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Old 04-17-2010, 03:59 PM   #1
jacobmuller
 
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Default It's not Rocket (limited super)Science

Well, actually, it is, kinda:)
A TL10 SM+9 transport vessel with a Fusion Torch drive has suffered a "fuel supply failure". It's drifting into a gravity well.
In emergencies the drive can manage 1G, 3G if they use water, but there is no fuel. There is, however, life support (Habitatx2).

Assuming they have the necessary skills and supplies, how many mps of fuel could they suck out of their life support system?
Would 15 tons of water per unit be too generous? Worth about 0.5mps @ 3G (a 3 second burn?) or 1mps @ 1.5G.

Thanks in advance for any feedback
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Old 04-17-2010, 06:40 PM   #2
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Default Re: It's not Rocket (limited super)Science

Quote:
Originally Posted by jacobmuller View Post
Well, actually, it is, kinda:)
A TL10 SM+9 transport vessel with a Fusion Torch drive has suffered a "fuel supply failure". It's drifting into a gravity well.
In emergencies the drive can manage 1G, 3G if they use water, but there is no fuel. There is, however, life support (Habitatx2).

Assuming they have the necessary skills and supplies, how many mps of fuel could they suck out of their life support system?
Would 15 tons of water per unit be too generous? Worth about 0.5mps @ 3G (a 3 second burn?) or 1mps @ 1.5G.

Thanks in advance for any feedback
Look at your water bill. It almost certainly tells you how many gallons you used last month. Then divide that by the number of people in your house and decide how long it takes the life support system to recycle water. Any Spaceships Life Support system assumes recycling most water.

My water data shows an average of less than 100 gallons per day (800 lbs) even counting old, non-efficient clothes and dishwasher plus long hot showers for my arthritic joints. The toilets are low flow (and stop up often enough to prove it).

You could easily get below my 100 gallon upper limit with newer appliances and a cure for arthritis. If you only used water for drinking and cooking the numbers get very low, probably less than 1 gallon per man-day.

So the answer (assuming your life support isn't open cycle and using stored water only with no recycling) is very little to virtually no reaction mass.
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Old 04-17-2010, 06:57 PM   #3
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Default Re: It's not Rocket (limited super)Science

However, If the ship has Full Lifesupport in the form of hydroponics, I'd be more generous with "emergency" fuel.

If you're really desperate, you could also use your atmosphere... although that's unlikely to provide you much more mass unless you have lots of open space. Pure O2 would give 4x thrust, but divide DV by 4. Standard Earth Atmosphere would give 3.8x thrust, but divide DV by an equal amount.

----------

This is why I design deep space exploration vessels with Ice Armor... reconfigurable to fuel tanks.
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Old 04-17-2010, 07:30 PM   #4
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Default Re: It's not Rocket (limited super)Science

Quote:
Originally Posted by jacobmuller View Post
Well, actually, it is, kinda:)
A TL10 SM+9 transport vessel with a Fusion Torch drive has suffered a "fuel supply failure". It's drifting into a gravity well.
In emergencies the drive can manage 1G, 3G if they use water, but there is no fuel. There is, however, life support (Habitatx2).
Unless the fuel ran out in the middle of some really drastic maneuver, it's unlikely that the ship would be in immediate danger with realistic rules that conserve energy, I'll have to review the gravity well rule to see if that is likely the case. But assuming some acceleration has to happen:

For a "Cold Equations" feel, about 330 transportees @70kg each would yield 15 tons of water if you had time to feed them through the waste recycler. With TL10 health care, perhaps twice as many could survive donating their limbs for conversion.
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Old 04-17-2010, 07:49 PM   #5
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Default Re: It's not Rocket (limited super)Science

They don't need much - just enough to alter course to "slingshot" the gravity well.
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Old 04-17-2010, 09:23 PM   #6
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They don't need much - just enough to alter course to "slingshot" the gravity well.
Maybe, maybe not, unless slingshot maneuvers work the way they do in Star Trek. If they did you probably wouldn't need reaction mass.

<shrug> Mix and match your rubber science how you like but realistically that sort of stuff doesn't work real well.
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Old 04-18-2010, 12:21 AM   #7
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Default Re: It's not Rocket (limited super)Science

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realistically that sort of stuff doesn't work real well
I'm not an expert, Fred, but it appears to have worked for Mariner 10, Voyager 1, Gallileo, Ulysses, Messenger, Cassini and Solar Probe +. It's quite possible those are unusual circumstances in some way I don't understand, I'll admit that, but we're talking about an RPG here. The GM might simply rule that his PCs find themselves luckily in such circumstances and give them a navigation skill roll to save themselves rather than have them go burn-splat or drift off forever.

Any of the PCs got Luck?
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Old 04-18-2010, 01:29 AM   #8
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Default Re: It's not Rocket (limited super)Science

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Maybe, maybe not, unless slingshot maneuvers work the way they do in Star Trek. If they did you probably wouldn't need reaction mass.
Some technically don't need reaction mass, but a little is often used to adjust the trajectory for maximum effect.

It's the physics of conservation of energy and momentum. The spaceship loses gravitational potential energy as it gains velocity and momentum by falling closer to the planet. Assuming it avoids an inelastic collision with the planet itself or its atmosphere or any satellites, it will then continue around and fall away from the planet, trading back velocity and momentum for potential energy as it slows due to the planet's gravity. It arrives and leaves with the same speed at the same distance, measured relative to the planet.

However, it is not the direction or the same speed relative to the sun, since the planet is orbiting the sun at numerous miles per second itself, and it is not the same speed relative to the other planets which have their own speeds and orbits of course, so the slingshot maneuver can effectively add or subtract as much as twice the difference in speeds between the slinging planet and the other planets, or the sun. The velocity change of the spaceship can be considerable, but since the mass is so much smaller than the planet, conservation of momentum makes some invisibly small alteration in the planet's orbit that will be lost in the noise of everything else zipping past it.
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Old 04-18-2010, 03:29 AM   #9
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Default Re: It's not Rocket (limited super)Science

ROFL - bodies as reaction mass...
We don't have water meters here (yet) so thank you for the numbers.
The PCs would like to keep breathing and vac-suits just won't last long enough to reach anywhere.

Fuel was sabotaged, along with comms, etc. Their priority will be repairs for getting past the planetoid. Survive that and then fix the rest.

I reckon my numbers are off by at least a factor of ten - possibly in a couple of places: 1.5 or 0.15tons water per habitat unit? But, at a high rate of use, 15tons of water is possible.
Draining the hydroponics would be a good idea - but that'd mean they end up not breathing...
But it does give them 2 options - recycled water only for small boost and eventually die of thirst or recycled water plus hydroponics water for bigger boost and die not so slowly from CO2 build-up.

Not going to tell mention the body-as-reaction-mass option (and less people = more water/air). Will wait and see how selfless/ish they are.
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Old 04-18-2010, 09:01 AM   #10
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However, it is not the direction or the same speed relative to the sun, since the planet is orbiting the sun at numerous miles per second itself, and it is not the same speed relative to the other planets which have their own speeds and orbits of course, so the slingshot maneuver can effectively add or subtract as much as twice the difference in speeds between the slinging planet and the other planets, or the sun. The velocity change of the spaceship can be considerable, but since the mass is so much smaller than the planet, conservation of momentum makes some invisibly small alteration in the planet's orbit that will be lost in the noise of everything else zipping past it.
Yes, this is most of the stuff I didn't have the time to explain before going to watch Dr Who last night. :)

There's also stealing some of the planet's angular momentum by traveling in the same plane and direction as its' rotation and getting pulled along by its' gravity as it spins. With planets like Jupiter (angular momentum king of the solar system) this can be significant.

Still, I stand by my statement of "singshots don't work the way they do in Star Trek". The possible benefits are limited and specific circumstance only. It's not a general purpose way to gain velocity.

If you're in danger of hitting a planetoid you need to change your vector enough to not hit and this may or may not result in a graceful looking arc around the planetoid. Particularly from a planetoid you're unlikely to gain any significant velocity by stealing some of its' orbital or angular momentum. There's too little gravity involved.
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