01-09-2015, 05:53 AM | #1 |
Join Date: Nov 2004
Location: Niagara, Canada
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Fixing up stats for cobbled missiles
Anyone want to point out where I'm making any mistakes, and how they might be corrected?
Our Post-Apocalyptic Heroes have a small fusion power plant, a minifac, enough pure alcohol to fuel some powered paragliders... and have just received word that something resembling some unmanned A-10 Warthogs have started shooting up the next city over. One of Our Heroes suggests making some anti-aircraft missiles to try to defend themselves. One problem: there's no rocket fuel, and by the time the minifac creates a chemical refinery that can make some, it'll probably be too late. But alcohol has /some/ use as a fuel, so why not try using the minifac's design studio to come up with something? General rules: GURPS 3e Vehicles & accessories, TL9ish, no superscience. Initial stats to shoot for: The present-day FIM-92 "Stinger" round, 70mm diameter, 0.2 cf, 22 lbs, 1700 mph, range 4500 yards. Minimal cost, to maximize the number that can be fabbed before the drone-hogs fly over Our Heroes' hideout. First thoughts: Ducted fans, light turbojets, or liquid-fuel rockets seem to be the best bets. Assuming radical streamlining and a surface area of 2.5 sf, the target aSpeed requires a thrust of 24 lbs. This rules out ducted fans for weight, and turbofans are relatively expensive, leaving some version of a rocket drive to work with. After finding the energy densities of both ethanol and kerosene, it appears kerosene contains roughly three times as much energy per unit volume, so as a first approximation, an alcohol-burning rocket engine will have triple the listed fuel consumption. Mangling the rules beyond their design, the least expensive missile guidance system available appears to be a cheap, compact, optical homing system. Also mangling the rules a bit, if wings aren't being used for lift, there doesn't seem to be anything about a minimum size for them; so I'm going to suggest wings of around 0.0001 cf, so the vehicle counts as "winged" for aMR calculations, and take all of the lift from Lifting Body. (And Underbelly Skids for launching. And Variable Sweep wings to lower initial stall speed and takeoff run.) No significant fuel also implies no significant explosives. I'm going to mutter something about thermite and suggest an API warhead. Which has led me to statting out, so far: TL 9. 0.2 cf. 2.5 sf. Radical streamlining, lifting body. Structure: Extra-Light, Very Cheap. 7.5 lbs, $150 Liquid Fuel Rocket, thrust 24 lbs: 0.24 lbs, 0.0048 cf, $6, 26.4 gph rocket fuel (80 gph alcohol). DR 5 advanced metal, 2.5 sf: 1.25 lbs, $25, 0.003125 cf Fuel Tank: 1 gallon, Ultralight: 0.05 lb, 0.15 cf, $25 Alcohol, 1 gallon: 5.8 lbs, $0.5, Fire 10 Optical Homing, Compact, Cheap: 0.25 lb, 0.005 cf, $250, skill 13 Warhead, 2.7 lbs, 0.054 cf, $108 Blackout paint, 2.5 sf: $0.5 Total: $565, 17.8 lbs, 0.216925 cf With Variable Sweep wings, takeoff run is 5 yards. (Without them, it's 20.) With 70mm normal-sized API warhead, traveling at full speed, damage appears to be roughly 58d6, for an average of 200 points, with a (2) armor divisor. With wings of 1 hp, aMR is around 12.5 gees. Without any wings, aMR would be a mere 0.25 gees. Toasting: Within 2 yards of engine: 0.024 d6 = no damage A little lighter, a little larger than the "Stinger", but possibly close enough to pass a reality check. Or maybe not. Which is why I'm posting; how many of my quick-fixes need to be shot out of the sky, and what would it take to create a design that really would pass a basic reality check? (Or, if you prefer, how could the design be tweaked to improve the odds of survival against one or more Warthog-like aircraft?)
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Thank you for your time, -- DataPacRat "Then again, maybe I'm wrong." |
01-09-2015, 02:34 PM | #2 | |
Night Watchman
Join Date: Oct 2010
Location: Cambridge, UK
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Re: Fixing up stats for cobbled missiles
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Suitable oxidisers for kerosene include nitric acid, hydrogen peroxide, and liquid oxygen. Unless the characters have good access to rocket design data, building something that works reliably and develops a decent fraction of its theoretical performance will take considerable time, and many explosions. It is not straightforward work. |
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01-09-2015, 02:42 PM | #3 |
Join Date: Feb 2005
Location: Berkeley, CA
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Re: Fixing up stats for cobbled missiles
Yeah, you won't be able to create an alcohol-fueled rocket without stuff you don't have on hand, and if you had that stuff on hand you would probably also have better fuels available than alcohol. Note that alcohol, because of a rather low flame temperature, not only requires more fuel, it also has lower power output (so the fact that your paragliders even run is a bit optimistic).
Note that A-10s are durable but not particularly agile, so there's no special reason you can't shoot them down with unguided munitions. Also, if you have the ability to manufacture explosives (without which, no warhead), you probably also have the ability to manufacture solid fuel rockets, since they involve many of the same compounds. Which is also a good way to blow yourself up, but has the virtue that someone else has already done the design work. Last edited by Anthony; 01-09-2015 at 02:49 PM. |
01-09-2015, 02:49 PM | #4 |
Join Date: Jul 2008
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Re: Fixing up stats for cobbled missiles
If I'm seeing this right, you've build a would-be surface-to-air missile that's trying to imitate the Stinger. But with an acceleration of under 1.4 g when full (climbing to almost 2 as fuel burns out?). The Stinger, on the other hand, goes from main engine ignition to mach 2 in 2 seconds.
An anti-aircraft missile with 1.4 g of thrust might well be able to catch a drone warthog, but it's not going to perform anything like modern anti-aircraft missiles. To give it a chance I'd strongly recommend more range. Remember that it has has to climb up to altitude of the target before it can attack. I don't know how Vehicles would treat this, but such a missile might be more viable with command guidance as opposed to self-guidance, since just flying straight at the target is not likely to work for it. EDIT: I second Anthony's unguided weapon suggestion. Low-flying, slow-moving ground attack craft, and perhaps especially ones that are drones, are susceptible to gunfire. Knock together some medium-caliber autocannon.
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I don't know any 3e, so there is no chance that I am talking about 3e rules by accident. Last edited by Ulzgoroth; 01-09-2015 at 02:52 PM. |
01-09-2015, 03:03 PM | #5 | ||
Join Date: Nov 2004
Location: Niagara, Canada
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Re: Fixing up stats for cobbled missiles
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Thank you for your time, -- DataPacRat "Then again, maybe I'm wrong." |
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01-09-2015, 03:05 PM | #6 |
Join Date: Sep 2007
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Re: Fixing up stats for cobbled missiles
The V-2 used ethanol and LOX. (74% ethanol/26% water, actually.) It was used in a number of other rockets as well, including Redstone.
Can Our Heroes build a compressor to liquify air? (They can build rockets, so presumably they have metalworking facilities and expertise.) |
01-09-2015, 03:09 PM | #7 | |||
Join Date: Nov 2004
Location: Niagara, Canada
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Re: Fixing up stats for cobbled missiles
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__________________
Thank you for your time, -- DataPacRat "Then again, maybe I'm wrong." |
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01-09-2015, 03:25 PM | #8 | ||
Night Watchman
Join Date: Oct 2010
Location: Cambridge, UK
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Re: Fixing up stats for cobbled missiles
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Edit: I recommend they don't try to use Oxyliquit explosives as warheads, because they're shock-sensitive, and home-made rockets tend to generate violent shocks. Last edited by johndallman; 01-09-2015 at 03:31 PM. |
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01-09-2015, 03:27 PM | #9 | |
Join Date: Jul 2008
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Re: Fixing up stats for cobbled missiles
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If they can't come up with some chemistry more vigorous than ethanol, I think their chances of pulling this off aren't great.
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I don't know any 3e, so there is no chance that I am talking about 3e rules by accident. |
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01-09-2015, 03:27 PM | #10 | |||||
Join Date: Nov 2004
Location: Niagara, Canada
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Re: Fixing up stats for cobbled missiles
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The standard minifac can, with appropriate supplies, build about $500 an hour. Going from OH to CG would more than double the cost, halving the number of missiles that could be built before any drones arrive, and use up those appropriate supplies twice as quickly. It's a change that /could/ be made, but I don't see a compelling reason to. Quote:
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Thank you for your time, -- DataPacRat "Then again, maybe I'm wrong." |
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