GURPS: Space:: reducing space launch prices

[Daraniya]

So, I've been reading about all of these great ideas for heavy lift airships

Skylifter - Link 1 Skylifter - link 2

LEMV

And it got me to thinking about how someone could use these to reduce the amount of fuel required for rockets to leave planets with heavy gravity, like Earth. The skylifter is capable of lifting 150 metric tons (150,000 kg or ~330,000 pounds) on it's own, and I had an idea that you could tether 3 or 4 together with a platform in the middle of them, that it would be possible for a hydrogen version of the Skylifter to take payloads up to 40 miles up. Could they even get that high? Would this reduce the amount of fuel needed to achieve a stable orbit?

I know rockets like the Falcon_Heavy are 1.4 million pounds (~635,000 kg) or so, but the majority of that is fuel, if you could reduce the gravitation tug before launch, you use less fuel, which requires less fuel to be onboard, etc... plus, a payload (shuttle-type craft) won't be nearly that heavy (I'm thinking... minus the weight of the balloon material, platform, etc, there'd be almost 50 to 100 metric tons)


And to add another question, do rockets have to launch vertically? I was wondering about either horizontal launch or launching at a 45 to 60 degree angle... to further reduce platform size/weight...




P.S> Sorry the above is so 'stream of consciousness'... this is the kind of stuff that bogs me down creating realistic settings... took me a month to decide on using a WHIP drive, and how one would power the wormhole (theoretically needs around 10^10 tesla to generate the field required for a stable wormhole to occur)...

[munin]

Originally Posted by Daraniya

... Would this reduce the amount of fuel needed to achieve a stable orbit? ...

A little. The Earth's surface is about 4,000 miles from its center of gravity. Adding 40 miles to that would reduce escape velocity by about 0.5% (1/200th). GURPS Spaceships (p. 37) has a short paragraph on determining escape velocity for different planets which can be used to make estimates here (Me = 1, Re = 4040/4000).

Originally Posted by Daraniya

... do rockets have to launch vertically? ...

Nope! Here's one proposal for a horizontal launch system.

[RogerBW]

The basic difficulty with this idea is that the important part of being in orbit - the hard part to achieve - is the huge horizontal speed, not the altitude. Yes, the Earth's atmosphere makes a difference, and a high-altitude launch would reduce the fuel budget somewhat - but probably not by enough to make a significant change to vehicle design.

For example, Spaceships gives Earth's escape velocity as 6.96 mile/second, and orbital speed as 80% of this, 5.6 mile/second - which would be valid if the planet had no atmosphere. Actual delta-V values for launches to Earth orbit are around the 5.8-6.2 mile/second range. So you can assume that around 0.2-0.6 mile/second of your delta-V budget is being used breaking through the atmosphere - so maybe 3%-10% of the whole thing. In return for saving that, you have this whole huge extra stage...

It's worth noting that most high-altitude-launch proposals deal with an aircraft capable of moving fast, not just getting away from Earth. In effect it becomes the first stage, perhaps using air-breathing engines (which give lower fuel consumption per thrust) and dropping away at an altitude where they're no longer useful.

[johndallman]

Originally Posted by Daraniya

And to add another question, do rockets have to launch vertically? I was wondering about either horizontal launch or launching at a 45 to 60 degree angle... to further reduce platform size/weight...

The reason rockets usually launch vertically is the huge amount of drag that you get when flying through dense atmosphere at high speed. It saves fuel to rise above the denser levels of the atmosphere before you start accelerating seriously.

[Daraniya]

Originally Posted by munin

A little. The Earth's surface is about 4,000 miles from its center of gravity. Adding 40 miles to that would reduce escape velocity by about 0.5% (1/200th). GURPS Spaceships (p. 37) has a short paragraph on determining escape velocity for different planets which can be used to make estimates here (Me = 1, Re = 4040/4000).


Nope! Here's one proposal for a horizontal launch system.

Yea, I saw the loftstrom loops... Freaking expensive, though... not as expensive as the US war expenditures, but still...

[Daraniya]

Originally Posted by RogerBW

For example, Spaceships gives Earth's escape velocity as 6.96 mile/second, and orbital speed as 80% of this, 5.6 mile/second - which would be valid if the planet had no atmosphere. Actual delta-V values for launches to Earth orbit are around the 5.8-6.2 mile/second range. So you can assume that around 0.2-0.6 mile/second of your delta-V budget is being used breaking through the atmosphere - so maybe 3%-10% of the whole thing. In return for saving that, you have this whole huge extra stage...

It's worth noting that most high-altitude-launch proposals deal with an aircraft capable of moving fast, not just getting away from Earth. In effect it becomes the first stage, perhaps using air-breathing engines (which give lower fuel consumption per thrust) and dropping away at an altitude where they're no longer useful.

I was just trying to get 120 miles or so up, enough to get a stable orbit... I thought the first 40 miles was the hardest... the last 80 miles is the more difficult...

RogerBW, I'll take a look at that... If the GURPS folks have it in there, it's gotta be correct... I'll read up on it, and work something out.

I just want something worked out in case a player wants to delve into that... maybe once space flight becomes more regular, the lofstrom loops would become possible...

[Humabout]

Originally Posted by johndallman

The reason rockets usually launch vertically is the huge amount of drag that you get when flying through dense atmosphere at high speed. It saves fuel to rise above the denser levels of the atmosphere before you start accelerating seriously.

And just to complicate matters greatly, launching at an angle helps you add the planet's rotational velocity to that of the rocket when attempting to achieve orbital or escape velocity.

Making things more complicated still is that it is more fuel efficient to blast all of your fuel into thrust in the first seconds of launch and coast the rest of the way than to accelerate all that extra weight up to escape velocity over the course of a longer burn.

Current rocket launches attempt to balance all of these wonderfully interconnected bits and pieces to achieve the most fuel-efficient launch trajectory possible.