Great pics, my keyboard is soaking. Pics of power tools have the same effect.
As the Low Orbit Helium Assisted Navigator (LOHAN) team preps our Vulture 2 spaceplane for a quick paintjob, we're still mulling just how best to connect the aircraft's rocket motor heater to its external battery supply. To rewind for those of you who may have inexplicably missed the LOHAN hot coupling excitement, we've got a …
bit of a repost but:
How about using strips of foil that have a notch in them? If you align the clamps at each end of the foil to be slightly non-parallel then you can make sure that the tearing force concentrates at the notch. Bit like tearing a sheet of paper by putting a fold in it, making a small tear at the top and then just pulling the top corners.
Should be fairly robust and flexible on the way up if the rocket wobbles with respect to the truss but should tear with pretty minimal force when the rocket wants to leave.
Seems like a good idea, but perhaps using copper foil tape rather than aluminium kitchen stuff?
You'll be able to solder to it, dirt cheap and reliable.
Ebay or garden centres (used for EM shielding and anti-slug warfare).
If you've got a little more room for insulation around the tube, perhaps that metallised stuff you put behind radiators (has a really thin polythene foam layer), polystyrene lining paper (I know you can get it in 2mm thickness), or super thin loft insulation from a DIY shed.
Hmm pre-torn foil sounds like a plan. Dunno what sort of current this heater draws, but perhaps some fine wire wouldn't introduce too much resistance if it were a short length - that would just snap as the rocket pulls away. Thinnish Lead wire or similar low tensile metal would snap easier.
If your launch temperature is below about 28 deg. C you could use gallium to fuse the contacts together and then insta-melt them with the rocket's exhaust?
Or maybe use a bimetallic strip that will disconnect the heater at a predetermined suitably low temperature - might not be accurate enough though. Perhaps it might be a means to weaken some other mechanical connection until the moment of release.
Or weaken yer magnets, teflon shims, different magnet material or something.
Or make the connection using high-altitude blu-tack. I have *never* used blu-tack to secure an electrical connection, but it *might* work... :)
... to hold hatches on things like model aircraft and boats.
If you have a magnet which is too powerful, you wrap a bit of insulating tape around it, or mount it a bit further from the steel plate using some kind of spacer. This gives you an infinitely variable force from full down to nothing.
But I would still like a little servo-motor to release spring-loaded clips, so that they fall away just like a NASA umbilical cable just before a Shuttle launch. The Russians use a swinging arm, and a spring-loaded one of those could be mounted somewhere to hold two contacts in place as an alternative...
For sheer simplicity use a magnet to hold a pair of slide-off contacts together:
For each connection put a polished steel shim on Vulture 2 as its contact pad. This doesn't move and can be glued flush to the fuselage side near the rear and the heater and/or thermometer lead soldered to it. The truss side contact is brass shim with the magnet (use something like a 3mm disc magnet) glued to the non-contact side of it so, when connected, the brass shim is in between the magnet and the steel contact. Use flexible wire between the truss and the contact (laquer-coated single strand wire is probably best because it has no plastic insulation to stiffen in the cold) so V2 moving about before launch can't pull the magnetic contact off the steel one. The idea is that, on launch, the magnetic contact will slide off the steel one: this takes much less force than pulling it away vertically. If the sliding force is too high, simply put thin non-magnetic shims between the magnet and the brass contact - 0.4mm ply or epoxy board would both work.
Make the leads different lengths. Apart from making shorts after launch much less likely, this will make sure that the rocket doesn't have to pull all the contacts apart at the same time.
Alternatively, go with the previously described pure mechanical slide-off connectors. Using a servo to disconnect them initially sounded like a good plot, but is probably a bad idea. Apart from the need to keep the servo from freezing up, there's always a chance that the linkage it uses to disconnect the connector will freeze solid. It will also add weight, since very small service don't have much power and you might need a servo per connector.
I assume you'll be using just three connectors: a common ground, a positive line to power the heater and the third is the second line to the thermometer's sensor thermistor?
Neodymium magnets permanently lose most of their magnetic strength by 60C. Put them in hot water till they are weak enough for your requirements. By molten solder temperature they've lost pretty much all their magnet nature. Just soldering stuff to the tails can make them lose significant strength.
I don't know anything about them though.
Would they be too heavy and power hungry?
Also if you had all the connectors from each side attached to their own plastic bar (lollipop stick) or something then you'd only need to hold the 2 plastic bars together with the electromagnet all the connectors lined up. It then also shouldn't be possible to short out the connectors.
Why not skip the physical connection altogether and go for wireless power?
I know that Murata has an automotive module for this (see http://www.murata.com/products/wireless_power/index.html for more information) that might fit your needs.
Looks like an interesting little project for an engineering student...
I still think my electric kettle idea still holds water (pardon the pun). You glue two flat contacts onto the rear section of the fuselage, connected to the heater and have two spring loaded contacts pressing down on them from the truss above. As the rocket lauches and moves up the launch rod, the contacts slide off the fuselage. If you want I can draw you a diagram.
I don't think i need to explain the Paris Hilton angle.
I agree, i was thinking the same principle.
But instead of having spring loaded contacts with flat bottoms, you could have something like multiple bend wires or pieces of steel wool pressing against the contacts on the fuselage.
This way, there would be a number of smaller contacts brushing and moving against the contacts plates during ascent, pretty much making sure that icing wont interfere with the connection. Also, this should lessen the forces required to make a good connection, therefore lessen the forces holding back the plane after ignition.
Another way to help making better contact resistant to icing and "bad spots" when the plane moves slightly during ascent would be to put some electrically conducting paste inbetween the contacts. Something along the line of silver based thermal paste springs to mind.
Of course, this would need proper testing in a local meat warehouse freezer or something...
Okay, how about this?
Two sockets on either side of the motor UNDER the heat blanket, so they will be nice and toasty.
Two rods (pick your material) that slide up into these sockets, connected via wires to the battery and properly secured to the base plate
INSIDE those heated sockets, pack them with conductive grease, then insert your rods.
That's right, some double penetration action going on here...
The rods are your means of applying charge to the heating blanket.
That blanket heats those rods in their nice, snug sockets.
The conductive grease gives you a 100% rod-to-socket contact for your electrical connection.
Because the whole thing is inside LOHAN (the innuendos going on here are rather thick on the ground) you reduce the chance of anything icing up. Use some decently insulated wires and the heat from that blanket will propagate along the wire, ensuring where it goes into LOHAN won't ice up either.
And when the rocket fires the rods simply slip free of their nice, warm, and well lubed sockets.
Find some conductive grease which is non-flammable, btw.
I made a picture!
A couple of thoughts on the photo of the attached thermistor...
- The ring-terminal appears to be floating in free space -- it should be thermally attached to whatever you wish to measure the temperature of (the thermistor itself is inside the crimp area of the terminal)
- You're on the outside of the heater -- will this not give you an artificially high temperature (as opposed to being thermally bonded to the aluminium tube)?
To expand a bit, why not combine magnetic coupling with the slolenoid I suggested here: http://forums.theregister.co.uk/forum/1/2013/04/04/lohan_launch_update/#c_1782429
If the igniter system has the juice, it could be used to power the solenoid as well as fire the igniter. A nice, clean disconnect, before the engine fires.
That's clever! I like the aluminium crimp lug on the end of the thermocouple wire. In this situation (and for the original aquarium heater application) the increased thermal time constant won't matter, and it makes it dead easy to attach. Seems like an effective way of protecting delicate thermocouple junctions. Brilliant!
On second thought, it might be a thermistor glued inside there, not a thermocouple (it's not been crimped). Still a good idea so long as the adhesive is thermally conductive.
The problem of excessive detachment force when using magnetic coupling can be reduced by holding the magnets apart with tape etc., as suggested above. It's also possible to remove one of the magnets by using a second pair which slide alongside as Vulture departs. In effect this switches off the magnetic coupling.
Connect from the heater via parallel copper foils which press onto contact plates. The foils/wires should be long enough to allow a bit of free movement. To hold the connections in place, fix a bar magnet (A) 'above' and between the contact plates. Use a second bar magnet (B) to press both foils from 'below' against the contact plates via a suitable insulator, the thickness being adjusted to give appropriate contact pressure. This second magnet is held in place solely by its attraction to the first magnet and is otherwise entirely free. Magnets A and B couple N-S-N-S.
Fix a pair of similar sized bar magnets (C & D) a short distance away so that as Vulture departs they straddle either side of the free magnet, B. Their poles should both be in the opposite orientation to A and B, S-N. As they come alongside B, they will largely neutralise the field from A, couple to B and steal it away. B's north pole will attach to C and D's south poles and it will decouple from A.
This idea is similar to magnetic base switching.
Two foils on the end of flexible wires make contact with pads on the Vulture's body. The foils are held in place using a horseshoe magnet. This is laid on its side, flat against the body, with the poles pointing in a forwards direction.
A thin sheet of plastic insulates the magnet from the foils, and it is attracted towards a suitably shaped piece of magnetic material behind the pads with sufficient force to keep the contact resistance down and to stop it falling off during the ascent. A bit of mumetal would be good; or pieces of transformer lamination. The attractor plate should have low magnetic remanence and high permeability
When the Vulture begins to depart, the magnet will move forward. As it does so it meets a suitably placed keeper, also made from high permeability stuff, to which it attaches quite firmly. When the keeper 'short circuits' the poles, the magnetic coupling with the attractor behind the pads is very much reduced; so the pressure on the foils is released and these are then easily pulled free from the pads on the Vulture's side.
If necessary an additional bar magnet can be suitably placed so that without the keeper in place the horseshoe magnet overcomes its field but when the keeper is in place the horseshoe is slightly repelled. The magnet, foils and wire will thus swing away enabling an unencumbered launch.
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