[Spaceships] Rocket Boosters and Other Modular Attachments

[Seneschal]

I've recently decided that I wanted to make atmospheres and planetary gravity wells a bit more significant in my sci-fi campaign, and to stick with (relatively) non-superscience means of reaching orbit, mostly so that the players wouldn't use their ship (and its 30-ton laser cannon intended for space combat) as they would an attack chopper. It would still be possible for them to land, but to get back up, the players would need help and money - contragravity lifters, a nudge from an electromagnetic catapult, or detachable/reusable rocket boosters. I'd also like to add modular features to spaceships; having recently seen Macross Frontier, I liked the concept of various detachable "packs" that are strapped onto a basic fighter for whatever suits its current mission - extra missiles, extra engines, extra armor, an FTL drive, etc.

Unfortunately, Spaceship's system isn't designed for this - there are no rules for recalculating specs (HP, accel, delta-v) with added mass. There doesn't seem to be a way to attach anything to a ship without counting it as a system or a separate stage, effectively having to redesign the ship every time you strap something temporary onto it.

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.

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?

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?

- If we ignore the engineering problems of designing universal sockets for ships of all shapes and sizes (the setting is not that hard sci-fi), would this essentially circumvent the need for single-piece ships completely?

So, how would you guys handle this? Is there perhaps another ship-design system better suited for this?

[cultureulterior]

Originally Posted by Seneschal

I've recently decided that I wanted to make atmospheres and planetary gravity wells a bit more significant in my sci-fi campaign, and to stick with (relatively) non-superscience means of reaching orbit, mostly so that the players wouldn't use their ship (and its 30-ton laser cannon intended for space combat) as they would an attack chopper. It would still be possible for them to land, but to get back up

The way I solved this was a transportable beanstalk with an attached helicopter at the end. The ship simply goes into orbit, unwinds enough of the beanstalk to dip into the atmosphere, and then the helicopter flies away. I made a spaceships module for this in some ancient thread.

[IrishRover]

One advantage to single piece ships vs. modular: Single piece ships are stronger. The hull itself gives strength, and everything is designed to work together.

A clamp-on accessory would be heavier than an integral one, and require extra mass on the ship itself for the hardpoint.

Something like the Traveller modular cuter might work well, because the ship is designed for a module--but won't fly without one, IIRC.

If adding attachements is more effective than one piece ships, then most ships will be designed that way. Note that, irl, almost everything is a one piece hull, except for certaion special needs. (Skycrane hellicopter, Space shuttle, etc.)

[Ulzgoroth]

Originally Posted by Seneschal

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.

Originally Posted by Seneschal

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:

Originally Posted by Seneschal

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.

Originally Posted by Seneschal

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.)