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 of …
"attached to something cheap and expendable"
May I suggest an eloquence of lawyers?
Lawyers are expensive.
How about X-Factor runners up?
re: "attached to something cheap and expendable"
Unfortunately Lawyers are not cheap, unless refering to their aftershave and dress sense.
Although I totaly agree with you about them being expendable.
Paris: Because she's cheap....
That would satisfy "expendable", but fail rather seriously on "cheap".
We may have to settle for Big Brother contestants to avoid breaking the budget I fear.
cheap != inexpensive
Look it up in your Funk and Wagnels. Or your OED.
Let me think
You need something cheap, expendable, bitterly cold and with no atmosphere... looks like a trip to Blackpool is in the offing.
DIY hypobaric chamber
Time to build your own simple hypobaric chamber?
Wouldn't have to be very large if all you want to test is ignition.
Just a length of steel pipe big enough to take the rocket, with a blow-off plate at one end that would release when/if the motor fired.
Re: DIY hypobaric chamber
As part of my mulling, I was thinking about something along the same lines. We evidently need to test possible motors at low pressure.
...getting some scaffold tower and some 4 inch plastic sewage pipe + fittings. The idea is to use the pipe like an old mercury barometer, but using water instead. Mercury is expensive. So, you have a 10m vertical pipe. Put the bottom in a bucket with some water in it. Put an end cap over the bottom, with some kind of stop valve fitted. Fill the pipe up with a hose pipe. Then cap off the top with a modified cap with the rocket motor epoxied into it, so that the motor is inside the pipe. Open the bottom stop valve under the water in the bucket. Then the rocket is at low pressure. The pressure is adjustable by how tall the water column is.
What could possibly go wrong?
Actually, I guess you don't need the whole column to be made of 4 inch pipe, one inch pipe would be good enough, and more flexible, and quicker to fill. You just need the rocket ignition chamber to be about 10m vertically above the bottom of that pipe. A steep hill might substitute for a scaffold tower.
OK, enough, I'll get my coat...
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.
"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).
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.
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.
> 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?
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.
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.
Just use some liquid nitrogen.
Of course, then you have to worry whether the bits holding the rocket still are going to fracture in the cold, or from the temperature gradient achieved after firing.
What could possibly go wrong?
But quite within the capability of dry ice.
Re: Borrow a hypobaric chamber?
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.
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?
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.
For -60 degrees, I would suggest you need to get hold of some liquid Nitrogen.
Having done that, you might as well get hold of some LOX as well....
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.
LN2 is great fun.
Don't do any of this. Get a two litre plastic Coke bottle. Put about 2 inches of LN2 in it, then screw on the top after smearing some silicone sealant inside the cap. Invert. Stand well back and wait about 5 minutes. Very, very loud bang! I couldn't believe how strong those bottles are.
Re: LOX is *much* more dangerous,
but it's also so_much_more_fun.
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.
LOX Makes Most Things...
On the other hand, Liquid Nitrogen is safe enough to store in a Styrofoam coffee cup.
Yes, of course it helps things to explode/burn, that is why it is _essential_ for any _proper_ rocket...
built a hypobaric chamber when they were testing whether or not bulgarian airbag implants will explode during flight.
This strikes me as the sort of test they would be delighted to assist with!
But sadly they're tossers...
Dry ice is about -70'c, rap that around the rocket and you would be able to test it's firing ability in the cold.
Cheap and expendable
Will you find an X-factor contestant that's up for it?
Find a friendly university.
I used to work in a Physics dept where the lab technicians would gladly knock up the chamber and provide liquid nitrogen to get down to -60 in return for a pint or two at he local. Especially as the whole project looks like fun.
Build your own cool box?
Put the ignition system into a tube or box, then wrap that in dry ice (-78C). Allow to cool. Take outside and play:-)
BTW, what's the price of one of these rocket motors?
> BTW, what's the price of one of these rocket motors?
About US$20 each, plus the HAZMAT fee for the "reload"
The reload able engine case is under US$90.
I thought titles were passe
Assuming a successful launch occurs at say 90000 feet, can any mathematicians out there advise how much extra altitude is the 9 seconds burn time likely to achieve? Presumably it is weight and drag-dependant, but some upper & lower estimates would be interesting.
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.
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.
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.
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.
You Wanna Gain Some Altitude?
Forget about 2-pound-thrust G class engines. Go with one of these babies:
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.
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.
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.
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:
They'll obviously get higher in the thin air at 80-90,000 ft but possibly not more than double the height.
"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?
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?
How to test a rocket at realistic high-altitude conditions.
Stick it on top of another rocket and see what happens.
I heard you like rockets
So I put a rocket on your rocket.
There's no way it's going to get even close to the desired altitude though, unless you use a frigging big mofo of a rocket as the carrier.
Low pressure tests
Just out the igniter + rocket in a small length of pipe, and suck air out with a hand-pump. Nothing fancy required. Doesn't matter if you destroy the test equipment in a single run.
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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