"attached to something cheap and expendable"
May I suggest an eloquence of lawyers?
LOHAN eyes hardcore partner's impressive girth
The dust has settled on our recent trip to International Rocket Week (IRW) and the dramatic launch of the Negligible Altitude Obstreperous Model Initiative (NAOMI) rocket, so it's time to reveal just what the assembled experts thought would be the best power plant for the Vulture 2 spaceplane. Click here for a bigger version …
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Monday 12th September 2011 15:14 GMT 8-{>
Might want to avoid epoxying the motor in to anything. The "RMS" bit of the Aetotech motor designation stands for "Reloadable Motor System" The reloadable motor bit is an aluminium motor casing with forward and aft closures (end caps) they dont come chea and are meant to be reuseable / reloadable. The reload is a bag of assorted cardboard tubes, washers, o-rings, propellant slugs or "grains" and a nozzle. These have to be assembled in the right order or the motor wont work as desired. Aerotech are the most open to failure due to operator error. I have done it. Forgot one o-ring, result rocket flambe.
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Monday 12th September 2011 16:22 GMT Tim Starling
Or maybe use a vacuum pump?
You can buy a new vacuum pump for about £250. I'm sure someone would lend one. No need for fantastic water column constructions.
A cheap pump would be slow so it would make sense to keep the volume small. You could close off the combustion chamber with a valve and disconnect the pump entirely if you were afraid of damaging it.
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Monday 12th September 2011 13:44 GMT Anonymous Coward
Re: Re: DIY hypobaric chamber
How about doing it the other way around by partially blocking the exhaust nozzle of the rocket motor?
In other words, raise the internal pressure of the motor rather than reducing the external pressure. If you can get the ratio of internal to external pressure the same as it would be at altitude then you should have a valid test.
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Monday 12th September 2011 15:12 GMT 8-{>
"How about doing it the other way around by partially blocking the exhaust nozzle of the rocket motor?
In other words, raise the internal pressure of the motor rather than reducing the external pressure. If you can get the ratio of internal to external pressure the same as it would be at altitude then you should have a valid test."
The problem is that pressure drops with altitude. The pressure inside the unlit motor and outside the motor will be lower at altitude than ground level. For a rocket motor to work, for the chemical reaction to work fast enough for the motor to burn properly, the motor has to reach the correct operating pressure. This is achieved by careful design of the nozzle so that, among other things, it maintains a pressure differential between the inside burny bit and the outside. A pressure gradient. When the propellant ignites it creates huge volumes of combustion products / gas which raises the pressure in the combustion chamber, the inside of the motor. The rise in pressure increases the speed of the burning reaction, which increases the pressure which increases the rate of reaction... This depends on the geometry of the nozzle, a venturi, being correct for the designed burn characteristics of the propellant, the internal geometry of the propellant grain (a slot, star, "C", bates grain, etc..), motor casing materials and air pressure range through which the motor will operate. The nozzle geometry for commercial model rocket motors is designed for ground level launches up to a couple of 10's of thousands of feet. This is why Aerotech are being cautious. Raising the internal pressure of the motor for a ground level test by blocking the nozzle will lead to the motor over pressurizing and failing (going bang one way or another).
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Monday 12th September 2011 17:12 GMT AlanB
> partially blocking the exhaust nozzle of the rocket motor?
When he says "without a burst plug or other means to retain internal pressure", presumably a burst plug is something that will block the nozzle during ignition but reliably rupture in a consistent manner once pressure has built up.
How hard would it be to add one to the design?
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Tuesday 13th September 2011 14:10 GMT Disco-Legend-Zeke
Building An In House Hypobaric...
...facility would save a lot of travel. Suitable test chambers can be added at scrap metal prices to primary investment in pumps and valves, plus a nice SCADA to run it all. The altitude sensors need to be developed. And it would be REALLY cool to have a Hypobaric Wind tunnel.
Sponsors can get advertising space on the gear (around your logo, of course.)
There is no need to get a long burn for ignition testing I am sure the motor manufacturer would be happy to supply short burning motors in return for test data.
There are chemical mixtures that are insulators until burned. If you are using electronic controls, adding electronic flame sensors along the propellant grain burn would give proof of ignition as well as end of burn detection to initiate other features such as wing deployment.
Paris because there would not be a LOHAN without her.
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Monday 12th September 2011 11:32 GMT Colin Miller
Borrow a hypobaric chamber?
"Unfortunately, rocket motors aren't something they allow you to fire up in a hypobaric chamber"
That was my first idea as well. Do you have any contacts at DARPA /QinetiQ/ESA etc who can lend you time with theirs?
As for building your own, you also have to keep it at -60C which is a bit beyond a domestic freezers capabilities.
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Monday 12th September 2011 13:22 GMT Colin Miller
ICBM test rig
"We used QinetiQ's hypobaric chamber for PARIS, but the chaps down there made it clear no inflammables, naked flames, explosives, etc, were allowed in."
I was thinking more along the lines of the chambers used to test the middle and upper stages of rockets. Although putting a few cm rocket inside a hanger-sized chamber does seem a bit of overkill.
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Monday 12th September 2011 12:16 GMT John Sager
Solid CO2 sublimes at -78.5C at atmospheric pressure, so it could be in the running to cool the rig down. However mild steel is probably quite brittle at those temperatures, so not the best material for a hypobaric chamber! Al alloy? I've used 2" Duralumin scaffold poles for an antenna mast and that might be the basis of a suitable chamber. I believe though that such scaffold poles are in short supply now due to changes in safety regulations?
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Monday 12th September 2011 12:15 GMT Anonymous Coward
LOX is *much* more dangerous
Having used both, believe me. LN2 requires relatively little in the way of safety precautions - it's chemically pretty much inert.
LOX is a nightmare. Never, ever again. It will make *anything* burn. Including things that you're convinced have already been thoroughly burnt. It's ludicrously easy to ignite.
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Monday 12th September 2011 15:10 GMT Jan 0
Re: LOX is *much* more dangerous,
but it's also so_much_more_fun.
My favourite:
Pack an empty baked bean* can with wire wool and leave 50 cm of fine wire wool trailing over the side and across your fireproof** surface. Fill the can with liquid oxygen. Now apply a flame to the end of the wire wool and watch the red glow crawl towards the can, turning into a bright yellow flame as it reaches the oxygen vapour creeping down the can, then marvel as the miniature volcano peppers the ceiling or surroundings with white hot molten droplets of iron and iron oxide. Notice how most of the can has burnt away as well.
Oh to work in a proper lab again.
Mine's the asbestos coat:)
*any similar sized can will do. Avoid larger ones unless you arrange remote ignition.
** don't use metal unless you want to weld the base of the can to it.
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Monday 12th September 2011 15:35 GMT rickyjames
Phoenix Rocket Glider
If the decision is made to go with a G class engine, there's not going to be very much altitude gained. Here's a link to a 25 ounce (just over 700 gram) rocket glider kit that only gets up to 1000 feet (just over 300 meters) with a Aerotech RMS-RC 32-60/100 class G engine.
http://www.skykingrcproducts.com/rcplanes/bob_parks_kits.html
Basically, the rocket engine selected would keep a two-pound rocket hovering at its relaese altitude for around eight seconds, neither rising nor falling back to Earth. If the rocket weighs less than two-pounds, it will go up. But since a rocket can't be made in this case for much less than two pounds, it's not going up very far. Especially in only 8 seconds.
The key for altitude gain is thrust-to-weight ratio. The selected engine is going to severely limit performance in this area.
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Monday 12th September 2011 17:00 GMT rickyjames
Doing It Old School
What a blast from the past I've found. Here's some links to the PDF copies of key Estes Model Rocket Technical Reports from the mid-1960s that I cut my teeth on back when I was a kid flying these things: TR-4 about boost gliders, TR-10 about altitude prediction, and TR-11 about aerodynamic drag and fin design. Anybody associated with or interested in LOHAN should read these things cover to cover as I did a million years or so ago. There is no better intro to these subjects.
http://www2.estesrockets.com/pdf/2266_TR-4_Boost_Gliders.pdf
http://sargrocket.org/Documents/Estes/TR-10.pdf
http://sargrocket.org/Documents/Estes/TR-11.pdf
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Monday 12th September 2011 17:21 GMT rickyjames
The Rest Of The Best
Woohoo - more research, more Estes Technical Report gems found. First link is a collection of TR-1 thru TR-7 (with TR-4 and TR-7 specifically about boost gliders) and my beloved TR-9 on "Designing Stable Rockets" which was the foundation of my 10th grade science fair project. No, I didn't win. Stupid judges.
http://www2.estesrockets.com/pdf/2845_Classic_Collection_TR-TN.pdf
http://www.ninfinger.org/rockets/EstesTR9.pdf
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Monday 12th September 2011 16:21 GMT John Sager
100 Newton-secs - about 12 Newtons for 8 sec with that motor, so a 0.5kg rocket would
be accelerated by 24 metres/sec^2 - about 2.5G - for 8 secs. Remember school physics? S = Ut+0.5at^2 and V = U+at. U is essentially zero and forget atmospheric drag for the moment and assume it's fired vertically up. As the motor fires, the upward acceleration is 24-9.81 ~= 14, so it's initial height gain before the motor burns out is 0.5*14*64 = 448 metres. and V = 14*8 = 112m/s. It now carries on up, but with gravity slowing it down. So 10t = 112 and the rocket reaches the top of its trajectory 11.2 sec later, having risen a further 0.5*10*11.2^2 = 627.2 metres. So total height gain to the top of the trajectory should be around 1075 metres.
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Monday 12th September 2011 16:48 GMT rickyjames
Fins.
What John Sager said. Also, realize that is theoretical performance which is only going to happen if the rocket orientation is controlled to keep the tip pointing straight up. In most amateur rockets, this is accomplished by fins - the "wind" going by the rocket works to "weathervane" the rocket via aerodynamic forces on the fins. For LOHAN, we are launching from a dead stop at 90,000 feet where the air is very thin. The fins are going to have to be really oversized (read:heavy) compared to what people are used to on a ground launched vehicle to generate the control forces to keep the tip pointed up.
Unless there's some kind of spin/gyroscopic stabilization....nah.
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Monday 12th September 2011 13:44 GMT Mark 62
Build options.
First is that the cold doesn't need to be supplied by the chamber. Set up the rocket with igniters, power supply etc, and put it on dry ice. When the chamber is about ready quickly load it in drop the pressure and fire it off. The chamber itself doesn't need to be able to take the cold just hold a vacuum and take the pressure.
PVC pipe is tough stuff and should work well. As for dropping the pressure, you can either do a plunger setup, or like someone suggested, the water column. No need for a scaffold, just strap it to the side of a building.
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Monday 12th September 2011 16:22 GMT Martin Gregorie
This sounds like a very good idea, though if you use a water column, something impermeable on the top, such as a loose-fitting polystyrene cylinder might be good: I just wonder how much the water vapour in the low pressure chamber may affect the ignition sequence - especially with the motor at -60C.
On second thoughts, water vapour in the chamber is probably a good idea because things could well ice up while LOHAN is on the way up under the balloon, specially if its a bit cloudy that day. If it ignites under icing conditions in the test chamber it should be OK on the day and less likely to fizzle when lit for real.
BTW, I agree that the rocket powered altitude gain will be much less than that under the balloon. I've seen the RC rocketry boys flying S8E boost gliders (typically 200g, 200 sq.in wing) using cartridges with similar thrust and burn time as Lester is considering. These models are capable of getting to 1100 ft on an Aerotech E6 cartridge. Here are some launches and an idea of the size and shape of these models:
http://www.youtube.com/watch?v=t52Ii55kgt4
They'll obviously get higher in the thin air at 80-90,000 ft but possibly not more than double the height.
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Wednesday 14th September 2011 11:36 GMT The First Dave
"They'll obviously get higher in the thin air"
Not entirely sure that is true - in order to maintain aerodynamic control, the flight surfaces are going to have to be much larger, and unless I am misunderstanding things, the basic point is not the size of the fins as such, but the drag that they create - presumably just as much drag will be required at altitude as is required as sea-level?
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Monday 12th September 2011 16:20 GMT David Given
Partially blocking the nozzle of a rocket doesn't just sound like a recipe for disaster, it sounds like getting disaster delivered wholesale by the lead manufacturer of disasters and then getting it installed by top-of-the-line disaster creation experts... I like the idea of the 10m column for a hypobaric chamber, though; simple, effective and less likely to explode in your face.
Might it be possible to avoid the temperature problem by just insulating the engine? A couple of shaped expanded polystyrene blocks either side of the engine, split down the middle so they fall away when the engine fires... would that keep the heat in for long enough?
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Monday 12th September 2011 16:47 GMT Neil Barnes
Mixing a number of options here:
A slight modification of Mark 62's method here: a four inch PVC pipe held within a six inch PVC pipe by suitable nylon bolts through the outer pipe to centre the inner: note that the inner pipe is only supported, not punctured. Let the inner pipe protrude above the outer so that a (possibly sacrificial) vacuum spigot can be attached. Cap the bottom of both pipes.
Insert the rocket and launching device (presumably a pressure sensor of some type) in the centre pipe and seal the end with e.g. non-setting silicone goop or a suitable o-ring and a flat plate.
Fill the gap between the two cylinders with liquid nitrogen. Subtract air through the vacuum spigot, after retiring to a safe distance.
When the air is being pumped out, the lid holds against no more than atmospheric pressure; when the rocket ignites, the exudates will rapidly fill the inner tube and blow the lid off. The intention is not that the rocket leaves the tube, but it may well - that's a bonus. What we're testing is whether the rocket and the starter will work, so we'll probably want to do it more than once.
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Monday 12th September 2011 19:10 GMT Anonymous Coward
What about a small scale test?
Rather than using the same size motor as planed for the LOHAN, why not buy the smallest, cheapest version that they sell? Since the test is just to see if this kind of engine will fire under the predicted conditions, save some of the budget and get a sample motor. You could also use an old icebox and run your vacuum pump to lower the pressure. Dry ice will help with getting the temp down, and the box will help keep the cold in. Just a thought.
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Monday 12th September 2011 20:22 GMT Hawkmoth
DIY is indicated, you bet!
This is definitely a case for a diy vacuum chamber. I like the 10 m pipe idea, but a 10 m high column of water's going to weigh a ton and be pretty unwieldy, leading to its own problems before you even get to evaluate the rocket's. I think you can get enough information by using a compressor tank and pulling as much air out of it as you can with a hoover or a hand pump of some sort. Cold is also important but you just need to ice down the motor with dry ice before you install it.
To the person who commented about plugging the motor being dangerous, unfortunately, pressure is important for proper ignition of gun powder (which is what the propellant is). Pressure is why gun powder just goes WOOF if you light a pile of it on a table but does BANG if you light the same pile while it is bottle up in the chamber of a firearm. Once the motor is burning, the back pressure from the nozzle will sustain the reaction, but it might be difficult to get going in a near vacuum.
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Tuesday 13th September 2011 12:12 GMT Alan Brown
vacuum chamber
1 length of heavy 4 inch PVC pipe with flanged ends. Vaseline the flanges, put flat 4mm thick aluminium lids on.
Suck out the air. External pressure will hold everything together and the lids will fall off once the motor ignites so you won't destroy the setup.
As for burst plugs: a bit of soft wax (beeswax, etc) will do the trick and what doesn't blow off will burn away in less than a second. You don't have to go overboard, just make sure whatever you use doesn't go rock solid at -60C.
I'm not sure why you're worrying about ripping the wings off. There's virtually no atmosphere at these altitudes, so very little friction. G forces are your only real worry and as long as Cp is behind Cg the thing should stay more or less vertical.
Having said all that, as others have said these motors don't have much sustained thrust and you might not gain all that much altitude. Laws of diminishing returns and all that stuff.
Terry Pratchett once compared current rocket technology to firing a gun at the ground and riding the recoil to orbit instead of riding the projectile. He has a point...
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Wednesday 14th September 2011 11:51 GMT LucL
Additional upgrades
Hello,
1) why not add a little pressure sensor in the rig. This is an easy proxy to determine if the motor is running fine (a sensor, three wires passing through and an ADC-datalogging - check Freescale MPX-range of pressure sensors and and the ADC datalog from Sparkfun electronic).
It is to be compared with an identical test without dry ice.
2) Why not add a cheap video (keyring type) in the test rig (just protect the the camera with a piece of plexiglass). This will show how quick is your ingnition and help debug if needed.
3) Don't forget to precool your motor until it reaches the temperature.
Good luck with the test.
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