Pre-heat sounds sensible
- no added weight to worry about.
Just don't pre-heat it too much!
We know you lot like a nice bit of kit, so you'll certainly enjoy the latest piece of high-tech gadgetry to turn up at the SPB's mountaintop headquarters. Click here for a bigger version of the LOHAN graphic As you know, we've been busy conducting experiments with the Rocketry Experimental High Altitude Barosimulator (REHAB) …
If we can preheat and the insulation is sufficient to keep the motor warm enough to fire after a couple of hours at -60C external temperatures, that's the cleanest way to go. Adding temperature checks and a heater switch and batteries makes things more complex and gives the processor (a little) more to think about.
We're working hard on the KISS principle here - the less there is to happen in the air, the less there is to go wrong!
Steady on - the heater weighs 15g. If we pre-heat, it's hardly adding much weight. What we're trying to do here is to get a rocket motor to fire at altitude. It's without a doubt the most critical and difficult part of the mission, so we make no excuses for covering all bases.
Can you power the heater using solar? Single figure Watts doesn't take much. That would save mass....
As for pre-heating on the ground, I would use an oven - KISS? initially, then try to maintain temp during assembly with this device. Start off with a core temp of 80C rather than 15C and you've got quite an advantage.
Assuming (and this is a big assumption) that the operating power levels end up being 10 watts, that's a rather large solar panel to be carrying up. Even just a couple of watts will be something on the order of suitcase sized.
That said, there's no clouds up that high, and I'm guessing this thing will be launched in Spain rather than Cloudsvill^Wthe UK.
Size doesn't matter, weight does. We're not exactly worried about wind resistance here.
The article talks about ~2W solar power - you can have a 3W panel at eg http://www.selectsolar.co.uk/prod/272/powerfilm-pt15300-200ma-154v-mini-solar-panel for ~£90. It weighs less than 100g and. To run a heater you don't need any fancy voltage regulation, just hook it up. If you don't want it on all the time, then a MOSFET switch between it and the heater will be enough to switch it on or off.
The article talks about 2.24 watts of power at 12v, with a comfortable margin for increasing the levels if needed. I'm guessing the amount of power required to keep things from freezing is going to be tested before the launch, and I'll hazard a guess that even with the best insulation, it'll be hard to keep the temperature within design parameters at only that power level. At least, not when sharing company with a whole bunch of dry ice in a bucket.
Another thing to think about is the risk of the heater making things a little too toasty near ground level where it's likely going to be a lot warmer. Single thermistor, one relay and a routine in the onboard computer to check temperature every few seconds, perhaps?
1) Too late to mount the rocket in the truss I guess.
2) Fuel cell, for example: http://www.horizonfuelcell.com/portable_power_minipak.htm (temps might be an issue)
3) Solar? probably too large an area needed to give the current needed.
4) Wind -hmm non-starter
5) A just big-enough (LiPo? - http://www.overlander.co.uk/batteries-chargers/lipo-batteries/lipo-batteries-6100mah-4s-14-8v-30c-supersport.html) battery that can be set to turn on at a certain time or altitude?
all for now.
You already have some sort of timing/computing system onboard to launch the rocket at the right moment, right?
Why not let the rocket cool to whatever temperature it will go, and then a few minutes before ignition, connect the heater to, let's say, a 9 Volt block battery via a transistor or relay. That way you don't need much power, you can probably save the weight of (most) of the insulation and the complexity keeps relatively low.
In my mind, it doesn't matter if the rocket temperature drops very low during ascent, as long as you can get it to reasonable temps just before ignition. And i doubt you get it insulated enough that preheating on the ground will make any difference.
^^ that. You've already got a power source in the mothership for the camera, GPS etc, so just run off that. You can connect it via a contact pad in a Scalextric style-ee, instead of using the daughtership's power supply (which I assume it has for its own GPS tracking, pictures and radio downlink?) to minimise the weight at rocket launch.
As an on board option, caps are pretty light and cheap. A set of 10F caps running through that Polyimide Thermofoil might keep LOHAN warm enough to light her fire. That said, I imagine it would be more of an insurance thing with the preheat on the ground and let the caps do what they can on the trip up.
Agreed, which is why the preheat would still be needed. The caps would just provide a means to postpone the big chill but it would really depend on what the rate of ascent is. There's always silica aerogel or hollow micro-spheres for good insulation but those get pricey pretty quick.
I'd suggest that airborne heating might be best done with some of those air-activated hand warmer heater packs. They're light, and cheap. Sure, they're technically producing heat through a form of combustion (oxidization of iron filings IIRC), but there's no open flame, and you've already proven that the rocket motors are bloody hard to ignite with the correct gear, let alone by accident.
They're safe enough to be used near skin, so I wouldn't think overheating would be a problem. They do need oxygen though, so their effectiveness will decrease somewhat with altitude I suspect. Might be a good insurance policy...
but I'd have gone for a suitable length of nichrome wire wound around the engine. Cheap, robust, no need for epoxy (inbetween the heater and the engine) reducing heat transfer to the engine body. You could even distribute the turns so that there are more around where the igniter sits
I doubt you'd be able to insulate the motor casing enough (On all ends) to prevent it from cooling. The business end, the nozzle, is where it matters. And that is exactly the bit you can't insulate as good as the rest. And even the rest can probably not be insulated enough to keep it nice and toasty for 30 minutes in an environment well under freezing.
If you really want to save weight, do some quick calculations on the amount of energy needed to keep the motor warm all the way. (For which you need a thermal resistance value for the motor insulation) and for warming the motor up from cold prior to launch. I am guessing warming it up from cold in flight is going to be more efficient than trying to keep it warm all the way.
Then also decide if you NEED it to be at +20 degrees C or if you can get away with only -5 C for instance. The lower the temperature difference with the outside air, the smaller the heat loss through conduction.
Extra piece of equipment to stick to the glider? Dangling wires? Don't like it very much.
Surely simply using more flammable material for the igniter will sufficiently heat up the part of the fuel that needs to fire up. After that it will heat itself. No?
Or, attach the motor to the glider in such a way that until launch it can rest inside a cylinder that is attached to the truss, and heat the insides of this stationary cylinder. (But this, too, fails the KISS principle.)
With low pressure, insulation relying on trapped air is nearly useless. This rules out fibreglass, wool, polyurethane foam, and even aerogel. Since radiation is the only source of heat loss at altitude, you need radiant insulation. Layers of infrared reflective material separated by a small gap performs quite well on satellites. A cheap version can be made out of a space blanket and some cheap lace.
Of course, you'll probably need a bit of conventional insulation before it reaches altitude.
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