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back to article LOHAN to brew thermite for hot ignition action

The first rocket motor tests in our magnificent Rocketry Experimental High Altitude Barosimulator (REHAB) rig went with a whimper rather than a bang earlier this week, as both solid propellants failed to ignite. Click here for a bigger version of the LOHAN graphic Despite the best efforts of the SPB's elite team, who occupied …

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Anonymous Coward

So, ignore the Mach problem and that you're going to rip the wings of your aircraft as it goes spinning off.

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(Written by Reg staff) Silver badge

I'm fairly convinced the Southampton University postgrad team designing the Vulture 2 know their business. They certainly know what motor we're proposing to use, what thrust it has and what the acceleration and ultimate top speed of the aircraft will be, so I wouldn't lose too much sleep over Mach numbers, if I were you.

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Anonymous Coward

And what did they say about lighting the blue touchpaper?

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re: Pratchett

Don't stick it up your nose first.

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Thumb Up

Thermite

As a mere admirer of explosions - certainly no boffin - thermite gets a huge thumbs up from me*

* for what it's worth.

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Devil

Re: Thermite

Make sure you _DRY_ it very well if you use this approach of building a thermite head.

I am speaking this as a chemist and someone who can now turn the sausages on the BBQ without a fork - I had an unfortunate incident with humid thermite during my first year in a university. It was wet and went into an air-dust cloud which burned instantaneously. I got lucky - 3a degree burns across most of my palms and a few spots of 3b resulting in losing most of the heat and pain sensitivities in them forever.

So based on experience - make sure it is in correct proportions (a bit difficult using the wet dip method) and is dried properly. Otherwise... Things can get funny...

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Re: Thermite

Dry and compressed thermite doesnt explode. It just gets very hot & produces molten iron. This is how they used to weld railway track, perhaps still do. Having said that, one of my teenage escapades was to mix a stoichiometric mixture of Al powder, & Ferric oxide, both readily available finely divided as pigments at the time, in a 1 gallon paint tin. However the mixture was very fluid, and I knew it needed to be compressed under vacuum but didn't have the kit to do that. So we stuck Mg ribbon in the top, lit it & retired immediately. The thermite caught, and the superheated air in the mixture blew the stuff into a fantastic fire fountain, followed by molten iron running over the ground (this was in a disused quarry). Sadly, today, I would be banged up as a terrorist for doing that:(

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Childcatcher

Re: Thermite

Thermite is GOOD. But you have to get that lit - same problem as before.

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Mushroom

"You'll get molten iron being sprayed onto the inside of the motor. This will get it started."

Yup, sounds good to this pyromaniac!

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Alternatively...

how about launching the ballon with the rocket inside so that the balloon provides both initial lift and the atmosphere for the rocket to light? I assume that the rocket would then force its way out...

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Re: Alternatively...

That is bad idea on a several count. First, it wouldn't work. The air pressure inside the balloon would be the same as the pressure outside of it, or at least close enough as to make no difference. Second, how would you get the rocket into the balloon without damaging it? Third, yes the rocket could probably punch through the balloon, but it would use a significant portion of its kinetic energy to do so. Put into perspective for you, forgetting to take the cap off of the launch rod in a more traditional model rocket launch will cut your altitude to about a quarter of what you would normally see (speaking from experience here). That cap offers far less resistance than the skin of a weather balloon would, so I wouldn't expect to see the rocket achieve more than a few meters after punching through the balloon.

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It's amazing the number of problems that go away when you use Thermite.

It's incredibly useful stuff!

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Boffin

Re: It's amazing the number of problems that go away when you use Thermite.

Hmm.

Eyes the stubborn stain in the bathtub.

Thermite, eh?

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@ John Gamble

Yep, use thermite, hence no tub, thus... no stain! Perfect!

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Pint

Re: It's amazing the number of problems that go away when you use Thermite.

The side benefit is that you get a second drain hole in the tub, in case the first one gets plugged...

(test it out with beer)

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@John

Do you want it done right? Or do you want it done fast?

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Lighting the lighter....

Will thermite be that much easier to ignite than rocket fuel?

I'm thinking the yank was right, loosely fill with a flash powder that will get the burn to the whole cavity quickly.

BTW From the pics it looks like the match wasn't in contact with the fuel, I guess sparks go out quickly without oxygen so closer contact might be the issue.

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Boffin

Re: Flash Powder

I disagree about flash powder - fairly sure that the solution is a longer burn from the igniter, rather than a slightly bigger, but very quick burn - think about standing snuffing out a candle with your fingers: a quick pinch does no damage, but hesitate a bit and you will get burned before you even touch the wick.

Close contact itself probably won't help either - again the analogy is a candle: the wick is totally flammable, but fails to burn because it is inside the flame, whereas a dry wick held just outside the flame will light quickly.

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Re: Flash Powder

I was thinking more about the rate the flame front moves around inside the motor. I think there's a risk of a slow start which gets the rocket out of the launcher but without the oompf until it's too late.

Anything in full contact with the fuel should trigger it, the candle analogy doesn't really work because there are no wicking effects here.

Obviously a longer burn will transfer more heat to the fuel so should help.

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Re: Lighting the lighter....

Actually this is where the magnesium ribbon comes in since you need this temperature to ignite thermite. I wasn't completely sure if I should publicly recommend the full monty of pyro powdered aluminum and ferrous oxide complete with the magnesium ribbon as the starter. Thermite also has the advantage that it doesn't "explode." It burns very rapidly and very hot. The amount you are going to be making is going to combine so fast that the cap does make sense to hold in the heat until the fuel catches and blasts it out the back.

Just went out poking around and there is the Magnelite igniter. I have no idea how well these things work, but they are magnesium and if the Copperhead cannot provide the heat to ignite magnesium ribbon or shavings from a firestarter (as a failsafe) to ignite the thermite, then a magnesium igniter in direct contact certainly should be able to do so. The temperature you have to hit to ignite Mg is 473°C. It will burn just fine in CO2 if you want to pre-test the ignition of just the Mg and the Copperhead in a cold cold dry ice pit.

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Flame

Thermite. Definitely go for the thermite.

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Mushroom

How about plugging the igniter itself?

I've got no experience here so I could be talking from my nether regions but . . .

If the problem is dissipation of the igniter flame, how abot putting a "plug" directly over the igniter. Presumably it's shaped like some sort of rod in contact with the fuel. Could you perhaps put a lump of something directly on top of the igniter?

Obviously the low-temperature is a consideration on what you use. I'm thinking something like silicone sealant, clay or even araldite. Whatever you use, all it has to do is confine the flame against the fuel long enough for the fuel to ignite. It will then presumably get burnt away. I'm assuming that the remains of the plug are not a massive concern given that there is presumably debris left in the combustion chamber from the igniter anyway?

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Anonymous Coward

Thermite

Despite occasional spontanious combustion, it's not that easy to light, and more so when it's cold (and oddly enough damp), a chopped up sparkler wrapped around visco (then wrapped in magnesium ribon) does the job consistently for me, the other option is to mix aluminium (or preferably magalium) powder with water then mix in plaster, fill a straw and you end up with a thermite fuse. Or you could melt potassium nitrate and sugar (gently!) slow the burn by chucking in some some bicarb or wax and create rocket candy sticks to use as an igniter "core"

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I am pleased! It's good stuff. However, I advise you to not make a large batch and store it, but instead make small amounts each time. And discard the leftovers. Even if you put the paste (it's pretty safe when wet) in a nice screw top jar, you'll get crystals growing up into the screw thread of the container which will go off from the frictional heating next time you open the container. It's fascinating to watch but worth avoiding. The base pyrogen paste is made from letting an estes motor dissolve in water. The cardboard tube will sort of collapse and go mushy (just fish it out, along with the little cermic nozzle that was inside it) and the propellant will dissolve. You might want to help it along by breaking it up a bit with a screwdriver once it's wet. I added about 20% IPA (not the beer) to the water to help it as a solvent.

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This post has been deleted by its author

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Flame

Alternatives to Thermite.

Thermite can be a bugger to ignite. Go For Flash Powder! much faster burn, bit easier to light than Thermite and just as easy to make. (it'll still need a bonding agent like the thermite to hold it in place.)

Powdered Magnesium (Filings) & Powdered Potassium Permanganate Crystals (yes the Deep Purple ones)

Job Done.

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Anonymous Coward

Re: Alternatives to Thermite.

Be careful what bonding agents you use, glycerine and Permanganate will set you off all premature.

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Flame

Re: Alternatives to Thermite.

I think Barium Peroxide may be a better-behaved oxidizing agent than permanganate or saltpeter. It's stable up to about 500C, above that temperature it releases neat oxygen.

Do take great care with this sort of research. I once read an article by someone who'd managed to set a piece of Titanium on fire.

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Re: Alternatives to Thermite.

You don't want faster burns! That's not a good thing. However you're right that a bit of it might help activate the thermite, but I've not found activating the thermite to be a problem so far.

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Anonymous Coward

Re: Alternatives to Thermite.

DON'T DO THIS!

Permanganate flash is horrifically unstable. There is a reason no self respecting pyro has used it in the past 60 or so years. Not that it is particularly relevant in this situation because flash won't solve the problem. As others have said, they want a slow burn that produces lots of slag. There are dozens of pyrotechnic compositions that are designed to do precisely that, they are called primes, Pinball prime and Fencepost prime are two fairly common ones.

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Transfer of heat from a match to a fuel grain is not usually done by conduction, but by hot gas.

Burning match composition produces hot gas initially at the density of the composition - in other words the gas would be at what we might consider a very high pressure. The gas then expands according to it's surroundings, very high presure if it's tightly contained, and lower if it is open to some surrounding atmosphere.

If the surrounding gas pressure is very low then the gas expands more than it would do at atmospheric pressure - and the important point here is that when gas expands it cools. At lower pressures it expands more, and therefore cools more. The gas which is supposed to ignite the fuel grain is now cooler, and also less dense (so it has less heat-carrying capacity), than it would be at atmospheric pressure.

The standard way to solve this is to use medium-grain silicon powder or something like that in the match. The outsides of the grains of silicon burn in the match and get hot, and red hot globules of molten silicon (note it's a liquid, not a gas, so it doesn't cool by expansion) splash over the surface of the fuel grain, igniting it.

Thermite would probably work just as well, the hot iron droplets produced would not cool by expansion as they are not gasses.

Incidentally IIRC (and IANAL) I think it would be legal to do this in Spain, but it would not be legal to make a thermite match composition in the UK. However if you wrote to the HSE they might well give you a dispensation, they are quite good about that sort of thing

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This post has been deleted by its author

The other half of this (which is still only half of the problem) is that it is good to have a match composition which doesn't produce much gas - for instance, for the silicon match described the desired reaction is solid_oxidiser plus silicon gives solid silicon_dioxide (aka sand) plus spent_solid_oxidiser. There are no gaseous products.

We need some gaseous products though, to spread the hot molten extra silicon about. That's not hard though, as silicon plus solid_oxidiser doesn't burn nicely for any normal versions of solid_oxidiser, and the usual compositions necessarily include some gas-producing fuels.

But match compositions for vacuum use can't produce too much gas, or they will burn too cold, and the spray of hot liquids will be cooled so they are not hot enough to ignite the fuel grains, especially if the fuel grains are very cold.

And here we come to the other half of our overall problem, which our gallant 'nauts may face soon. Even if the fuel grain is ignited - it may then go out. The cooling from the expansion of the gases from the ignited face may be enough to cool the burning surface of the grain to the point where it cannot ignite the next layer of grain below.

By the way, if you doubt that cooling by expansion can do that, have a look at this:

http://www.nasa.gov/mission_pages/constellation/multimedia/cece.html

It's the exhaust of a liquid hydrogen / liquid oxygen rocket engine, which was about 6,000 F when it was burnt, and which is below freezing point by the time it expands enough to get to the nozzle exit. So much so that icicles develop on the end of the nozzle.

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FAIL

Rocket exhaust temperature is not below freezing

Typical rocket exhaust temperature is about half at exit of the nozzle than it was at entry, so it's still thousands of degrees (in this particular engine 2760C). Those icicles were formed due to cooling the nozzle with liquid hydrogen. A thin layer of steam along the inside of the nozzle condensates into water. At high power icicles wouldn't form, but when you switch to low power the excess water will form icicles, see eg. http://www.nasa.gov/multimedia/nasatv/on_demand_video.html?param=http://anon.nasa-global.edgesuite.net/anon.nasa-global/MARSHALL/CECE_Engine.asx

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Facepalm

I'd guess that's why they call it Rocket Science

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I'm just wondering...

How many of the correspondents here are now candidates for the various terrorism laws? If it all goes quiet, we'll^%$^"££$&*%^*(*(&)

+++NO CARRIER+++

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Go

time

I imagine that the vacuum pump achieves the target pressure much faster than the balloon ascent to launch altitude will take for the actual exercise. I mention this because if you use additional igniting material that contains volatile compounds, those compounds will have dissipated to a greater degree in a balloon launch than in your ground level testing. Perhaps you should take a roughly equivalent time to draw your vacuum so that conditions in the test chamber more closely match those in the forthcoming blessed event.

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(Written by Reg staff) Silver badge

Re: time

It's a fair point, but unless we use a full-scale hypobaric chamber, that level of control isn't available. Sadly, I don't think QinetiQ will let us blow up their Farnborough facility: http://www.theregister.co.uk/2010/09/15/qinetiq_result/

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Pint

Re: time

Perhaps it would be good enough if you draw your vacuum and just let the apparatus sit for while, turning the pump back on from time to time to make up for any leak losses. I think the time it takes to drink two or three pints or glasses of wine ought to do it.

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(Written by Reg staff) Silver badge

Re: Re: time

Provocative. So, we're looking at a 90-minute simulated ascent, which would be around eight pints at the standard Vulture Central Friday night rate of consumption...

This could prove very expensive in terms of liquid fuel.

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Mushroom

Re: time

As long as it took out A1 building, I don't think you'd find many of the staff at QQ complaining about the noise and disruption...

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Mushroom

You need to take into consideration the launch attitude of your test rig is inverted - if you ignite some thermite in there, the molten steel end result will fall down onto the fuel grain.. This will not happen in actual flight use! You may want to build a frame of some sort to do a final test in flight orientation.

I'd suggest a flash powder based igniter - but there are some things you need to know. Under no circumstances use a chlorate based composition - ammonium chlorate could form at the interface between the grain and the igniter comp, an extremely unstable substance. I'd suggest a 60/40 (by weight) potassium perchlorate and magnesium/aluminum alloy 200-325 mesh, bound with nitrocellulose("NC") and a solvent. A suitable alternative to NC is the easily-obtained ping pong ball. What you would do is make a slurry of this stuff, and paint a bit on the end of the fuel grain. Bound like this, it will burn rather slowly and extremely hot. You wouldn't want to coat the entire exposed fuel grain, the idea is to produce a good sized high temperature flame envelope to ignite the still exposed fuel grain, rather than the fuel grain directly underneath the comp. You'd also want to use the slurry to glue the igniter on to ensure ignition, or coat the igniter in the slurry.

An alternative comp would be potassium perchlorate and aluminum 325 mesh (70/30 respectively). I suggest flake aluminum for this but spherical would work as well.

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Flame

Way back in the bad old days, certain air-to-air missiles used nichrome wire and black powder to fire the propellant. Always worked from near ground level to max altitude of the aircraft.

Flame.. cause that's what we want to see....

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h3
Bronze badge

Best way to detonate thermite is barium peroxide.

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Pint

Burn baby burn

I hope Playmonaut is off his holidays and back on training again;

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Looks like it will take a few shots to get a good ignitor

..so it may save a bit of cash to experiment with smaller, cheaper motors until you can get reliable low pressure and low temp ignition. Have a word with your suppliers. Explain the problem and ask them about cheaper motors that use the same propellant and if there's a volume discount.

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Boffin

Use a different fuel

I think you will be disappointed with the performance of a black powder motor, even if you can get it to light. BP doesn't work well at low pressures. You would need a high volume chamber to reveal this, as in a small one the exhaust will rapidly raise the pressure, but good luck with that, as no self-respecting chamber owner would let you test a BP motor on account of its remarkably filthy solid exhaust products. There are plenty of alternative fuels known to work at low pressures, but most of them use perchlorates and you probably can't buy them off the shelf. Potassium perchlorate/potassium benzoate works well, or ammonium perchlorate/aluminium. There are plenty of people who know how to make these if you ask around. Best of all would be a nitrocellulose fuel like cordite, but since this is happening in Britain I suppose that's out of the question.

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Facepalm

Lots of interesting complicated ideas, but...

I posted this on the previous thread, but then thought you might not check the old stuff...

The real problem is the transfer of heat through an expanding, burning gas. In a near vacuum, the hot gasses see the vacuum outside the plug and goes racing towards it like a drunken Hollywood starlet in a Porsche heading for the back of the nearest semi.

The trick is to slow that escape down. A silicon plug seems like a good idea, but putting it in place will seal in the 15psi/1 bar/1 atmosphere (pick your metric/imperial/generic terms as you will) for the entire pumping down to vacuum or flight in the real case. That means it's got to be in the nozzle awfully tight, and you're begging for a container over-pressure, also known as a "CATO", or catastrophic failure. (No, not O.J.'s house guest.)

But, there's good news. What you really need is to keep that gas in the nozzle just a little while longer, to slow the escape out through the nozzle, yet still allow the whole thing to equalize as the chamber is pumped down.

My recommendation? Simply pack the igniter into the nozzle using -- simple cotton.

The cotton will allow the pressure to equalize as the chamber heads to vacuum, yet will still hold the hot gasses in place for those needed fractions of a second. And there's no way that a little ear-bud's worth of cotton will stand up to the high pressure as the chamber fills with burning fuel gasses.

Simple, cheap, and whatever cotton you have left could be used for other things, like peroxiding your hair or something...

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Devil

The reason why

I suggested the guitar pickup wire was because bridge wire initiators, tend to go best when very fine and the limitations of the power supply would deem that guitar pickup wire is actually "hair thickness"...

MMmmmm actually, One needs thicker wire to hold the bridge wire inside the mass of ignitor, and holder wires leading into the mass as well.

It's just the motors ARE very small, so we need to size proportionally.

But yeah... there is some very interesting material on the development of high altitude aircraft during WW2, and the problems they faced on every front, due to the very low atmospheric pressure and the extreme cold...

It was the issue of getting the bombers (mostly) above the height of the AA fire and the fighters - their time to launch and gain the altitude.

So with the rocket motor's ignition, what we take for "face value" at ambient pressures and temperatures - completely goes out the window at very low pressures and temperatures.

The reverse of this is to compare in reverse, how black powder lights up under different temperatures.

If a small teaspoon of it is lit with a match, it "burns and flares" and that is not such a big deal.

But hold the same teaspoon of powder about 8" above a candle, at arms length, so the mass of powder more or less slowly heats through until it ignites by decomposition - from heat alone - and you will have an extremely intense fireball that flashes in an instant - and removes ones eye brows..

"Don't ask how that happened!"

Since chemistry is all electronics and the transfer of energy -

Good ideas / observations:

On the basis that the problem is due to the low ambient pressure resulting in the too rapid dispersal of the initially vaporised fuel, before it can ignite the rest of the fuel

Bad comments.

"For example, black gunpowder is typically regarded as an explosive" - it's a propellant - that propogates by ignition, not by concussion - and the way it burns, that depends upon grain size and the degree of confinement.

A dip cast coating or rice grain sized pellet of black powder, will burn completely differently to FFFF grain powder inside a sealed chamber behind a bullet.

My prognosis...

The copper head ignitors seem to be fine and reliable... I would be inclined to simply add to their mass of combustible material by using a ground up match head slurry mixture, or some black powder slurry, with a small amount of binder, say 3 - 5% gelatin by weight to make it stick on and hold it there without unduly quenching the flame as a wax coating / binder may.

Assuming the mix was 20% black powder, 75% water and 3-5% gelatin - and dipping the copperheads in that mixture.

Then dry them promptly and properly on top of a simmering pot - away from the steamy vent...

That should work.

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Headmaster

Stratosphere cartwheel worry

I'm concerned about the comment from a poster in the previous thread who pointed out that even if you master the ignition problem (which you will), then you're got the problem that the rocket will blast off into air so rarified that the fins at the rear will offer no useful stability.

This will result in the rocket cartwheeling uncontrollably and going almost nowhere. There's a reason that Von Braun's A4 (aka V2) rockets had graphite vanes in the path of the exhaust, coupled to gyroscopes in the nosecone. Later, more powerful designs achieve the same control effects by gimballing the entire engines.

LOHAN is sufficiently low-power that graphite vanes would work just fine, but adding all the actuators and gyros will make the whole endeavour far more complex than it is at present. And crank up the costs of course.

But will it even work without such measures?

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