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Old 07-26-2021, 07:29 PM   #11
Fred Brackin
 
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Default Re: Spaceships: How to take off?

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Originally Posted by Michael Thayne View Post
A

HEDM engines are 2G each, so it only takes one.
Indeed 1 engine and then you get 9 Fuel Tank modules whihc would be 4.5 mps of Delta-V except for the table next to the Fuel Tank entry which explains that with 9 tanks you multiply Delta-V by 1.4. That gets you 6.3 and you have not only enough to get to orbit but enough for a vertical landing SpaceX style too.

That leaves 10 spaces and you must have a Control Room. Then you probably want 3 Armor systems to give you a decently durable hull. The remaining 6 spaces you can use for Cargo and you're HEDM shuttle can haul 30% of its' weight into orbit. That's why i rated it best. It's much better than any chemical rocket such as we have now.

For Superscience drives you just take enough engines to add up to over 1 G and then one or more Power Plants to supply the needed energy.
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Old 07-26-2021, 09:47 PM   #12
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Default Re: Spaceships: How to take off?

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Antimatter thermal might make sense for an aerospace fighter but they're not very cost-effective for routine passengers and cargo.
The price of antimatter in Spaceships looks as though it is based on Robert L. Forward's estimate¹ of the cost of producing antimatter at TL8 combined with GURPS' posit that things' prices don't change with TL. I have tried alternatives, such as Georges Dupont-Roc's generalisation that energy technologies' real prices per joule diminish by an order of magnitude or a little bit more from their commercial introduction to their maturity. But it doesn't help very much. You need a superscience or at least limited-superscience way of making antimatter to have affordable AM-powered rocket operations.

I did a study of the minimum cost of orbital launch using GURPS Spaceships, and concluded that far the cheapest means at TL10 is to use ground-orbit lighters propelled by limited-superscience fusion torch engines using water for their reaction mass. For the median human-habitable world (as produced by GURPS Space 4e), which has 0.81 gee surface gravity and an orbital speed of 4.15 mi/sec, one engine with normal thrust and one tank of water minimises the cost. In the case of the the highest-gravity habitable worlds (which have about 1.6 gee and 7.7 mi/sec) cost is minimised by one high-thrust fusion torch and four tanks of water.
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Old 07-27-2021, 01:14 AM   #13
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Default Re: Spaceships: How to take off?

The realistic answer is interplanetary drives and launch drives are different drives, and most likely are attached to different vessels. A ship with a high-impulse drive and an interface shuttle with a high-thrust drive.

If you must have a dual-purpose drive the least superscience option that's not economically disastrous like antimatter or environmentally disastrous like external fission drives is probably the fusion torch.
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Old 07-27-2021, 01:25 AM   #14
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Default Re: Spaceships: How to take off?

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If you must have a dual-purpose drive the least superscience option that's not economically disastrous like antimatter or environmentally disastrous like external fission drives is probably the fusion torch.
The fusion torch has a power output of bit over 2 gigawatts per ton of thrust. Even if the radiation isn't a problem, they're going blast giant craters every time they take off.
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Old 07-27-2021, 05:23 AM   #15
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Default Re: Spaceships: How to take off?

Anyway, I know it is super science at this point in our history but solving the unified theory problem is not something completely unthinkable.

As soon as you solve that problem, one type of force can be turned into another type of force. This means those reactors that turn the propellor on an aircraft carrier and push it across the ocean can be used to turn a gravity propeller and push a ship up into space.

For me, if I were going to start a space campaign, this is one of the breakthroughs I'd want to put in my history. Have a FTL drive is far less realistic than a unified theory breaththrough. In fact, I've long posited that just such a breaththrough is what we are waiting for to make our next "big" scientific jump.

What I find less realistic to be honest is small craft jetting all over the solar system on reaction based fuel the way they do in the Expanse. I hope there is a gas station on every single corner in that solar system.
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Old 07-27-2021, 06:05 AM   #16
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Default Re: Spaceships: How to take off?

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Originally Posted by Michael Thayne View Post
HEDM engines are 2G each, so it only takes one.
Errr, right. I was going off of MaryAnn's numbers rather than checking the books for myself, and misread "0.5 mps" as "0.5G."

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The fusion torch has a power output of bit over 2 gigawatts per ton of thrust. Even if the radiation isn't a problem, they're going blast giant craters every time they take off.
I'm envisioning a setting where, out in the middle of some desert or similar, there are a series of deep craters that get deeper with each launch. I wonder how deep you could get before the debris would start bouncing back with enough force to damage the ship during liftoff...

A possible way to avoid this would be to use a more traditionally-powered aircraft to get up into the sky, then break off and launch with your own drives, much like with the VSS Unity (and associated mothership VMS Eve) that recently took Richard Branson to the edge of space. So, you have a ship that's capable of traveling through space and typically uses a towing ship to help it get into space, but is capable of getting there under its own power if necessary.

Of course, I agree that it makes a lot more sense to have separate space-travel craft and interface craft. The former would never touch down on a planet - rather, they'd park up in orbit, either docking with a space station or by themselves, and use interface craft to ferry personnel and goods/materials between the ship and the surface.
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Old 07-27-2021, 08:00 AM   #17
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Default Re: Spaceships: How to take off?

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A possible way to avoid this would be to use a more traditionally-powered aircraft to get up into the sky, then break off and launch with your own drives
Or a sled launch, or merely limit power output near the ground. As long as you have some kind of aerodynamic lift (wings, lifting body, etc.), you can take off using just as little power as a conventional aircraft of the same weight, then fly out over the ocean and throttle up the torch once you're safely clear of things you care about.

On a world without atmosphere this is obviously more of a problem. In a setting I built some years ago, the rare cases of places with enough gravity that you couldn't just dock with them, but not enough atmosphere for wingborne flight (planets like Mercury), were dealt with via magnetic launch/landing tracks: fly your ship into the hole, and the magnetic field grabs it and decelerates it for landing. The numbers for aborts get a bit hairy (you need several ships in the pipe at once to get sensible traffic levels) but I think it's doable.
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Old 07-27-2021, 08:44 AM   #18
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Default Re: Spaceships: How to take off?

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Originally Posted by Fred Brackin View Post
That leaves 10 spaces and you must have a Control Room. Then you probably want 3 Armor systems to give you a decently durable hull. The remaining 6 spaces you can use for Cargo and you're HEDM shuttle can haul 30% of its' weight into orbit. That's why i rated it best. It's much better than any chemical rocket such as we have now.
Laser thermal rockets (not to be found in Spaceships 1, unfortunately) are a very attractive candidate for launch vehicles. They're equivalent to HEDM in performance, but use cheaper and safer fuel. They require substantial ground infrastructure, of course.
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Old 07-27-2021, 09:06 AM   #19
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Default Re: Spaceships: How to take off?

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Laser thermal rockets (not to be found in Spaceships 1, unfortunately) are a very attractive candidate for launch vehicles. They're equivalent to HEDM in performance, but use cheaper and safer fuel. They require substantial ground infrastructure, of course.
Yeah, I think they're in both Spaceships 7 and Spaceships 8, though, if you have either of those books. Spaceships 8 even has a worked example of an interface vehicle using that system. Otherwise, my personal preference is to use nuclear thermal designs like the Condor Spaceplane in Spaceships 2—the payload per trip is lower, but the fuel is both cheaper and safer.
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Old 07-27-2021, 09:07 AM   #20
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Default Re: Spaceships: How to take off?

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Laser thermal rockets (not to be found in Spaceships 1, unfortunately) are a very attractive candidate for launch vehicles. They're equivalent to HEDM in performance, but use cheaper and safer fuel. They require substantial ground infrastructure, of course.
And, of course, that ground infrastructure can be multi-purpose - a laser powerful and precise enough to get a ship into orbit can also deflect meteors, destroy ICBM's (or other airborne hostiles), deorbit space junk, etc. This makes it an expenditure that is much easier to justify.

EDIT: Of course, the international community may well object.
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