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Scott Martin
SOC-13
The problem with ion drives has always been getting an extension cord with enough length 
(well OK, power supply and vaccum arcing...)
Specific impulse ratings are pretty damn good though, now if we can just get HEPLAR exhaust velocities (another 1 - 2 orders of magnitude increase in exhaust velocity) we'll be off to the races!
Scott Martin
(well OK, power supply and vaccum arcing...)Specific impulse ratings are pretty damn good though, now if we can just get HEPLAR exhaust velocities (another 1 - 2 orders of magnitude increase in exhaust velocity) we'll be off to the races!
Scott Martin
That's the real world conundrum, getting both high exhaust velocity (Ve) and high thrust. In an ion drive the Ve is going to come from the E field which is powered by a power plant. Unless the power plant is insanely efficient (far, far better than 99%) the waste heat will be enough to melt the enclosing structure. The same applies to fission, fusion, etc. drives.Originally posted by Scott Martin:
The problem with ion drives has always been getting an extension cord with enough length
Specific impulse ratings are pretty damn good though, now if we can just get HEPLAR exhaust velocities (another 1 - 2 orders of magnitude increase in exhaust velocity) we'll be off to the races!
Scott Martin
I've been fiddling around with trying to design a reaction drive with both high thurst (enough propellant mass ejection to give a high acceleration (e.g., 1G+))and high Ve using a 99% efficient power plant/reactor and a structural material 10,000 times better than daimond with respect to the total energy required to melt 1cm. I still can't get it to work. It looks like Orion is the only way to go.
Well, the issue isn't the energy required to melt the material -- the issue is how hot you can run the engine, and thus how much heat you can dissipate. Orion drives don't actually solve this problem.
Using real materials, you might be able to have a graphite torch bell which can shed around 5 MW/m^2 (3064K). 10MW/m^2 (3644K) is probably not possible.
If we assume that the drive bell has a radius of 10 meters, and the drive only emits 1% of its total power as waste energy (wildly unlikely), sending the other 99% out the back, this gives us a total possible drive power of (5MW/m^2) * (10m)^2 * (4pi) / 0.01 = 628GW.
Drive power is equal to Isp * Thrust * 0.5, so if we want a thrust of 1,000T (10^7N) exhaust velocity cannit exceed 125 km/sec, or an Isp of about 12,600.
An Orion drive is likely to produce vastly more waste heat, which means either a much larger drive bell, vastly lower thrust, or vastly lower Isp.
Using real materials, you might be able to have a graphite torch bell which can shed around 5 MW/m^2 (3064K). 10MW/m^2 (3644K) is probably not possible.
If we assume that the drive bell has a radius of 10 meters, and the drive only emits 1% of its total power as waste energy (wildly unlikely), sending the other 99% out the back, this gives us a total possible drive power of (5MW/m^2) * (10m)^2 * (4pi) / 0.01 = 628GW.
Drive power is equal to Isp * Thrust * 0.5, so if we want a thrust of 1,000T (10^7N) exhaust velocity cannit exceed 125 km/sec, or an Isp of about 12,600.
An Orion drive is likely to produce vastly more waste heat, which means either a much larger drive bell, vastly lower thrust, or vastly lower Isp.
Hi Anthony,
I think we're saying the same thing.
I'm working from assumed performance parameters and trying to see if I can fit such a drive within the hull. I think your working from hull size and determining performance parameters. I was just going to try that angle tonight.
Oh I agree 99% drive efficeincy is wildly unlikely, but I'm viewing everything possible in favor of the engine to set an upper theoretical limit on performance. I think 99% efficiency is more than generous in this regard.
In what I did last night:
I'm fixing the max size of the vessel and hence engine compartment, assuming a certain fraction of volume for payload, a certain payload density (and thus payload mass)and drive efficiency.
I then look at some assumed exhaust velocities and accelerations.
Then I assume a ship of volume T, an exhaust velocity of Ve, and an acceleration of A for a payload mass of m, and assume the engines take no volume, and the drive is 99% efficient, then ask can I make this thing with current materials? If not, how much better than current materials does the material need to get.
I see what you are saying on the radiative aspect of the shell. As heat is constantly being thrown at the shell, the real question is how much can it radiate as this will determine the steady state temperature or how hot the engine can get. As the engine gets hotter it just ends up eventually melting the shell as excess heat can not be radiated in time.
This is what I did: I asumed an acceleration then calculated the rate of propellant mass ejection needed per unit time to achieve that acceleration assuming the total mass was the payload mass only (which overestimates the performance of the engine). With propellant mass ejection rate and Ve I calculate the energy of the exhaust. I assume that this exhaust energy is 99% of the energy produced by the engine, the other 1% being waste heat. I do a quick ratio to get the power wasted as heat. Then I calcualte the energy/m^2 for a sphere of radius r and see how big r gets before the energy/m^2 is below a certain value. Since I couldn't get r small enough I started seeing what all this extra heat could melt, hence my comment on melting diamond. Technically, it was how much daimond would have melted in 1 second.
edited: to check calculation
If you wnat lots of real world data on various rocket engines (and anything else relating to spaceships) take a look atAtomic Rocket
Gearhead heaven, I promise!
Gearhead heaven, I promise!
