Fixing up stats for cobbled missiles

[DataPacRat]

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?)

[johndallman]

Originally Posted by DataPacRat

... 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.

A rocket needs oxidiser as well as fuel. Trying to make an air-breathing rocket leaves you with an inefficient ramjet or a turbojet/turbofan.

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.

[Anthony]

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.

[Ulzgoroth]

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.

[DataPacRat]

Originally Posted by johndallman

A rocket needs oxidiser as well as fuel. Trying to make an air-breathing rocket leaves you with an inefficient ramjet or a turbojet/turbofan.

Suitable oxidisers for kerosene include nitric acid, hydrogen peroxide, and liquid oxygen.

LOX might not be out of the question, depending on quantities. Some atmospheric separation and liquification gear is already on premises, to produce dry ice and liquid nitrogen for cryonic preservation of the dead.

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.

Hm. The full capacities of the design software are unknown, but were originally based on near-Singularity-level principles, and appear to have been modified post-Singularity with additional data. (Which is its own issue...) But "significant amounts of testing", whether virtual or physical, is a good point to consider.

[Anaraxes]

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.)

[DataPacRat]

Originally Posted by Anthony

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).

I think I already posted a thread about PPGs somewhere on the board; TL8 multifuel engines from Vehicles seem to work out for the purpose.

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.

The most powerful weapons that have been acquired so far are compressed-air needleguns (which haven't even been used yet), explosives made from geneered goat's milk (used up, and species possibly extinct), and a scientific laser stuffed into a vaguely rifle-shaped holder. It's been a very "munitions"-lite plot so far.

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.

I mentioned in my original post a certain lack of explosives, leading to the choice of an Armor-Piercing Incendiary warhead, which I could easily be argued down to simple AP.

[johndallman]

Originally Posted by DataPacRat

LOX might not be out of the question, depending on quantities. Some atmospheric separation and liquification gear is already on premises, to produce dry ice and liquid nitrogen for cryonic preservation of the dead.

The boiling point of nitrogen is below that of oxygen, so it looks promising. But liquid oxygen is dangerous stuff: all sort of things will burn very enthusiastically or even explode when it's applied to them. See the classic barbecue video.

Hm. The full capacities of the design software are unknown, but were originally based on near-Singularity-level principles, and appear to have been modified post-Singularity with additional data. (Which is its own issue...) But "significant amounts of testing", whether virtual or physical, is a good point to consider.

Rocket design is a very specialised art, there are only tens of organisations in the world that are good at it. Unless someone was deliberately making software with design expertise for everything, rocket engines are unlikely.

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.

[Ulzgoroth]

Originally Posted by DataPacRat

I mentioned in my original post a certain lack of explosives, leading to the choice of an Armor-Piercing Incendiary warhead, which I could easily be argued down to simple AP.

If they can't come up with some chemistry more vigorous than ethanol, they probably can't manage API.

If they can't come up with some chemistry more vigorous than ethanol, I think their chances of pulling this off aren't great.

[DataPacRat]

Originally Posted by Ulzgoroth

If I'm seeing this right, you've build a would-be surface-to-air missile that's trying to imitate the Stinger.

The Stinger is more of a guideline of an unattainable ideal, and an example of what tradeoffs work well in an existing design, but in general, yep.

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.

I'm not sure that anything like modern anti-aircraft missiles is possible, given the constraints. The fact that it seems to be possible to get as close as I have so far is actually a little astonishing. :)

To give it a chance I'd strongly recommend more range.

Now there's a design change worth pursuing.

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.

The least expensive I think I can make a Command-Guidance package is about $1025, compared to $250 for a skill-13 Optical Homing package. Adding "Brilliant", so a missile's initial course can be preprogrammed before it starts homing on a target, would add thousands more to the cost.

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.

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.

What would the autocannon use for propellant? Compressed air may work for a needlegun, but seems unlikely... could alcohol and/or LOX be used in some sort of electrothermal kitbash?