[Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.

[Pectus Solentis]

When I played Kerbal Space Program, only 1.2G-1.6G was sufficient to escape Kerbin. And I know that TWR of real-world Saturn V is 1.15G.

https://forum.kerbalspaceprogram.com...s-initial-twr/

But at GURPS Spaceships p21, it says that TL7 Chemical Rocket provides 3G accelerations. It seems that it is an astounding overspec. It is greater than real-world TL7 Chemical Rockets, and it is greater than even TL9 HDEM Chemical Rocket (it provides 2G).

[Rupert]

Originally Posted by Pectus Solentis

When I played Kerbal Space Program, only 1.2G-1.6G was sufficient to escape Kerbin. And I know that TWR of real-world Saturn V is 1.15G.

The initial acceleration might have been that low, but maximum acceleration for the Saturn V's first stage was about 3.5G. What's more, the system was designed to not have too high an acceleration - both the first and second stages cut fuel to the centre rocket well before they did to the outer ones to keep acceleration within set amounts.

Also, the F-1 rocket used in the first stage has a thrust/weight ratio of 94-to-1 on its own, so Spaceships is actually being conservative - an F-1 weighing 8.4 tons in a ship weighing 168 tons (twenty times as much) would have an initial acceleration of 4.7G.

Likewise, the 2nd stage J-2 had a 73-to-1 thrust/weight ratio.

Russian launch rockets have thrust/weight ratio of anywhere from 75:1 up to at least 137:1.

I'm not sure how generous (or not) Spaceships is with delta-vee, but it's conservative with rocket engine accelerations.

As for the TL9 HDEM rocket - it trades raw thrust for efficiency. I doubt anyone would think it very unreasonable for you to decide that chemical and HDEM rockets can use the High Thrust option available to most other reaction engines (x2 acceleration, x1/2 delta-vee per tank).

Now, if you want numbers that don't make much sense, consider jet engines. Being generous a modern jet manages a thrust/weight ratio of 5:1, or 8:1 with afterburning, which should give 0.25G per system (0.4G per system with afterburner). Also, they should burn an entire system of fuel per quarter hour of normal thrust and every 7.5 minutes on afterburner.

[Ulzgoroth]

Just to make it explicit, that means that if you want a chemical rocket with less thrust, the natural approach is simply to use a smaller system as described in Spaceships 7.

[Pectus Solentis]

Originally Posted by Rupert

The initial acceleration might have been that low, but maximum acceleration for the Saturn V's first stage was about 3.5G. What's more, the system was designed to not have too high an acceleration - both the first and second stages cut fuel to the centre rocket well before they did to the outer ones to keep acceleration within set amounts.

Also, the F-1 rocket used in the first stage has a thrust/weight ratio of 94-to-1 on its own, so Spaceships is actually being conservative - an F-1 weighing 8.4 tons in a ship weighing 168 tons (twenty times as much) would have an initial acceleration of 4.7G.

Likewise, the 2nd stage J-2 had a 73-to-1 thrust/weight ratio.

Russian launch rockets have thrust/weight ratio of anywhere from 75:1 up to at least 137:1.

I'm not sure how generous (or not) Spaceships is with delta-vee, but it's conservative with rocket engine accelerations.

As for the TL9 HDEM rocket - it trades raw thrust for efficiency. I doubt anyone would think it very unreasonable for you to decide that chemical and HDEM rockets can use the High Thrust option available to most other reaction engines (x2 acceleration, x1/2 delta-vee per tank).

Now, if you want numbers that don't make much sense, consider jet engines. Being generous a modern jet manages a thrust/weight ratio of 5:1, or 8:1 with afterburning, which should give 0.25G per system (0.4G per system with afterburner). Also, they should burn an entire system of fuel per quarter hour of normal thrust and every 7.5 minutes on afterburner.

Thanks for your generous explanation. But then, Is the G that Reaction Drive provides not an initial G? Then, How can I set initial G for my Reaction-Drive-Driven vessels?

[Ulzgoroth]

Originally Posted by Pectus Solentis

Thanks for your generous explanation. But then, Is the G that Reaction Drive provides not an initial G? Then, How can I set initial G for my Reaction-Drive-Driven vessels?

It is an initial acceleration, and also a final acceleration, because while Spaceships doesn't entirely ignore the way decreasing rocket mass in flight affects delta-V, it does entirely ignore the way it affects acceleration.

[Pectus Solentis]

Originally Posted by Ulzgoroth

it does entirely ignore the way it affects acceleration.

well... understood.

[Aldric]

Originally Posted by Ulzgoroth

It is an initial acceleration, and also a final acceleration, because while Spaceships doesn't entirely ignore the way decreasing rocket mass in flight affects delta-V, it does entirely ignore the way it affects acceleration.

Could you calculate the acceleration every time you empty a fuel tank for example?

[Rupert]

Originally Posted by Aldric

Could you calculate the acceleration every time you empty a fuel tank for example?

You could, though if you're doing that you should probably also do a more precise and correct calculation of the delta-vee in each tank, as it increases as the vessel gets lighter too, and Spaceships treats is as constant just as it does acceleration.

At some point you may as well just do all the calculations from scratch, and use real rocket science. (But not me - that's too much effort given the minimal return I'd get.)

[Pectus Solentis]

Originally Posted by Aldric

Could you calculate the acceleration every time you empty a fuel tank for example?

Well, I understood that there will be a lot of calculation problem for computing accelerations of every times.

[Agemegos]

Originally Posted by Pectus Solentis

When I played Kerbal Space Program, only 1.2G-1.6G was sufficient to escape Kerbin. And I know that TWR of real-world Saturn V is 1.15G.



But at GURPS Spaceships p21, it says that TL7 Chemical Rocket provides 3G accelerations. It seems that it is an astounding overspec. It is greater than real-world TL7 Chemical Rockets, and it is greater than even TL9 HDEM Chemical Rocket (it provides 2G).

Are you taking into account that the reaction engines in Spaceships are 5% of the mass of the entire ship including propellant?

A Saturn V all-up launch weight was 3,270 US tons. Its first-stage engines, five Rocketdyne F-1, totalled 46.25 US tons mass. That's one seventieth of the vehicle mass (1.4%), not one-twentieth of vehicle mass (5%).

In Spaceships' terms a Saturn V had less than one third of a system of reaction engine, chemical rocket (TL7). Build it as a smaller system using the rules in Spaceships 7 on page 4 — then you will be complaining that the TL7 chemical rocket is actually slightly under-spec¹ and that a Saturn V cannot take off until it has burned part of its fuel and if you write house rules for the changing acceleration as ship mass diminishes.

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¹ TL7 chemical rockets in Spaceships have a thrust-to-weight ratio of 60 g₀, Rocketdyne F-1 engines had a thrust-to-weight ratio of 84 g₀.