actually help protect Lohan from any lateral buffetting she may receive during the ascent...


    1. Stoneshop

      Re: Why not use the blast plate for the power connection

      I was just thinking along the same lines. As Lohan's guide rod is supposedly pointing heavenwards, the craft itself will 'lean back' against the launch plate. With a pair of springs mounted there pushing against contact pads on Lohan's posterior, you should get good enough contact to run the 0.2A the heater requires.

      Another option is indeed the pyro-electrical cutting of the wires, as close to Lohan's body as feasible. And it's no big deal to first cut the heater wires, and then, a few seconds later, start the engine. One could even think up a simple circuit that senses the loss of current through the heater wires and uses that to ignite the motor.

    2. David Pollard

      Re: Why not use the blast plate for the power connection

      If there is a defunct cordless 'phone to hand which can be cannibalised, the connectors for recharging when it's in the cradle might be appropriate. Some have sprung pins designed to tolerate a certain amount of misalignment.

    3. robarm

      Re: Why not use the blast plate for the power connection

      I like the idea of using the blast plate, since you'll already be nestled up against it. I also like the idea of using the Mac connection method, so why not go whole hog and use an induction power supply (like on the HP TouchPad) That way there is no physical connection needed, only proximity and no trailing wires.

      1. imanidiot Silver badge

        Re: Why not use the blast plate for the power connection

        The problem with using the blast plate is that a fair amount of rocking and bouncing is expected during the ascend. The mechanism would have to be able to tolerate quite a lot of misalignment and abuse. I doubt simply resting LOHAN on a couple of springs will provide the needed contact. Not to mention the problems with condensation, dirt, etc mentioned before.

        1. Stoneshop

          Re: Why not use the blast plate for the power connection

          Well, Lohan doesn't just rest against the blast plate,there's also the stops that holds the wings roughly equidistant from the truss, so there may be bouncing along the shaft, but little rotational movement. Which can be dealt with by the right contact plate design.

      2. Stoneshop

        Re: Why not use the blast plate for the power connection

        Induction power supply would require extra electronics in Lohan's body, and I would want to see it working at minus whatever first.

  8. TeeCee Gold badge

    Contact patches.

    Something else occurred to me after writing that: A very light smear of low-temperature grease on those contact patches prior to sticking the magnets on, if you please.

    That should prevent any condensation interfering with contact and also potentially freezing the things into place and providing rather more "pull to disconnect" than planned.

  9. Pete 2 Silver badge

    Look out for ice

    Just make sure that your magnets don't get dewed up during the ascent. The dew could freeze the magnets together, so that the pull-apart force is no longer defined by the strength / size of the magnets, but by the amount of ice holding them together.

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

      Re: Look out for ice

      Yup. We've got some Molykote 33 low temperature grease, which should do the trick.

      See here:

      1. Andrew Newstead

        Re: Look out for ice

        Would this affect the conductivity at all?

        1. M Gale

          Re: Look out for ice

          I guess connect the magnets first then lube 'em up.

          If it's sending power on the ground, chances are it'll continue to do so on the way up.

  10. Spook


    Always with the magnets....

  11. BigNose

    High contact, low adhesion, easy.

    Take a wire, cut it in two, strip wires back 1cm, overlay exposed wires, don't twist together.

    Take lightly oiled paper and apply to 1 side on the covering and heat-shrink the whole.

    The connection is made and enforced by the heat-shrink and the oiled paper ensures that the separation of the 2 wire-ends enclosed is eased. A little tinkering with the heat-shrink maybe required to validate tensions.

    For safety, to avoid exposed wires touching, repeat the process for the other wire, but place the oiled paper on the opposite covering and ensure the 2 wire cust are offset by 2 or 3 cm.

    For each side of the junction, you are left with 1 exposed wire end and 1 covered end, offset from it and the weight added to the whole is neglible (gms).

    Job done.

  12. Iain Thomas

    May be missing something here...


    But where is it written that the heater supply connections have to be the same? If it's possible to arrange conductivity via either the launch bar or the rubber back-stop, that would seem half the solution...

    Additionally, a common theme seems to be concern about shorting the lipo battery; some sort of over-current cutoff or post-launch cutoff would seem to address that concern.


    Regards, Iain.

  13. GreenOgre

    MY shed includes ELECTRO-magnets.

    In MY shed I have relays and other electro-magnetic devices.

    Small electromagnet on the truss retains a ferrous contact plate on the rocket. Triggering the motor igniter would drop the current to the electromagnet (via relay or other, more devious, means) allowing the rocket to depart with zero friction or magnetic drag and zero trailing wires.

    Cheap, simple, effective. What more could a ballocketeer ask for?

    1. M Gale

      Re: MY shed includes ELECTRO-magnets.

      A sodding big battery to keep that electromagnet activated?

      What kind of power do you need for a few hundred grams of stickiness anyway?

      1. This post has been deleted by its author

      2. GreenOgre

        Re: MY shed includes ELECTRO-magnets.

        Sodding great battery already present to supply heater. A coil from a small solenoid would draw less than the heater.

        Gravity would do most of the work but a small latch-plate on the rocket (sprung to allow some movement) would stick pretty well. I will see if I can find some current ratings ...

  14. Bumble
    Thumb Up

    Virtual test facility, available for free.

    In the patient waiting period while the Reg team are blowing things up and magnetizing their nads, why not play an up-and-coming Indie Game / Rocket launch Sim - Kerbal Space Program.

    If you haven't seen it or tried it, you, gentlemen of the rocketry-fancying fraternity are in for a real treat.

    A youtube fan's exploits can be found here:

    The actual game (free demo) here:

    You're welcome.

    1. M Gale

      Re: Virtual test facility, available for free.

      Got the full version, as I'm sure many others here do.

      Currently figuring out how to get 7 mapsat-mod-equipped probes to the Jool system all at once without all the palaver of forming a space station out of multiple bits docked together in LKO (Low Kerbin Orbit). Also mechjeb, because it's an essential mod pack.

      An amazing game (inasmuch as a pre-beta sandbox can be called a 'game'). Shame it's entirely written in C#, so even a moderately-sized rocket stack makes this machine hate me.

      Native Linux version now, though! Go go gadget penguins!

  15. CaptinRik

    Magnets and Electromagnets

    Why not use magnets and metal plates on the rocket side and an electromagnet with trailing wires on the truss side.

    Use conductive gel (think hospital ecg gel / or from REHAB clinic) or copper paste to ensure the connection at low temp.

    The electromagnet would be powered by the inline current supplying the heater.

    Then use a lightweight relay to disconnect the battery or crowbar an inline fuse triggered by the ignition pulse.

    Pros: Clean disconnect, no trailing wires on aircraft, no live wires on truss, control over disconnection.

    Cons: Implementation of low current electromagnets may be challenging

  16. Tempest8008


    Isn't there any way the heating circuit can be energized via induction?

    All you'd need then is an external cradle under the LOHAN with the induction mat wirelessly sending the power through to the heater.

    It would require close proximity, but suitable low temp lube would (as always) allow for vigorous, unimpeded thrusting.

    All the grunt work remains on the truss, and there ARE no cables between the launch vehicle and the launch platform.

    1. Pookietoo

      Re: Isn't there any way the heating circuit can be energized via induction?

      Yes, but it involves a big heavy copper winding on the rocket.

  17. mark_nottellingyou

    Breakable conenction

    How about two relatively thin strips of ordinary tin foil, with a V-notch cut into them. When you pull on them, the notch will quickly run and cause the strips to tear.

  18. Pookietoo

    re: When you pull on them

    That's launch thrust wasted.

  19. Anonymous Coward

    Gravity sucks - use it

    The rocket will, if I read the drawings the right way around, be pointing upwards from LOHAN's mighty rod, will it not? If so, can't the contacts be made with a washer or similar flat metal surface on one, and a spring on the other, to maintain the contact held in place by the sucking action of the planet?

  20. DogTrack

    Maintain contact using rocket mass

    Re: the cordless phone. It rests in a cradle, charging away using only it's own mass to maintain contact and require no extra force to escape. Position simple contacts at the point the rocket rests most force on the launch platform.

  21. Anonymous Coward
    Anonymous Coward

    5 looks good.

    Perhaps a 3.5mm jack plug? Should allow both positive and negative inputs.

    I feel confident that you won't get any problems with LOHAN when you've got a Dual Input-Link Drop Off up her jacksie.

    1. M Gale

      Re: 5 looks good.


      In addition, wrap some nichrome wire around the socket with a bit of insulating fluff as a small heater to stop the jack freezing inside it.

      Nichrome wire is cheap. Look up e-cig suppliers.

  22. M Gale

    Oh yeah..

    Tugging magnets apart requires more force than sliding them off each other. Just a thought.

  23. evs

    Ever played with a slot car?.

    Something resembling a motor brush is the traditional solution to making reliable electrical contact without too much mechanical resistance in one direction. Light, cheap, good vibration resistance. I'm thinking the contact springs out of a large slot car might work though I expect by the time you get everything just so you might find there's an off-the-shelf breakaway connector that does exactly what you need.

    The super low tech solution would be to just twist the wires together or use aluminum wire bent just so (hard to get reproducible characteristics with those two though).

  24. Clinton
    Thumb Up

    A few thoughts

    I didn't see it mentioned. You may want to cover the magnets and contact points with some sort of conductive low-temperature grease to ensure they don't freeze together.

    May also be a good idea to rig the wires forward of the connectors (in relation to the plane) so you have some forward momentum at the break-away point.

  25. Richard 12 Silver badge

    Gold sprung contacts onto gold pads

    Sorry, I missed the last thread!

    They provide 1A (and USB data) in our application, tested in a multitude of low-altitude environments around the world. They're intended for 'industrial' phone charger type applications.

    Zero disconnect force. To be more accurate, a little less than zero...

    I'll dive through the WEEE bin on Monday, see if we have any in there and drop you a photo.

  26. Anonymous Coward


    twas me that suggested an incendiary, so don't want to appear biased.

    But I hope the effect of cold on the magnets is being considered. For example, first result I dug out of Google

    In these tests, a magnet that pulls 225 at 0 degrees pulls 275 when at just -21. You're going colder than that by quite a long way. So you better be testing the pull strength of your magnets at the right temperature, or you might find your rocket goes nowhere.

  27. Charles E

    Creeping Featuritis

    This whole LOHAN project is a classic case of Creeping Featuritis. The more features you keep adding, the less likely it is that ALL of them will work perfectly during unexpected conditions. And the way you are setting it up, every single feature has to work perfectly the first time. What could possibly go wrong?

    1. John Brown (no body) Silver badge

      Re: Creeping Featuritis

      "every single feature has to work perfectly the first time. What could possibly go wrong?"

      Isn't that pretty much what the NASA guy said about the "sky crane" used to land Curiosity on Mars?

  28. Gordon 8


    The whole point of any addition is it has to be KISS (Keep It Simple Stupid)

    Lohan is resting on a metal Rod - One connection can be done via that, it just needs a LOHAN based pickup.

    Only one other connection is needed. What ever it is (Magnet, foil, Spring loaded thingamejig.....) locate it away from the Rod.

    Therefore if the rod is 'live' and the other wire is swinging around in free space they can't touch (unless the Truss has a really bad landing)

    Don't forget the fuse in the power circuit just in case. Solder it in line with a rating of say 3A fast blow. LIPO packs can take that much short circuit with out causing major damage.

    Lester - a list of current items under investigation would be nice. Then the White coat brigade of readers can start giving you reams of reading to do as to how to fix each one ;-)

    Beer - SPB contributors need at least one each

  29. Martin Budden


    I was being serious!

    However, I have since been convinced by springs resting on the blast plate: this idea is simple and reliable.

  30. Neil Barnes Silver badge

    My electric toothbrush

    charges quite happily at low AC current with a zero force connector, and not too much weight in the pickup coil... AC isn't a problem for the heater so there's nothing more than the coil in the rocket, and an AC (more likely square wave) driver isn't a problem. Added bonus in that the input impedance of the output coil increases as the input coil moves away, so it self-regulates...

    1. M Gale

      Re: My electric toothbrush

      Same here.

      However, it also takes something like 15-18 hours to charge an internal battery that is only going to be 1Ah at best. The more current you want, the bigger and heavier those coils are going to be.

      Effectively you're building a transformer in two halves. Even a one amp transformer is quite a hefty beast for a model aircraft to carry!

      1. Grogdor

        Re: My electric toothbrush

        I have the same toothbrush and the same thought. The heating blanket draws <200mA at 12V which isn't much for induction. The physical layout of the rod/guide might be well suited for a couple coils of wire, nothing resembling a bulky transformer. It wouldn't depend on contact, magnetism or separation, but it would need proper alignment of the coils or over-wrapping.

        If you can't handle his voice for half an hour, here's a quick summary: 13Vpp at 22kHz on two inversed coils, 4.5Vdc from the existing simple circuitry pushing 100mA charging into the Sanyo AA 2700mAh NiMH 1.2v battery plus probably another 30mA+ for the diodes, transistors and low-power microcontroller. He doesn't dismantle the base so we don't see the driver circuit/coil but it's specced at 2W which adds up. I doubt it's much more complex than the receiver and I know the base doesn't weigh much.

        Simple, reliable technology in a $20 consumer device. There's also the Duracell Powermat and even chip-level implementations. Stick the magnets to the fridge, don some gloves and dig a couple broken toothbrushes out of the bin!

    2. David Pollard

      Re: My electric toothbrush

      A possible advantage of using transformer coupling is that it would be possible to step up the power. Although there would be 10% or maybe 20% loss through inefficiency, the battery capacity is presently not fully used.

      Also it would be worth delaying the heating. If the heater goes on at the start this wastes power: both increasing the surface temperature unnecessarily, which leads to faster heat loss, and keeping things warm when the outside temperature is still reasonably high. It would be better to wait until the rocket has cooled somewhat and then turn it on.

  31. Larrykfr

    small change to the optimal plan

    I'm an old slot car fan, the slot cars used a light spring loaded contact to get power from the track, you could use a very light spring loaded contact to connect to contacts on the space plane, no friction from the contacts to slow anything down and no wires left hanging after launch!!


  32. Faye B


    Oh for F***k sake, haven't you guys ever picked up an electric kettle. It's so simple, to have spring loaded contacts that engage when you slide the plane down onto its launch plate. Magnets, sheesh! Do I have top draw you a diagram.

  33. Barely registers

    Magnets - electro magnet?

    I may show a shocking lack of grasp of the physics of magnetism here....

    Use a small electro magnet on the truss and a small "bar" magnetic stripe (like a shower door seal) on the LOHAN, separated by a non-stick membrane. Push current through the electro magnet to attract it to the bar magnet.

    At the point of ignition, reverse the current in the electro magnet, providing an actual repulsive force to disconnect the power supply.

    The electro magnet means the magnetic join can be made very strong, reducing the risk of premature separation, but there will be no static magnetic attraction to be overcome on launch. Indeed, you might even get a push forwards.

    Or I'll just get my coat.

  34. TRT Silver badge

    Just catching up...

    Conductive paint? The type you use to repair car windscreen demisters?

  35. John from the USA

    Turn the magnet off at launch.

    I'm not a magnetics boffin, but I'd like to minimise the resistance at launch and maximize the altitude attained. Rather than trying to find the perfect strength magnet, can an electromagnet of sufficient strength be fitted to the truss? That would allow the Vulture 2 to be released decisively at the moment of launch, while being held securely until that time.

    Apologies in advance if a credentialed boffin has already proposed this, but I didn't have time to read very far back in the comments.


This topic is closed for new posts.