Oh no boys,
You'll have to phone NASA.
It's been a lively couple of days down at the Special Projects Bureau, with the first of our Rocketry Experimental High Altitude Barosimulator (REHAB) experiments, designed to determine whether solid propellant rocket motors will fire under simulated high altitude conditions. Click here for a bigger version of the LOHAN …
Fancy crushing some rocket fuel? I'm not sure I do.
However, the idea of casting the igniters into a block of something fuelly, pressing into place, and then popping a smidge of silicone outside that seems rather appealing. Then the silicone seal is against something solid, and there's no pressure differential across it until you press "go!".
A small bit of magnesium ribbon wrapped around the Copperhead would generate a nice sustained heat for a second if the heat of the propellant in the engine is an issue, but it would need a source of free oxygen for it to burn. Sort of a secondary igniter. I am trying to think of a powder that might do the trick. Finely powdered ferrous oxide?
Any other suggestions?
I bigger actually better?
If ignition requires air to transfer heat from the head to the fuel there is no way it will work. What you need is direct heat impingement from say a small motor with integral igniter if there is such a thing. Looks like you might be stuffed.
It is a good job you did a ground test.
Not sure a sacrificial silicon seal would be strong enough, it's gonna be seeing close to 14 pounds per square inch which will probably mean it'll just pop off before you get a chance to fire the motor. Maybe a weakened perspex disc bonded to the nozzle would be better but if it is too strong it could make the motor go bang rather than burn, bit of a balancing act getting it right.
Even if the seal does not pop off, the chances are that the air will slowly leak from inside the motor. The body of the motor may not be leak-proof in the first place, and as the temperature drops imperfections of the sealing will be getting worse. Even it works in REHAB it may fail at launch as it will spend hours in near vacuum and in the cold.
I would rather try to find some substance that could act as "ignition booster" to put between the igniter and the "main" fuel.
We have suffered the same problems as you with low pressure ignition. The flame front from the burning igniter products dissipates too quickly (thanks to the low ambient pressure), before it's had enough time to transfer enough thermal energy into the propellants to get the party started.
You can either plug the motor (would have to be done very well to hold a seal for the couple of hours of balloon ascent) or modify the igniter. We have in the past used some special sauce that was a bit of pyrogen dissolved in water (which makes a paste) with aluminium powder and iron oxide powder (rust). This paste (I call it rocket bbq sauce, but I am silly) can be applied to the igniter head in several layers (dip, let dry, dip, let dry, etc for 5 or 6 times, like coats of paint) and then will produce a lot more energy, the thermiting ali and rust powder especially are in liquid phase (molten iron) rather than gaseous as they react, which helps with gets you round the hot gases dissipating in the low pressure. You'll get molten iron being sprayed onto the inside of the motor. This will get it started.
The flame front from the burning igniter products dissipates too quickly (thanks to the low ambient pressure)
Rather the inverse. AIr conducts heat (by convection, mostly). Lack of air means lack of convection means lack of heat transfer: the principle of the thermos flask.
I'd say the igniter has to be well bonded to the rocket fuel, ideally with some goop that burns (and fairly easily ignites) while supplying its own oxygen.
1 ohm might be a little too low. It would probably burn out before it can inject enough heat.
Hackaday recently featured a post about using resistors to launch fireworks: http://hackaday.com/2012/07/04/a-resistors-fiery-death-used-to-launch-fireworks/
I'm not sure however that these rockets are launched by just heat or if there is more at play here.
To ignite the propellant components must first be vapourised, then the vapours should mix to form fuel, only then the ignition is possible. The igniter will vapourise the propellant even at low pressure but then the gases will be vented immediately before they have a chance to form fuel mixture.
I should think a simple plastic or rubber plug in the igniter hole would be enough. It should not be gas-tight and be able to withstand the full pressure differential. In fact, I'm afraid that if it will be so strong it will blow up the motor. The task of the plug will be to prevent just enough gas escaping the motor to initiate the burn...
Probably all true from Vladimir above, I'm no expert on this but bear in mind that when you fit the plug you'll be doing so under normal atmospheric pressure (close to 1 bar) but you'll be evacuating the chamber to around 20 mbar pretty soon afterwards. You'll want the plug to be loose enough or porous enough that the ambient pressure air inside the motor can be extracted along with the surrounding air.
There is also the question of how the copperhead igniter works... does it try to eject a small amount of burning gas in the general direction of the motor's fuel core? If so, it might be that the burning hot gas dissipates too fast in the 20 mbar environment (and in doing so, cools down too fast).
I'd think that if you laid an elongated horseshoe of manganin resistance wire physically inside the hole that runs through the rocket fuel core, then you could energise that. It would heat up to red or orange heat if you got the sums right, and being in contact with the rocket fuel *ought* to set it off.
Caveat: I have a lot of experience igniting engines, but just the black-powder type, not composite engines.
That being said, in my position of knowing just enough to be stupid, I have a suggestion.
Estes now ships little plastic igniter plugs with its engines, but they didn’t 20 years ago. Back then I found out, through plenty of trial and error, that physical contact between the fuel and igniter is essential to ensure reliable ignition. (Using the plastic plugs they ship now, I have *never* encountered an igniter firing without lighting the engine.) My solution was to place the igniter in the engine and stuff a wad of tissue paper tightly in place, and then capping the whole deal with adhesive tape.
Now, of course, the physical construction of the composite engines you use here (and the fragility of the igniters) make that approach impractical. However, this might work:
1. Before assembling the engine, thread the igniter through the ignition slot such that the end protrudes out the far side.
2. Place a small wad of tissue at or slightly below the igniter chemicals on one side of the igniter; pull the igniter and paper into the fuel. The chemicals should be held firmly against the AP composite propellant.
3. Assemble the motor really carefully to avoid wrecking the igniter.
4. Apply burst cap silicone and let dry.
This should (a) maintain nearly ground-level air pressure and (b) ensure physical igniter/fuel contact.
Ah - you think you want maintain circa 1 bar pressure inside the motor do you? I was assuming (see post further up) that the pressure plug was just to cause pressure to rise and stop gas being lost as just the first couple of grams of fuel ignited.
If you rely on holding 1 bar pressure in the motor, it might work in REHAB but would be hard to maintain those conditions under a helium balloon that might take an hour to reach launch height.
If you have ever looked at the CCTV footage of the ignition phase of a LOX rocket, you'll notice an industrial strength sparkler is used to maintain a strong ignition source. Sparklers are essentially "Thermite" which burns hot enough to melt Iron.
Humor me and try this, take a few Ohio Bluetip or other strike anywhere Matchheads and gently peel the head from the wooden match. Crush them gently and use them to fill the end of the rocket motor nozzle with the ignitor making good contact. That matchhead stuff is typicallly a form of ammonium perchlorate and fuel that burns very well since it is both fuel and oxidizer in one. This "ignitor" MUST contact the rocket motor "Fuel". You can use alot more matchhead than is provided with an "ignitor" so there is way more flame that lasts longer. Then follow the proceedure from "Poor Coco" in covering the materials so they don't fall out.
Before electric ignitors for Estes model rockets were commonly available, we used to make them ourselves using a loop of thin stranded bare copper wire and electrical tape for insulation. We also tried using cannon fuse wrapped with copper wire. We used alligator clips attached to a 120 VAC extension cord and a light switch for power. The wire vaporized, broke the circuit and never even blew the circuit breaker. This ALWAYS provided reliable ignition (even when damp or windy) and worked with car batteries as well. For even more heat, the "Ni-Chrome" heating element wire from an old toaster worked quite well and glowed white hot.
Blimey... You just reminded me of what I used to as a, errm, teenager engaged in making things go bang... (it really was a long time ago).
I'd 'developed' an explosive mixture using potassium permanganate and magnesium powder. For some reason i also had a small supply of used 12 bore cartridge cases (I really can't remember where I got them - the chemicals came from *cough* school).
I used to run a loop of light gauge copper wire inside the case and pack the mixture round it. Then, with a very long cable (enough to reach the bottom of the garden!) I'd connect it up to the 240 ac in my bedroom. Pressing the switch would elicit an almighty bang, a satisfying purple mini mushroom cloud and several very irate neighbours + dogs.....
Ah.... Those were the days!!
You and I seem to have shared similar aspirations as teenagers. Yes, those were the days indeed, halcyon days of summer blowing up the sandbox with anything that could be made, bought or "liberated". I certainly gave the old man a few gray hairs. I like the purple smoke concept. I doubt any of us will ever have as much fun as back in those days. They'd shoot us all dead on sight today.
Copper pipe and shotgun powder worked best for me. See I had been making my own black powder since I was 11 but never seemed to get the ratios correct. Premade powder worked best. Matchheads came in a close second but were way too energetic for safety let alone the first time I tried it and just cut the head off the match instead of peeling it off (with the wood still below the blue stuff) when the "fireworks" went off, little flaming matchheads also flew all over the back yard and me.
The stuff you were using was almost Thermite (aluminum powder mixed with iron oxide instead of magnesium) but you had the ratios off (not so much potassium permanganate ). An old encyclopedia we had from the early 1900 had all sorts of great recipes for interesting combustibles. If you had added a few drops of Glycerine to the potassium permanganate it would have begun spontaneous combustion which would have made enough heat from the aluminum and rust mixture to melt through an engine block. They used this stuff to weld railroad rails. Your mixture sounds very "energetic"!
We had a great pharmacy that stocked sulphur, activated charcoal and potassium nitrate all of which could be bought by a teen ager with nothing more than a forged note. Mind you they each had to be bought at different times and the excuse for the potassium nitrate was it was for the sausages we were making.
That could never happen today.
BTW, others comments about the "sparkler" being for excess hydrogen doesn't jive. There is NO SUCH THING AS EXCESS HYDROGEN on a NASA rocket engine, If you had any significant leak, the launch did not happen. Yes, there was "Venting" but that happened at the top of the liquid fuel and oxidizer tanks, NOT at the nozzle of the engines.
You ALWAYS have to light the hydrogen BEFORE you open the valve for the oxygen, otherwise the whole thing goes BOOM like a cutting torch will with the wrong gas mix. My old man told me that was what they were for and he should know. He was a project manager for Thiokol and Bell Aerospace during the 60's and 70's working on Saturn, Agena, various ICBM rocket engines (See where I get it from?). There were SEVERAL methods for ignition used back then but on the pad they electrically ignited the "sparklers" then had different ways to ignite the liquid fuel boosters in space for secondary burns (High voltage spark ignition) Lighting the solid fuel boosters involved what amounted to a fireworks "Fountain" of sparks designed to contact a large surface area of the solid fuel inside the booster to get it burning evenly (thus even thrust). You know what happens when there is not "even thrust" combined with leaking O rings.
Nooooo very cold liquid hydrogen is COLDER and DENSER than air, so it sinks from where ever it's venting / leaking from - including things like priming the engines etc... and before they get lit...
As it warms up... it rises and forms pockets of very explosive gas mixes in what ever will confine it.
Rockets are fairly huge, and very expensive and fairly dangerous if they get damaged when full or fuel or when in flight and a few cubic meters of hydrogen / air mix.... underneath the rocket, around the engines, panels, bulkheads etc... and it tends to migrate UP inside of things...
Get a cupful of petrol and spray that around inside an abandoned car - that has all of it's windows, then arrange to have some sort of ignition from some distance or with a time delay (long piece of twisted up news paper going down into the partly open window?)
That is ONLY the volume of the inside of a car - but it will go off with a hell of a bang and it will shake a building 1 block away... "Fucking WHAM"
Hydrogen has a wayyyyyyyyyyyyyyy faster flame speed, and while it's energy density is lower, it's pressure wave is far more intense - if there is enough of it, it will just "shatter" things....
Remember that reactor with the confined hydrogen gas in the concrete containment building in Fukishima?
Granted that there was a lot of it... but Frightened to Death of it....
This is what the sparklers are for - to ignite any hydrogen gas, (or other fuel / oxygen mixtures) so it does not form "explosive pockets of gas" inside and underneath things like the shuttle or other rockets...
This is not correct I'm afraid. The sparkers are there to burn off any leaking fuel before the engines start, so it doesn't gather somewhere and explode at some later time.
All the ignition is done inside the combustion chamber itself by the rocket engine. The sparkers play absolutely *no* role in the ignition.
I like the video! Very cool.
The commentator comments that your motor is a "100N" motor which I thought might mean that it can generate 100 Newtons of thrust. If so, it would be able to lift 10 kg at ground level, which I think looks a bit optimistic...
Your kitchen scales bears this scepticism out showing a reading of 1797 grams when the motor lights (notice though that for most of the burn, the reading is more like 1400 g). Please note that thrust shouldn't be measured in grams as claimed at the end of the video. Keep thrusts in Newtons please, or you'll likely be headed for an accident.
The scales are actually measuring force, but "helpfully" converting this to equivalent mass assuming normal earth gravity of 9.81m/s². You ought to convert back. If you get a reading of 1797 g (which is 1.797 kg), then that's 1.797 * 9.81 which is a thrust of about 17.6 N
However, 1.4 * 9.81 - i.e. 13.7 N would be more representative of most of the burn time.
[ Additionally, since the motor was sitting on the scales and firing upwards (thrusting downwards) you needed to subtract the dead weight of the motor from the indicated scales-readings before multiplying by 9.81 to get the generated thrust. ]
This brings me back to the original comment. Why is the motor apparently claiming 100 N thrust. It's not even generating a sixth of that. Is the "100N" just an arbitrary part of the model number, nothing to do with the thrust?? Or what?
100Ns = 100 Newton-seconds, which is how rocket motors are rated. Thus, it can produce 100 Newtons for 1 second, or, more usually, 10 Newtons for 10 seconds., or any mathematical combination there-of.
It's a measure of the impulse of an engine:
P.S. It's been a LONG time since I've done much with rocket engines.
Been a genius Pyrotechnica...
Me thinks that the HUGE drop in temperature, like some 80+*C below normal ambient, will soak up enough of the heat from the bridge wire, as it heats up the ignitor compound, and that with the air gap and all between it and the very cold propellant, is enough to quench the ignition process.
Get some enameled copper wire from an electric guitar pickup, polish up a thin toothpick or polished steel wire from a base guitar or electric guitar string core, make a copper wire tail of about 3cm, then wind say 10 - 12 fine pitched turns around the tooth pick - be careful of the tension, too tight and it breaks and too loose and you don't get a neat winding, and you want a little "spring back"...
Then mix a paste of either some flash powder formula, or finely ground black powder whetted with some dilute gelatin in water (weak jelly mix) , and then dip your coil in the slurry.... enough to make a small pellet or rod of powder inside and around the core, then dry it and let it set.
Then use this as your electrically driven bridge wire ignitor.
You need a bigger, slower and longer lasting flash to ignite your very COLD propellant.
Did you attempt to fire the rocket motor again at ambient after the experiment? I only ask because, not knowing the fuel composition, it might be that one of the ingredients or binders that make up the solid fuel sublimates at low pressure leaving a non-viable mix in the combustion chamber. I know it isn't fun just burning through rocket motors (actually it kinda is) but it would be an easy test to see if the life was sucked out of the poor thing.
One of the concerns with sealing the end of the engine is that too firm of a plug may cause a blowout at the other end of the engine (Been there, done that, got to see the engine flying backwards out of the body of the rocket, whoopsie!).
For that matter, igniting a solid fuel rocket engine (or, for that matter, even a liquid fuel rocket engine) is a study in thermodynamics as well as acoustics. When the fuel ignites, you're going to have a pressure wave radiating outwards from the point of ignition. As more and more of the fuel ignites, the pressure wave may build. It will also reflect off of solid surfaces (other wall of the fuel, nozzle, sealed top end of the engine, etc.), resulting in a constructive interference pattern. The magnitude of this pressure wave can, under the wrong circumstances, build to a value that things give way. It's very possible to either blow out the nozzle, blow out the top end of the engine seal, or even fracture the grain of propellant. Note that you do NOT want to fracture the propellant grain, since this results in an increased surface area, which is where the burning will occur. A fractured propellant grain will cause a more rapid consumption of the propellant, resulting in increased pressures, perhaps even enough to blow out the engine casing (which is VERY bad news).
Thus, before stuffing anything in the void in the center of the solid fuel propellant grain, give some thoughts as to pressure waves, and how they may behave. You not only want to cause a reliable ignition; you want to do so in a relatively gentle manner that does not cause an echoing pressure wave.
There are various propellants and explosives readily available. Unfortunately, some propellants can turn into explosives under the wrong conditions, and you do NOT want an explosive as an ignition assist. For example, black gunpowder is typically regarded as an explosive, so you probably don't want it in the engine. Other propellents, such as nitrocellulose, with a bit of nitroglycerine added as a stabilizing agent (I'm being serious here!), may function as you want. Maybe. But, remember burn rates, pressure waves, etc.
There has been quite a lot of research done on this. Sadly, most of it is classified due to the potential applications to military weapons. Thus, you may have to redevelop some of the theories on your own, and do some experimentation.
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, I started thinking along the lines of using an equalising chamber to maintain pressure in the combustion chamber. For example, imagine two small balloons attached neck-to-neck, with one balloon inside the chamber and the other threaded through the nozzle to the outside, this double-balloon then being inflated to just above ambient ground-level pressure: as the ambient pressure drops both balloons will start to expand, the inner balloon expanding into and filling the combustion chamber and maintaining pressure within it. Once the rocket motor has fired I wouldn't expect it to have problems burning through the balloons as it's got to burn out the igniter wire anyway.
The tricky bit is ensuring an air-tight seal around the ignition wires and the balloon neck as they pass through the nozzle and enter the combustion chamber although as the balloons expand they'll tend to occupy any gaps and improve the seal. Of course, you'd also need to take into account the fact that the igniter wire will have to run around the inflated balloon within the combustion chamber i.e. against the walls which will mean more wire in the chamber than usual.
Alternatively, if you think that the motor would be able to burn through a fine nylon mesh, just stick a small balloon inside the chamber that's prevented from expanding out through the nozzle by a fine nylon mesh placed over the inside of the nozzle opening. However, the same issues re the routing of the igniter wire apply.
Ah, sort of like putting a balloon on the nozzle end of the engine. Interesting! That way, it'll hold pressure in the engine, and have enough capacity to handle small leaks. Plus, once the engine fires, it should blow/burn away from the nozzle quite rapidly. The upper end of the engine will have to be sealed, of course.
There'll need to be a bit of calculation as to how large the balloon can swell to in the reduced pressure of high altitude without bursting itself. But, it is an interesting idea!
I had the same thought when the too-rapid dispersal of gases was mentioned, but what popped into my head was not balloons, but a condom. A balloon would be better due to the tighter neck. Either should burn through quickly once ignition has been achieved.
Were I doing this here in the US, I'd just half-fill the cavity with a little loosely-packed, slow burning double-base smokeless rifle powder, put the igniter in, put some more double-base over it (again: loosely), and then some electrical tape (or Duct tape) over the bottom.
DO NOT USE HONEST-TO-PETE BLACKPOWDER. Or fast-burning stuff like for a pistol.
Lighting it is going to be the least of your problems (burst diaphragm). You have very low air density to provide the stability your rocket is going to need, it's not going to be moving fast enough by the time it leaves the launch rod, asymmetry in the rocket's centre of gravity will cause it loose stability and tumble. According to research I've read, at 100,000ft you need 71.6x the travel distance before your fins are going to provide the stability that you get at sea level. The speed of sound is lower at that altitude, 35.5meters per sec @ 100,000ft so your rocket needs fins and nose cone designed for super sonic flight too. I guess the long and short of it is spin the rocket up to get stability, design it for supersonic flight and to overcome transonic drag, shoot it straight up _through_ the balloon.
The International Civil Aviation Organization has the speed of sound at 100kft being 303.0 m/sec, not 35.5 m/sec. You seem to have a decimal misplaced.
Were Mach 1 35 m/sec at 100kft, then a U-2 would only be able to go about 35 m/sec (being a sub-sonic craft), and we'd be seeing more of them shot down.
You both seem to be right and wrong at the same time :-)
The speed of sound decreases with temperature and by coincidence air temperature decreases with height, so the speed of sound also decreases with height. But not by order of magnitude, though.
At the same time, aircraft's indicated airspeed decreases with decrease in air density, i.e. with increase in altitude. As the result, aircraft climbing with constant true airspeed will first see its indicated speed go down until at some stage it will reach stalling speed. On the other hand, the speed of sound will decrease with altitude, so if the plane will try to speed up to escape stalling condition it will reach transonic speed at ever lower indicated speed numbers. Eventually, at some height, the indicated stall speed and speed of sound will become the same and, unless the plane is designed for supersonic flight, it will enter the coffin corner - where flying slower means stall and flying faster means compressibility, shockwaves and again stall.
So, the plane's pilot will see that, apparently, the speed of sound became as low as his plane's stall speed but it will be more because his instruments underreport the speed than due to the actual reduction of the speed of sound...
Sorry if I confused you even more with that :-)
No, that actually cleared it up - but the original post isn't being clear.
IMHO, what he should have said is:
"Given how thin the air is up there, the rocket will have to be moving almost the speed of sound before the fins create enough force to stabilize the rocket. At that altitude, even moving at 300m/sec, the fins will only create as much force as they would at sea level going 35m/sec - not enough to stabilize the rocket. They had better plan on the rocket going supersonic, and design it accordingly."
Firstly bear in mind im guessing
I reckon the problem is a lack of O2 not for the rocket motor but for the igniter,
I haven't looked hard enough but from what I've read the copper head is essentially a fuse that you blow with or without something wrapped round it. now in an ordinary atmosphere it will glow red hot then burn providing the threshold energy to your rocket motor.
however 20mbar is quite good a vacuum with only 2% of the normal oxygen, so you copper head gets red hot then melts breaks the circuit, the lack of combustion reduces the peak energy generated to about 1100C the melting point of copper.
Trouble is the motor compound as it is designed to be used in relatively large volumes and be safe is designed not to ignite with any old heat source it needs to have sufficient energy to make it very unlikely to susceptible to static discharges and alike. Unfortunately the common methods of improving copper heads like taping magnesium ribbon to is or using thermite wont help you as 1100C isn't hot enough to to ignite either (~1600 C is required for thermite) and magnesium still needs oxygen.
Personally i find the idea that you will some how retain the atmosphere inside the vacuum unlikely producing a truly air tight seal like that is difficult.
More likely is finding some sort of igniter that will combust without O2 im guessing something more pro or using a fuse made of a higher temperature metal such as a tungsten light bulb filament as this will reach the temperatures you need.
It sounds like most well-informed commenters are agreeing that the problem is the fuel temperature in the fuel contacting the igniter. It's hard to imagine the problem fundamentally being pressure. If the rocket could only burn at 1 bar then it's not going to do well once the blast plug has popped off and the thrust has settled down to ~2 lb, since that will only provide a pressure back on the fuel of ~30 mbar. If it's going to work at all, it has to be self-sustaining in a vacuum.
So the solutions come down to either:
1. Use a blast plug, to contain the flame and help it to heat the bulk fuel to the autoignition temperature.
2. Heat the fuel externally, e.g. with thermite paste.
My preference is for 2, since it won't waste energy popping off the blast cap, so it should give a higher final altitude, and it seems like there's less that can go wrong with it.
The experiment was at ambient temp, no serious cooling yet, just the low pressure.
I think the addition of additional explosives / black powder .... may be hard, as Guns are harder to come by legally in Europe, and therefore the 'fuel' for them may be similarly controlled.
It may be LOHAN may have to have REHAB of two different types to demonstrate the location of the problem (s).
1 - low temp - Try to start a rocket when cooled
2 - low Pressure - Tried, and so far failed. LOHAN Boffins and drunks are working on some ideas for this
3 - Low Pressure & Low Temp. Combine what we have learned from 1 & 2.
I think we need to work out how to 'stuff' an improved ignitor in to the cavity and keep it there a BUTT plug (Barometric Uniform Temperature Thingy" may be required. (Slightly corney, but best I could come up with)
I'm aware that the experiment was at ambient temperature. I am not referring to the temperature of the whole rocket before the igniter fires, I am referring to the temperature of a small quantity of fuel immediately after the igniter fires.
I drew a diagram to illustrate what I am talking about:
What is relevant is the temperature of the "warm fuel" that I have drawn orange in that diagram. If the vapour created by the hot electric match escapes too quickly, due to low pressure, that "warm" area won't get hot enough to sustain ignition.
No It's pressure, there's nothing for the flame front to push against and there's not enough heat transfer to the propellant. You could try a proper pyro ematch/igniter, easy to get hold of, put a bit of quick or black match around it, stuff it in the motor and seal with a couple of layers of aluminium foil tape. The tape won't seal the motor on the way up, but it might hold enough pressure from the igniter for long enough as it fires with the added fuel from the match. It's worth a try as it will only cost you 50p for the bits. If you're in Spain, you can probably walk into a local pyro and get the stuff for less.
With the big ones, the igniter for a solid is basically a small motor that maintains both the temperature and pressure necessary to get the motor's reaction going. Normally it's at the fore end of the motor. If your motors can't accommodate a smaller, easier to light motor attached to the forward closure, putting one on the aft end with a bit of a gap between the two might do the trick. Mount it on the launch mechanism. What you're calling the igniter on the model motor does the job of our initiator (which lights our igniter.)
Going back to the sparking with liquid engines, yeah, the ones you see on the launch pads are for loose propellants. But in test, if we can't get the thing to light with its inbuilt ignition system we use other methods. Including duct taping a road flare to the aft end and opening up the propellant valves.
Whilst the thermite scheme will probably work it's a bit of a BF&I solution.
I like the idea of using a smaller motor to start the main motor though: form a miniature version of the main motor fuel charge that sits just inside the main combustion chamber and seals the main motor nozzle, facing backwards so that it fires towards the main motor fuel when ignited. Even if the seal isn't perfectly air-tight, it would provide enough of a seal to prevent the excessive expansion and cooling of the fuel vaporised by the igniter.
You'd need to calibrate the miniature so that it burns just long enough to start the main motor fuel before burning itself out to clear the nozzle; you could also add some thermite to the mini charge just for good measure, the thrust from the mini charge ensuring that the thermite is directed towards the main fuel instead of possibly just falling away (due to the motor being angled upwards on the launch rod, which it will be when Vulture2 is actually launched).
Is there any way of using something like a hand warmer to keep the fuel block at a higher temperature?
If it's any consolation the American rockoon attempts of the 1950s had similar problems when their clockwork mechanisms kept freezing. The Navy's solution was to pack the machinery with cans of hot orange juice.
As others have said, the issue is heat transfer to the propellant, in order to kickstart its oxidation process.
At Standard Temperature and Pressure (ie, sealevel on a calm day), the air around the igniter will transfer enough heat to the propellant to get the burn started, when coupled with hot gasses from oxidation of the ignitor (in general atmospheric oxygen effect is negligable when compared to the oxidants in deflagration mixtures)
The suggestions which strike me as worthwhile are
1: Make sure the igniter is actually touching the propellant. Convection doesn't work in near vacuum.
2: Use multiple igniters (for even burn/redundancy)
3: Put a small amount of oxidiser on the igniter to promote secondary burn.
You may need some form of tamper to physically hold the igniter(s) in place. A small wad of light tissue is likely to be enough - and it'd also allow gas pressure to build slightly before being ejected. This was standard practice back in the 1960s when nichrome loops were the igniter of choice.
You might want to consider limiting the voltage/current profile to the igniter so that it doesn't burn out instantly, but can transfer heat to the propellant long enough to start the reaction. (What's the actual ignition temp of the propellant?)
As for oxidizing agents (if actually needed), any suitable chemical which the copperhead can be dipped into (liquid form) and then allowed to dry out (like a matchhead) should work. You only want a tiny amount to kickstart the rocket's own chemical process and it strikes me that anything aimed at trying to preserve gaseous pressure around the engine at altitude before ignition seems doomed to failure.
Unless you're at cryongenic temps, secondary ignition is unlikely to be an issue. You can test that part easily enough by simply cooling and igniting an engine without using vacuum in the chamber (you'll need to do it in a closed chamber, otherwise condensation will kill things.(*))
I'm still wondering how you're going to stabilise the rocket in flight, short of having a 30 foot long launch rod.
(*) Water in spacecraft is a bitch to deal with. Instruments and chassis are left for weeks in hypobaric chambers to get as much out as possible because even atmospheric water is problematic(**). Once dried out everything tends to get shipped in sealed containers with an inert nitrogen atmosphere.
(**) It's probably iced up airbags that killed Beagle. The test articles were also the flight articles (BIG no-no) and there were huge amounts(***) of water coming out of them for months, right up to the moment they were packaged for launch
(***) Enough to seriously damage the vacuum pumps used for the drying process.
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