"lack of atmosphere at altitude may prevent heat radiating away"
Surely, convecting away? You'll get more or less the same amount of heat loss by radiation in a vacuum as you would in sea-level-pressure air, I assume.
Our audacious Low Orbit Helium Assisted Navigator (LOHAN) spaceplane project continues to advance on multiple fronts, and while Neil Barnes and Anthony Stirk put together the Special Project Electronic Altitude Release System (SPEARS) control board, we've been doing some more work on the Vulture 2 rocket motor heater set-up. …
I'd be using something like a thin aluminum tube (old wand from vacuum cleaner) and seating the engine inside that, and putting a 2 watt bulb underneath it... with an air gap of about 20 - 30mm... and a little insulating around the tube.
Everything else is just weight and complexity.
A new thought......
Now I am not saying this "IS" the answer.... but it may be AN answer or PART of an answer.
Many years ago, I had a combustication powered pocket warmer.
It was a premixed and pressed rod of carbon, and a few other things, and it was lit at one end, and stuck inside it's case, which was like a case for ones reading glasses, coated in blue felt, made thin pressed steel, and lined with fiberglass wool, in which the smouldering rod was placed.
The idea is that this ROD may in fact be chemically balanced enough to be a SLOW self contained source of heat - thus enabling it to work in the minions of satan land.
These sorts of things.
The light bulb method is good. I worked at a vending machine factory who had a redundant light bulb in the depths of the machine will away from punters eye line, it was to keep the machines destined for Korea slightly warmer inside than out to reduce condensation and humidity problems.
The silly sausages did go and custom design and manufacture and spec their own custom light bulb at a staggering price, whereas any of the shelf one would have worked, but I was to junior too argue against it at the time, likewise precision washers custom made at a high price just to level the feet. Ah well, I am now older and wiser with less fight in me to care any more!
Planting the heater between two polystyrene blocks, I would have thought would tell you nothing, other than that it doesn't take much to heat up polystyrene. I'd have thought it better to wrap the heater round something with the dimensions of the rovket motor, and about the same mass, and stuff the whole thing inside a domestic freezer. Make your test as near the physical makeup of the actual object as possible. You could do the same test inside a domestic fridge and then you'd have two sets of results from which you could extrapolate for even more exttreme temperatures with at least some hope of getting near the real thing. As the earlier poster remarked, radiation is not markedly affected by atmospheric pressure.
1. Put the assembly in your barometric chamber (with the dry ice to cool it) and reduce the pressure to the max altitude.
a: Check there are no air pockets in the batteries or other components that may explode out.
b: Check that the battery performance remains at these temperatures and pressures.
c: It should help provide more accurate delta T information as well.
2. Does it have a closed loop control system to regulate the temperature? i.e. while the heater is needed, no point if it overheats everything.
Actually it's worse than that: at 100000ft you still have about 1% of the pressure at ground level and that means heat transport by the surrounding gas is hardly reduced!
estimate the cooling power with some formula (using temperature differential and time and materials and dimensions as input) and work out if your few watts will hope to make the blindest bit of difference.
Or put it in the freezer "with" and "without" heat blanket and measure how much difference it makes!
There's a way to keep it warm using the latent heat of freezing of water. Surrounding the motor with a water-filled coat would keep it above 0 deg C until the water had frozen. The coat could be something like lengths of narrow bore layflat tubing, or even a palisade of straws if some simple and efficient means could be devised to keep the water in them and allow for expansion.
The heater puts out 2.24 W for 60 min, say, which is 3,600 x 2.24 = 8,000 J (approx.) The latent heat of freezing for water is 334 J/g. So it would only require 25 grams of water or so to freeze to provide an equivalent amount of heat to that supplied by the battery. But check my sums first!
Clearly a bit more thought is needed on the actual design, how to add a conducting inner layer and insulation on the outside etc., but it might be a reliable solution.
One of the goals of the heater is to reduce the amount of ice formation, so using ice formation to do it probably isn't such a great idea.
Also, I'm pretty sure the rocket motor (and especially LiPo cells running the electronics and the heater) need to be kept warmer than 0C.
If the water/ice is appropriately contained it shouldn't cause any problem. The thruster/LiPo will be approaching 0 ºC asymptotically. These could be warmed to perhaps 30 ºC to start with so that they only reach, say, 10 ºC at their coldest.
The crucial thing is energy density. Wikipedia gives that of lead-acid batteries as 100, alkaline as 590 and non-rechargeable lithium as 1,800 J/g. This doesn't take into account the extra mass of the heater and wiring, de-rating because of low temperature or non-maximal use. Freezing water provides 340 J/g latent heat and just over 4 J/g per degree C as specific heat, so about 400 J/g total.
A lithium battery may therefore be better provided that it is reliable, stays warm enough and that most of its energy is used.
I don't have any solid figures for the following but from what I know they will cause problems:
The capacity and output will be severely reduced by the extreme cold. Just as an example some DSLRs are rated to -10C operation but they suffer quite a reduction in shots per battery. Only a warm battery will provide that kind of output.
The test isn't valid for the expected conditions because you have only tested that it works at a relatively warm temperature at a low altitude and relatively high pressure.
I hope you have a way to address these issues.
There are a couple of fundamental errors in the design and assumptions here. Sadly they pretty much nullify what has been done.
There are three sources of heat loss - radiation, conduction, convention. The design and tests have not addressed these correctly.
Radiative losses are independent of atmosphere - they remain essentially identical in a vacuum or at sea level (for those wavelengths that the atmosphere is transparent to - which are those that matter here.) Heat loss due to radiation won't be noticeably less at altitude.
Heat losses by conduction through air are independent of pressure until the mean free path is longer than the distance between objects. For the dimensions and pressures of this project you can assume that conduction remains about the same. Use of an aerogel insulator would help significantly here.
Convention might matter. Even at 0.01 of sea level pressure, the air can move, and thus can convey heat from the motor to the body of the aircraft and thus to the outside. However the vastly lower pressure reduces the heat capacity of the air equivalently, so the energy moved reduces considerably. You may need to consider how to prevent convection cells of air forming. Making sure the cells of air are small (where small is a few mm) is the way to do this. Aerogel is good here too.
The critical one is radiation. Space blanket only provides useful insulation against radiative losses. It does this in two ways: it reflects radiation back to the source, and being a highly reflective material, it radiates heat very badly, and so does not lose heat by itself radiating energy. In order to work it must not be in contact with anything - it must have a clear space around it. Sandwiching it between two layers nullifies its entire function.
To use space blanket you must wrap the outside of the assembly very loosely - possibly in more than one layer, with a minimum of contact points between the rocket motor and the blanket, and if more than one layer minimal contact points between the layers. If you want an example of how it is done, look at a picture of the Apollo Lunar lander's legs. Indeed check out any picture of spacecraft and observe how the blanket is arrayed. Multiple loose layers of blanket will trap small cells of air, and thus also effect a reduction of convection.
As mentioned above, you won't know how well the system performs until you test it properly, and this means into the baro-chamber and packed with dry ice for an extended soak. It is worth applying a bit of basic physics here too. You know the energy drain of the heater - power = volts time amps. You can work out the thermal coefficient of the motor - (so many grams of aluminium, so many grams of propellant - or use a surrogate of similar known material) thus you know how the temperature of the motor should rise with time when the heater is energised. You can compare the observed temperature with the ideal case, and work out the thermal losses. You may discover that a carefully insulated motor will not require a heater, or if it does, you can work out the minimum heater current required, and appropriately size the power source.
A layer of aerogel then a couple of loose layers of space blanket and I very much doubt a heater would be needed. Insulating the system batteries would similarly benefit - enough that with internal losses naturally heating the batteries you may obviate any thermal problems in the batteries.
I use LiPos extensively for model aircraft electric motors. Even at a few degrees above zero their performance is very noticeably degraded, and they're useless below zero (although I'm taking one or more 10s of amps from them, so YMMY but I suspect not). These days I keep my LiPos in a "cool" box with a hot water bottle to keep them toasty.
Why not replace the helium with hydrogen and then burn some of the hydrogen for heat
The advantage of hydrogen increases as you get higher.
The fire risk is exaggerated
Hydrogen is cheaper
Hydrogen isn't a limited resource
BTW, Put the electronics inside the balloon, nice and dry, lower heat loss and any heat loss increases the performance of the balloon
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