Spaceships Chemical Rockets

[davester65]

According to the rules in Spaceships the delta-V for a chemical rocket is 0.15 mps per tank. From what I can figure, it would be impossible for a chemical rocket to achieve a delta-V high enough to make low Earth orbit even if it were multi-stage.
By my calculations, any size rocket with 1 space for engines, 1 space for armor (because of streamlining) and 18 spaces of rocket fuel could only manage a delta-V of 2.7 mps.
Am I doing something wrong? Or, is this an error in the book? The Midnight Sun's rocket engines seem to have a delta-V of 0.24 mps if you do the math with the stats they give with that example.

[Ulzgoroth]

Originally Posted by davester65

According to the rules in Spaceships the delta-V for a chemical rocket is 0.15 mps per tank. From what I can figure, it would be impossible for a chemical rocket to achieve a delta-V high enough to make low Earth orbit even if it were multi-stage.
By my calculations, any size rocket with 1 space for engines, 1 space for armor (because of streamlining) and 18 spaces of rocket fuel could only manage a delta-V of 2.7 mps.
Am I doing something wrong? Or, is this an error in the book? The Midnight Sun's rocket engines seem to have a delta-V of 0.24 mps if you do the math with the stats they give with that example.

You're doing a couple things wrong.

For one, you're missing the extremely important rule on page 17 about delta-V for ships with high fuel fractions. Factoring that in your all-out booster gets a delta-V of 6.75 mps.

For another, your calculation in no way would bar multi-stage rockets from getting to orbit. A multistage rocket can have an arbitrarily high delta-V, by achieving an arbitrarily high ratio of reaction mass to payload mass.

[Anthony]

Spaceships is highly simplified; as such, you can't really expect it to be able to replicate systems that operate at the ragged edge of the performance envelope.

[David L Pulver]

Originally Posted by Anthony

Spaceships is highly simplified; as such, you can't really expect it to be able to replicate systems that operate at the ragged edge of the performance envelope.

It nevertheless does allow multi-stage rockets to reach orbit without difficulty.

As the previous poster indicated, the relevant rules are found in the fuel tank section, which incorporate a rounded-off version of the rocket equation.

[davester65]

Originally Posted by Ulzgoroth

You're doing a couple things wrong.

For one, you're missing the extremely important rule on page 17 about delta-V for ships with high fuel fractions. Factoring that in your all-out booster gets a delta-V of 6.75 mps.

For another, your calculation in no way would bar multi-stage rockets from getting to orbit. A multistage rocket can have an arbitrarily high delta-V, by achieving an arbitrarily high ratio of reaction mass to payload mass.

Ooops! Guess I missed that chart. Thanks for cluing me in.

Originally Posted by David L Pulver

It nevertheless does allow multi-stage rockets to reach orbit without difficulty.

As the previous poster indicated, the relevant rules are found in the fuel tank section, which incorporate a rounded-off version of the rocket equation.

Again, thanks.

[Peter Knutsen]

Originally Posted by David L Pulver

As the previous poster indicated, the relevant rules are found in the fuel tank section, which incorporate a rounded-off version of the rocket equation.

You've talked a bit about plans to make one more GURPS Spaceships supplement with all sorts of extra-detail rules.

Could you please consider including a more detailed rocket equation table in that supplement, e.g. so that the multiplier for 11 fuel tanks is different from the one for 12 fuel tanks?

[SCAR]

Originally Posted by davester65

According to the rules in Spaceships the delta-V for a chemical rocket is 0.15 mps per tank. From what I can figure, it would be impossible for a chemical rocket to achieve a delta-V high enough to make low Earth orbit even if it were multi-stage.
By my calculations, any size rocket with 1 space for engines, 1 space for armor (because of streamlining) and 18 spaces of rocket fuel could only manage a delta-V of 2.7 mps.
Am I doing something wrong? Or, is this an error in the book? The Midnight Sun's rocket engines seem to have a delta-V of 0.24 mps if you do the math with the stats they give with that example.

The sample Midnight Sun on Spaceships p7 is a TL9, Chemical Rocket, two-stage spacecraft capable of lift-off to high-orbit!

Spaceships 2, p17 has the Baikonur Launch Vehicle which is a TL7, Chemical Rocket, three-stage spacecraft capable of reaching low-orbit

[Ulzgoroth]

Originally Posted by Peter Knutsen

You've talked a bit about plans to make one more GURPS Spaceships supplement with all sorts of extra-detail rules.

Could you please consider including a more detailed rocket equation table in that supplement, e.g. so that the multiplier for 11 fuel tanks is different from the one for 12 fuel tanks?

I took a few minutes and re-derived the rocket equation, have a table:

1: 1
2: 1.027
3: 1.056
4: 1.088
5: 1.122
6: 1.159
7: 1.200
8: 1.245
9: 1.295
10: 1.351
11: 1.415
12: 1.489
13: 1.574
14: 1.677
15: 1.802
16: 1.961
17: 2.176
18: 2.494
19: 3.073


(Table based on: log(20/(20-x))/log(20/19) * 1/x)

[Barenziah58]

Rocket sciencist use Isp to compare rocket engine and fuels performing. Isp stand for pounds of thrust per pounds of fuel burn per second. Most liquid fuel use LOX get than Isp in the 200 to 400 range.

[Fred Brackin]

Originally Posted by Barenziah58

Rocket sciencist use Isp to compare rocket engine and fuels performing. Isp stand for pounds of thrust per pounds of fuel burn per second. Most liquid fuel use LOX get than Isp in the 200 to 400 range.

But the Space Shuttle main engines have an ISP of 455.