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Old 03-20-2012, 11:46 AM   #4
Ulzgoroth
 
Join Date: Jul 2008
Default Re: [Spaceships] Rocket Boosters and Other Modular Attachments

Quote:
Originally Posted by Seneschal View Post
The rules for towing (under External Clamp, Spaceships p. 15) are the closest alternative, but they have problems - if you have a 100-ton ship with 3G accel, and strap a 30-ton laser onto it, its accel drops to 2.3G. If the laser is functional and works like any other ship system, then this thoroughly breaks the ship design mechanic - a weapon that would take up 6 spaces on a 100-ton ship can now be mounted at the cost of a mere 0.7G.
This isn't broken that I can see. This is exactly how physics says things work.

Problems with this include:
-By the book you'll need a clamp system to hold them together.
-The externally mounted laser is not inside the 100 ton ship's armor.
-Similarly, you need power from somewhere.
-You're on your own for determining the firing arcs for this.

Really, my question would be what you think the benefit is. It's not like there's any special reason you shouldn't be able to design a 130 ton ship if that's what you really wanted, it just happens to be below the granularity of the system.
Quote:
Originally Posted by Seneschal View Post
For a reusable booster to use the towing mechanic (counting it as a ship that clamps onto your ship to tow it), would I add together the "tons of thrust" of the two, and calculate a joint acceleration and delta-v?
Acceleration, yeah, just add together tons thrust, divide by combined tonnage. And make sure the thrusters can be arranged in such a way that they don't put a net torque on the ship or it'll be nearly unflyable in space.

Combined delta-V of different thrusters, clamp-on or otherwise, is bad business. Unless there's some rocket scientist trick that I don't know you don't want to be figuring combined delta-V of dissimilar rockets if you can help it. A brief analysis suggests that if the rockets have different specific impulses (different delta-V per tank) the order in which they're burned will influence the total delta-V.

Run and don't look back would be my advice there. You can do it, it's not even really complicated math, but you'd need to use the rocket equation a lot more than you probably want to.

It's not so bad to combine rockets if either:
-They all have the same specific impulse.
-They'll always be burned in a pre-determined order.

But even in the simplest case:
Quote:
Originally Posted by Seneschal View Post
Questions:
- The towing rules say nothing about reducing delta-v. Presumably it would be decreased by the same amount as the acceleration (in our above example, the 100 ton ship's delta-v supply would drop by 23%). Though I'm no rocket scientist, so I'm not sure; thoughts?
You can't just take a weighted average, you've got to calculate from the fuel fraction of the combined vehicle. Basically, that adjustment given for having a high fraction of fuel tanks on the ship needs to be applied to the aggregate ship, not to each component of it individually.
Quote:
Originally Posted by Seneschal View Post
So, how would you guys handle this? Is there perhaps another ship-design system better suited for this?
Not currently for GURPS 4th edition. You can pretty much bolt this stuff onto spaceships but you've got to work it out yourself (or get the forums to work it out for you.)
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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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