Our Low Orbit Helium Assisted Navigator (LOHAN) project is beginning to work up a good head of steam, as we begin to consider the practicalities of launching a rocket-powered spaceplane from under a whopping helium balloon at a not inconsiderable altitude. Click here for a bigger version of the LOHAN graphic Thanks to all of …
Have you considered using a mercury switch to determine and correct the orientation of the plane ? Its quite simple, a small capsule (think of your standard antibiotic size pill) filled with mercury. One contact on the bottom, where the mercury sits when flat, one contact on either side. This can then be used to move a flap or rudder whenever the mercury makes contact (ie. whenever the plan pitches or rolls, depending on which way you install the switch).
Lightweight and cheap. Mercury freezes at -38 degrees C, so this may be an obstacle depending on how high you intend to go.
Anyway, implementing such a switch could enable you to maintain a steady flight path of 45 deg, or even allow you to start at horizontal on release, can correct to vertical over a few metres, avoiding the ballon then moving to a vertical path.
Or an accelerometer
Better yet, tiny accelerometers are quite cheap now and can detect what direction "down" is quite reliably (this is how phones/tablets determine orientation flips).
Not so great if the plane is swinging, or once you light the engine, but neither is the mercury switch - and it is easier to detect swinging from an accelerometer than it is from the mercury. Plus, we can easily log a peak acceleration figure :-D
How complicated do you want LOHAN to be?
Its not just a question of putting in tiny accelerometers but building the flight control computer behind it and then writing the software that will change the control surfaces as needed.
You then have to test it, retest it, and that takes time, effort, and of course money. Essentially you'll end up with an auto pilot capable of landing a plane.
I'm not an RC modeler, but assuming that something exists that could be put in to a model plane, it would be interesting. Assuming that it doesn't exist but could be built, you'd have to shrink it to be small enough and light enough to fit in your plane. That too takes time, expertise and money.
You want to go 'low tech' you could do the following:
Create a detachable weight and make the tail of the aircraft heavy so that when released from the balloon it drops tail first. When ignition happens, the weight detaches and the plane is angled up.
Assume that if you time the delay long enough, your probability of hitting your balloon decreases to a small enough number.
I haven't followed LOHAN enough to know just how complicated of a flight system that El Reg wants to build. The launch scheme I suggested could be done with models (gliders) that contain no flight control systems dropped from a balloon at a controlled height. They key is working out how to get the timer to detach your weight and start the ignition. I'd suggest explosive bolts but I don't know if El Reg has access to aerospace tech like that, but the timer and electronics would be very small and simple.
I have used these
in water rockets. It is harder than you think determining which way is up when you are accelerating, it is also a non trivial task to determine apogee (which is the ideal with water rockets to deploy chutes at that point) which may not be much of a problem with LOHAN
Easier than you think
Hang on a moment. We're talking about a trigger used before flight here, not something that will move control surfaces.
Not that there are not loads of people doing the latter now - google ardupilot. One of the great miracles of the modern age is that these things are not only cheap, they are very easy to use.
Apogee WOULD be impressive
I have something very similar. It's easy when you're not accelerating, hard when you are... but when simply swinging gently under slow movement it's not so bad.
I wonder how heavy the weight would need to be and if there would be a problem dropping it from a great height where there might be people walking.
Not a good idea....
I suggest you do a test of your theory by building a rocket yourself. If you put the weight at the bottom it is unstable, usually you have to put weight at the top to make it stable. Doesnt sound right does it....
So by putting a weight at the bottom you are ensuring that the Centre of Gravity will go below the Centre of pressure and if that happens it will try to fly tail first.
Now of course you might quite like LOHAN tail first.
Detecting Apogee is very hard.
When the rocket is firing you will have +ve Accn upwards.
As soon as the motor burns out you will still be accelerating just at 1g downwards (drag would make it higher but at that altitude not by much), however you are still moving upwards.
When you reach apogee, from an acceleration point of view, nothing changes. So using an accelerometer how do you detect apogee?
You need to detect the point the rocket tips over, assuming LOHAN aint a tail slider.
The point is to make the plane tail heavy on launch
Assume that the plane is perfectly balanced.
All you need is a small weight large enough to force the tail to drop as the plane falls from the balloon.
the idea is to make the tail just heavy enough to force the nose up in free fall and when the engines ignite the weight is lost.
Depending on how much weight is required, you could use something like dry ice or something combustible that gets burned on ignition. (The later would be more difficult and more dangerous.)
Physics fail there
Gravity alone cannot cause objects to rotate*, because it acts on the centre of gravity - thus there is no moment.
From the point of view of the accelerometer, it will see the rocket thrust, then zero once that burns out.
It'll stay at zero until atmospheric resistance starts to become noticeable (going fast enough in dense enough air), at which point it'll read the atmospheric drag acceleration only. That's not necessarily in the direction of 'up/down' though!
Gyros are the only way to get an orientation.
*I'm excluding very high spacial curvature here, we're not near an object that massive!
Ok... you have a plane that is perfectly balanced. That is the center of the plane is also the center of gravity.
You add a weight to the tail. This shifts your center of gravity.
Add to this...you're not doing free fall in a vacuum. ;-)
(Do I have to spell that out to you? especially when you're talking about lifting bodies? :-)
One contact on the bottom, where the mercury sits when flat
except under acceleration ...
put the plane inside the balloon.
when it bursts it will launch
when it bursts it will launch
and when it launches it will burst. Win-Win.
If the ballon were filled with hydrogen it would be an even more spectacular launch.
(OK, OK, I know)
Two balloons, of course
After all, El Reg. coined the "Bulgarian Airbags" term, so I think it would be most fitting (ooer, missus!)
Seriously, one. Worked last time.
Three balloons better
Three balloons could be attached round a launching tube for the rocket, which would automatically point upwards with the business end of the rocket pointing down, away from the balloons. Simple.
Re: Three balloons better
There is an Occam-esque appeal in the simplicity of this.
Not having any physics/rocketry/engineering training isn't going to stop me commenting though.
How tough are the balloons? As they rub on the tube/each other will it make them burst prematurely?
Should one burst at (say) 100 feet, how do we deal with the now crazy angle of the launch tube? Does that matter?
What would the tube be made from?
Do we want to launch vertically, I thought the optimum angle was 45degrees for some reason?
As I recall the PARIS craft froze to its platform - a tube infers a lot of potential contact points, is it likely to freeze in there?
Does one launch these rockets from inside a tube (like a mortar) or from a milk-bottle type thing (like a firework) or does it not matter?
Nice and simple
It does sound like the simplest one, to me. However, you'd have to make sure each of the balloons provides the same amount of lift.
Also, you could name the balloon(s) stage "Eccentrica".
"Do we want to launch vertically, I thought the optimum angle was 45degrees for some reason?"
45 deg is when you want to lob a projectile the greatest horizontal distance. Which is not a design criterium for LOHAN (apart from that, it's not a projectile but a glider). Altitude is what we want, hence you want as much power as possible to be spent in a vertical fashion.
In favour of hydrogen
If hydrogen explodes, it explodes upwards. Unless you plan to be standing on top of the balloon when it explodes, you have no problem.
Hydrogen is mucho cheaper
In addition, if you were really clever, you could have the hydrogen gradually empty from the balloon into the rocket tank with rising altitude and use if extra flight time.
"In addition, if you were really clever, you could have the hydrogen gradually empty from the balloon into the rocket tank with rising altitude and use if extra flight time."
But you'd maybe need to burden LOHAN with some tanked oxygen to burn that Hydrogen...would there be sufficuent oxygen at that height to combust the hydrogen
Hydrogen plus oxygen
Plus a pump to get that hydrogen from the balloon into its tank, because pressure in the balloon is just over atmospheric. Which it has to do in a few seconds.
Doing the math on the energy equations shows you're seriously way better off leaving that hydrogen to dissipate into the upper atmosphere unburned. Really seriously way better off.
three balloons vertically
just use different lengths of cable to tether the balloons. Find the video of that guy who launched himself skywards in patio chair to see the idea configuration.
He went up to about 15 kfeet, IIRC. If PARIS' performance is anything to go by, LOHAN's jubs will lift her to well over five times that, meaning way more expansion and thus weaker skin.
Mount the rocket motor on gimbals and attach it to the same servos controlling pitch and yaw. Launch horizontally (or even down) but program the autopilot to pitch to vertical after launch.
The autopilot will clearly need different goals during different phases of the 'flight' (remember that it won't in any real sense be 'flying' when above 80k feet, not unless its doing SR-71 speeds).
Thrust vectoring 2
Or put vanes into the rocket exhaust. If they are only needed whilst accelerating to a speed where more conventional control surfaces are effective they don't have to last very long.
This is not a title
How about a long spar suspended under the balloon, with one side counterweighted and perhaps vaned, and the other side extending far enough horizontally to clear the balloon on launch?
Not a bad idea...
On the one end you have the connection to the balloon. The connection is a spring loaded hinge so that upon release it launches the plane in an upward arc like a catapult.
I'd say depending on the weight of the plane, look at some graphite tubing like from a golf club. Its light and durable. The only downside is that you've increased the size of your debris that could come falling down...
Thinking of something like this myself
Not necessarily a long spar but long enough to act as the launch rail for the aircraft.
The aircraft having a couple of loops on the top, or a hollow spar running through which fits over the launch rail. Rail doesn't need to be very long since unlike a Black Cat Super-whizzer you will have some degree of aerodynamic control.
Perhaps an additional "carrier" element with substantial fins could be used to get the aircraft free on a striaght path, thereafter falling away. More payload, but which is better - a guaranteed launch or maximum altitude.
Counterbalance for the launch rail would come principally from the rest of the launcher ( camera, telemetry etc) but a single guideline running out from the balloon tether point to a second spar in the opposite side from the launch rail should fix it at the right angle.
You could put the plane on the end of a long enough pole that the balloon doesn't get in the way, as suggested on the last thread. If you know what height you want to release at, Boyle's Law tells you how big the balloon will be. These balloons have huge lifting ability, so a reasonably strong bit of wood will not stop it going up.
But hydrogen is fun. Hydrogen goes *boom*.
Being serious for a moment, if you can't have a platform on the top of the balloon, you could launch from the side. From the top, it would look like a + sign, with strips bending downwards along the sides of the balloon. Three of the strips would have a weight on it equal to the plane, with the fourth holding LOHAN.
Stability is the problem
You cannot have a stable balloon configuration with anything but the payload UNDERNEATH. Furthermore, the skin of the balloon CANNOT repeat CANNOT support a load; it’s not a consistent shape during the flight, and it’s really fragile.
And now you only have 1/4 of the mass avaliable for the aircraft. C- must try harder.
Ah, but when *helium* goes boom...
there's enough energy around to get to orbit. And there won't be too much of that nasty beryllium byproduct lying around.
What /is/ it with you lot at SPB and hydrogen?
Stuff ain't as scary as you think -- especially when it's in something as flimsy as a balloon.
Hell, just get yourselves a little bit, a few party balloons, a pole with a lighter on the end, and make some bangs, you'll get over your namby-pamby woosiness soon enough.
soft whumping noise
I used to work with a bu ch of lads who would inflate bin-bags from the oxy-acetelyne welder, and let them drift upwards. Lighted fag-ends flicked at them would eventually burn through and produce a soft, wet-sounding ball of flame and a lot of soot.
Re: soft whumping noise
Dunno about your wimpy acetylene, but a hydrogen balloon makes a much more impressive sound.
Scene from my freshman chemistry class: Prof has graded exams, a floating balloon, and a lit torch (not slang for a battery-powered flashlight) on his desk. Prof says: "According to the exam, a depressing number of you did not know whether an exothermic reaction produces energy or not. For those of you who don't...", and waves the torch (which is on a long handle) to the balloon.
LOUD BOOM. Also, the flash of light was impressive.
This should not be considered an anti-hydrogen posting, but do be careful with lit cigarettes.
Re: soft whumping noise
I suggest that the balloon was not only filled with Hydogen, large volumes of Hydrogen do not explode they burn from the outside in as the oxygen gets to it.
Look at the video of the Hindenburg, that is not an explosion it is just burning fast, pretty damn fast I admit. However if the balloon has the right amount of Hydrogen AND Oxygen in it then the balloon will explode. (Technically it is still burning just very very fast, just like gunpowder burns very fast, C4 etc really do detonate)
Before H&S stopped the practice in Schools we had to open the windows before doing this experiment...........
Re: soft whumping noise
Perhaps -- I didn't ask the professor. But a toy balloon is not a large volume of hydrogen, and mixing of the hydrogen and the oxygen-containing outside atmosphere could happen very quickly, unlike the very large volume of the gasbags of the Hindenburg.
And the larger point, of course, is that it doesn't take much hydrogen to create an impressive explosion.
Arc shaped launch rail
One big giant balloon.
Suspend a big hoop below it, hanging vertically, with a few threads of fishing line like bicycle wheel spokes to keep it circular.
Fit launch rail from the bottom of the hoop curving up one side 'til it reaches the desired angle, then sticking off for a little bit (the last bit will need a rigid brace)
Rocketplane sits at the bottom, no need for any kind of counterweight. Light the motors and it accelerates along a curved path along the rail to the right angle, missing the balloon, then flies off at a tangent to the circle on the intended path.
Shouldn't weigh as much as a long rigid pole with a counterweight (oo-err?) as it's smaller and shouldn't need to be too hefty to be rigid enough (as it's efficiently braced)
Also means you have a bit of speed before being totally untethered so aerodynamic stability will work better.
No, no, no, no, no, no, no.
The centripetal acceleration of the plane would throw the entire thing cockeyed. Zero chance of success.
I'd better do the sums again
It will certainly drag the rail a bit... but unmanageably? As you're only using the bottom part of the arc, most of the force is in the vertical plane and pulls against the balloon (which can stand it); the remainder will try to turn the wheel about its axis but it's a comparatively small force and hasn't long to do it in.
Hmm, I must test this! For science!
While doing the sums...
...don’t forget dynamic forces. You know, Newton’s Third Law, the action/reaction one.
We’re talking a multi-kilogram plane — the MAJORITY of the weight of the entire ballocket — suddenly moving along a curved path with nothing to anchor it! The rail with thrash about and the rocket will leave at an utterly random angle, it’s worse than useless.
Happy with the laws of Newton, it's the laws of Sod I'm worried about
The rail exerts a force on the rocket to keep it on a curved path.
Maybe I've got my sines and cosines the wrong way 'round, but it seems to me that the larger part (cos @ less than quarter pi) of the opposing force exerted on the rail (on an axis through the wheel hub) pulls down against the balloon, and the smaller part (sine @ less than quarter pi) will tend to generate a moment that will rotate the (top-anchored) wheel in the direction of launch (as if the rocket were "dragging the rail around with it", although this is a separate force from the friction term, which I'm hoping I can keep nice and low)... this moves the wheel centre away from a vertical line through the anchor point, so its weight (and the cos part of the force) partly counteracts this moment.
The actual launch angle will be slightly higher than the set angle.
On a balloon, laterally there's not a lot of force if the thrust is nice and symmetric; there is no crosswind (we're drifting at jolly close to wind speed). On a ground level test the wheel definitely needs anchoring against crosswinds rotating it about the vertical axis, but I don't think this would invalidate the test results.
It's quite probably not right for LOHAN but I'm confident enough in my analysis to try it on a smaller scale (admittedly standing behind a wall)
Have you decided on the exact aim of the project?
I reckon there are 2 targets:
1 - Go as high as possible
2 - Go as far as possible
If you actually want a vertical launch
And I suspect that the 45-degree angle is better, but one approach would be:
One (or two so it doesn't twist around too easily) balloons at each end
Rocket attached to middle of pole.
Shape something like an H but with the middle of the H being 20-30 metres long and the cross bars only a metre or two each. Then attach a balloon to each corner. Make sure you know how big the balloon will be at launch so they won't meet and you get a vertical launch platform.
The problem is that it's not a stable launch platform, so you still need control surfaces, and the rocket backwash has a <b>much</b> higher chance of trashing the balloon than on a 45-degree launch.
Incidentally, if you're really afraid of trashing payload on the balloon from the backwash, you might be better off with a glide launch to get some separation before firing the rocket.
Two balloons - One half inflated
As the baloons rise and expand, the first one will eventually burst and the second one can 'take over' lifting duties until it has expanded to bursting point therefore acheiving more height before launch. I don't know if this is scientifically possible but I thought I would add my 2 pence worth.
As to how to fire the rocket - Fire it sideways. As long as you have a piece of buttered toast attached to the top (butter side down) it will naturally point skywards. Now I know this is scientifically correct - I would suggest using a cat (same scientific principal) but the RSPCA may take a dim view of this.
update on the cat
If we don't use the cat for directional control we could always use it at ground level - it would boost team moral and provide useful pest control in the command centre. As to the pet cats name - it has to be LOHAN's Pussy....
Go on moderator - I dare you!
Swing til you're winning
Could you use the release timing so that the rocket swings around the balloon and the balloon's inertia can be used to orient it to a more vertical angle before releasing it?
I know nothing about rocketry or physics, but in my imagination this works perfectly and I believe hard enough that I think it just could work...