Readers, we need you... for LOHAN ignition failsafe brainwaves The Low Orbit Helium Assisted Navigator (LOHAN) team is inviting experts among our super readers to submit final proposals for a back-up ignition system for our Vulture 2 spaceplane's mighty rocket motor. Click here for a bigger version of the LOHAN graphic As SPB ballocket regulars know, the primary system designed to light …
OK, but then, if it didn't have any (or just a few short) runs, the whole kaboodle would be falling for those 30 seconds before firing. That's quite a distance (2..4.5km, at 20..30 secs runtime) and it will have picked up an impressive speed too (200..300m/s, not counting drag and the deployment of the parachute). I don't think that would be a suitable condition to have LOHAN in at the moment supreme.
Set up a constant force spring, you know a coiled strip of metal like inside a tape measure, so that it is extended when it feels some gravitational pull. Once the balloon pops and enters free fall the spring will be able to wind itself up into its coiled state and make the contacts.
In ascii imagine the following sequence with the screen rotated clockwise;
:_________@ Spring extended under gravity and contacts (colon) on top. Balloon pops >POP<
:_____@ Spring starts to coil up as it doesn't "feel" gravity in free fall.
:@ Fully coiled, contact, ignition, Bob's your mum's brother.
I think you might snag yourself something larger than a duck were that to happen :-D
Still, a pint (or three) will solve any problem in existance.. world peace, starvation, cold fusion, and also why bags of nuts have that silly little label on them "May contain nuts"
In the words of Benjamin Franklin "Beer is proof that God loves us and wants us to be happy"
I'll drink to that!!!
If your gravity detector is (close to) horizontal, vector math shows that there's some fierce accelerating going on, and as a result of that there's a pulling force on the spring + cord, significantly larger than the pull by gravity. If the acceleration stops, the weight will drop into a vertical position with respect to the rig again; it will not 'overtake' the rig (except for a short moment when it will not yet have started to fall, and the force of the spring will start to reel it in). Only if the turbulence is such that the rig gets actively stopped and/or pushed downwards there's a possibility that the weight can slam into the rig. But I think those are conditions you wouldn't want to launch under.
I'm thinking winglets and a horizon-sensing "autopilot" just set to make sure the launch angle is optimum as best as it can.
Hell, a compressed-air or some other compressed-gas RCS might give you a few seconds of attitude correction and stabilisation - certainly enough to launch. It doesn't need to lift the rig - just spin it the right way after the balloon bursts.
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My idea is this:- you attach a weight to the balloon (hopefully not very heavy), attached to this weight is a cord (of whatever length), on the other end of which is an insulator between two sprung switch contacts. The idea is, the balloon pops, the weight falls, it reaches the limit of the cord length and snatches the insulator from between the two contacts, contact is made and the rocket fires.
This will keep constant force on the switch
It does not. As soon as the distance between balloon and rig decreases, the tension on the shock cord decreases too. Which means you may still have some tension where a normal cord would have gone slack, but constant it is not. And it may induce oscillations in the line, so it may not even keep any tension at all.
OK, let's see if I understand the requirements. We need a backup firing trigger in case the balloon bursts prematurely. The backup must fire the rocket within a second or so after the burst, must be (relatively) immune to turbulence induced false positives, and must be a mechanical system, the simpler the better. OK?
My proposed solution will take a little trial and error testing but seems sound in concept - at least to me. Take an ordinary security/ID badge reel of the sort that clips to your belt and allows you to pull the badge out on a string for normal use and then reels the badge back in by means of a spring powered reel. At the reel end, place two electrical contacts which, when shorted, activate the firing mechanism. At the badge end of the string, place a metal washer or equivalent. Place the reel on the truss in such a position that when the string is extended it is parallel to the upper tether, the part above the Y, and attach the washer at or just below where the tether attaches to the balloon.
When the balloon bursts, the reduction in tension on the main tether must be sufficient for the reel-in spring to bring the washer down to hit the contacts on the reel, firing the rocket.
The trial/error part is finding such a badge reel with just the right strength spring. If you can locate one, the time it takes for the string to reel in should be, if not right on one second, a fairly short time. An advantage of the time it takes to reel the washer down to make contact is that temporary turbulent induced reduction in main tether tension ought not to be long enough for a false activation, and when the turbulence passes the badge reel mechanism will tend to reset itself.
Very ggo! May I suggest a small modification?
Find a badge reel strong enough to hold the entire truss+rocket and replace a section of the main balloon tether with the badge reel. During ascent the reel is extended and the contact is open. During turbulence there is enough length in the reel to allow for bumpiness without the contact closing. When the balloon bursts the reel zooms back in and the contact closes, launching the rocket.
You could put your badger eel inside the launch rig, where it's warm and cosy. Run the cord up to the swivel joint and adjust everything so that when the two lines of the inverted 'V' are taut, the cord of the badger eel is just a wee bit off from maximum extension, so that it won't ever need to take the weight of the rig. Then of course the conducting disc, and a bunch of contact pairs around where the badger eel cord passes through the lauch rig fuselage.
Swap the payload swivel with a hollow one, an inward flanged tube sitting inside an outward flanged tube with ball bearings sandwiched between the flanges.
That way you can pass the safety tether through the middle of the swivel.
(A ball retaining plate like used to support the platter in a microwave oven can be used to reduce the number of ball bearings needed without having the all bunch up at one location.)
Use a length of line for your tether in a "Y" configuration between opposing shroud lines of the chute and place a small fishing line type swivel at the junction of the Y. That way, as the chute opens, the "Y" becomes a "T", pulling the safety pin.
You could rig a switch plus a tether that pulls the switch if LOHAN slides off the rod, but that would require LOHAN's batteries to be able to supply the juice for firing the igniter.
And I think the idea is to get LOHAN to fire in an upwards attitude if at all possible. Firing the motor when going down would be necessary only if there's concern about having the craft landing with an unused motor.
Find a badge reel strong enough to hold the entire truss+rocket.
During ascent the reel is extended and the contact is open.
During turbulence there is enough length in the reel to allow for bumpiness without the contact closing.
When the balloon bursts the reel zings back in and the contact closes, launching the rocket.
p.s. Thanks to Alan Esworthy for the inspiration. If I have reeled further than others etc...
Thanks, Martin. I did think about using the zinger line as the main tether line but was unsure of its tensile strength and the total weight of the truss, electronics, and spacecraft.
On further thought, running the zinger line from the truss to the Y in the tether might work even better/faster and eliminate or at least greatly reduce any chance of tangled lines interfering with the backup function if needed.
...Paris, because she's the real zinger!
Attach a suitable switch (of the pull-teflon-strip-from-between-contacts type) at one of the parachute's attachment points, and attach a cord at the opposite attachment point. Make its length so that it triggers the switch with the parachute roughly half to three-quarters open.
Less chance of binding/freezing than with the cord-through-rings-on-parachute setup; may require some experimenting to find the right switch and get the setup right; it also requires wires running down from the parachute to the launch rig.
is an indication of how much of a problem turbulence actually is under average launch conditions, at what heights it tends to occur, and whether it actually causes sufficient loss of pull to cause a false trigger when using a tension switch on the balloon tether. There's a video of some Lego craft getting up into the stratosphere, and not being shaken to pieces in the process. Or did they glue the thing together?
Because when turbulence stops being much of a problem over, say, 5000m, then you could hook a pressure switch set to the equivalent of 7500m into the circuit (parallel to the tension switch if the pressure switch opens at altitude, in series of it closes).
It's really a last resort system. We're not planning to use it unless absolutely necessary. If the SPEARS boards fails on a first flight, we'd probably bring the Vulture 2 down with the truss, and go for a second flight, when it's all or nothing. In that case, it wouldn't really matter what way the aircraft is pointing.
is to produce a mechanical trigger whic will only fire after the GPS altimeter has failed to either detect launch height or it has detected launch height but the ignition process failed. This implies that the mechanical trigger must react to an event other than GPS altitude reading, which could be either temperature (difficult to get right), r balloon burst).
A free-fall detector would be my sugestion, such as a weight on a spring enclosed in a box or tube. When free-fall is detected the spring should return to its unladen orientation. A problem with this idea, and any other free-fall detector, is turbulence during the ascent causing the mechnaical device to fire. The difficulty with any external cords or strings is the possibility of icing up and friction.
Here is my two pennies worth:
https://docs.google.com/drawings/d/1_7DDbiSNEaioELLo3h8gCKkSIJmK6d7MawU1ONUOJds/edit?usp=sharing
This is based upon the fact that distance "a" reduces to distance "b" after balloon pop.
I'm assuming "a" is at least 5m and "b" will be less than 3m.
A set of electrical contacts on long, self closing arms that touch when the parachute starts to open. The upper open end of the arms would be tethered to the top inside centre of the chute, and the lower end attached to the platform below (pretty much where length "b" arrow is).
The overall length of the open arms (which would be 2 x 1m = 2m) and the two bits of tether would be equal to distance "a".
During ascent, the weight of the platform and the lift of the balloon keep the arms open (and so the electrical circuit open). In the event of turbulence, the distance "c" would have to reduce by 2m before the circuit would close, hopefully dampening temporary reductions of "c" < "a" and stopping any premature firing. (I know the un-deployed chute will go squiggly as well - couldn't be bothered to draw it.)
After the balloon bursts, the arms will have freedom to close completely and so close the backup firing circuit .
Have the arms below the closed chute material (where the squiggly bit is in measurement "c") so they can't become tangled in the chute.
The firing system could also be a tube like you show in your "pin pull" system, but with the cylinder being upright, about 2m long and the electrical contact being made as the spring pulls the piston all the way in to the tube after about 2m of travel.
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