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

[Pectus Solentis]

Originally Posted by Agemegos

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, Rocketdyne F-1 engines had a thrust-to-weight ratio of 84.

Thanks. Understood.

[Ulzgoroth]

Originally Posted by Pectus Solentis

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

A lot, yes. Most of them are very simple if you wanted to do them, but they would add up.

The easiest approach would probably be to keep track of remaining delta-V and use that when needed to calculate remaining fuel fraction, then remaining total mass, then maximum acceleration.

[Agemegos]

Originally Posted by Agemegos

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

On the other hand the TL7 chemical rockets in Spaceships produce 0.15 mps/tank, implying an effective exhaust velocity of 3 mps. Rocketdyne F-1 engines had an effective exhaust velocity of 1.83 mps in vacuum (1.64 at sea level), which would give less than 0.1 mps per tank.

That comes down to the F-1 burning kerosene and LOX, whereas the TL7 chemical rocket is probably based on a LOX-LH₂ upper stage or shuttle main engine with less thrust but better specific impulse, more suitable to operations in space.

[jacobmuller]

Originally Posted by Rupert

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.

TQ I love this kind of info.

[Anthony]

In general Spaceships is a tad generous with rocket performance, mostly because if you aren't generous it will be inordinately difficult to design something useful (in part because it really is inordinately difficult, in part because the design simplifications of Spaceships makes it even harder).

[Fred Brackin]

Originally Posted by Rupert

T
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.

I've seen the SpaceX Merlin engine listed at 150 to 1.

There's also no magic in any given number of Gs. The shuttle had to throttel back when it hit 3 Gs because of mechical limits of the vehicle. A Titan II second-stage carrying a Gemini capsule ended its' burn at just under 7 GS. So did an X-15 rocket plane.

The msot important thing to remebr is that though Spaceships does give you a G number and a Delta-V figure that you can use in combat those things are mostly calculated on an average per mission basis not as second by second simulation.

What does Spaceships do to simulate how your Gs of thrust go up as you burn fuel and/or how your fuel use goes down as mass decreases while holding a given aceleration? It averages your acceleration and fudges your Delta-V with that adjustment in the Fuel tank section for the number of tanks.

Spaceships is a simple system, an abstact system and not a second by second simulation. Even Ve2 wasn't really and it was far more complex than Spaceships.

[AlexanderHowl]

There is a reason why 'rocket science' is an analogy for something that is really, really hard. It is a highly complicated field that has to take into account four dimensional space-time, material sciences, aerodynamic drag, gravitational drag, etc.

[Ulzgoroth]

Originally Posted by Fred Brackin

The msot important thing to remebr is that though Spaceships does give you a G number and a Delta-V figure that you can use in combat those things are mostly calculated on an average per mission basis not as second by second simulation.

What does Spaceships do to simulate how your Gs of thrust go up as you burn fuel and/or how your fuel use goes down as mass decreases while holding a given aceleration? It averages your acceleration and fudges your Delta-V with that adjustment in the Fuel tank section for the number of tanks.

Spaceships is a simple system, an abstact system and not a second by second simulation. Even Ve2 wasn't really and it was far more complex than Spaceships.

This is another of those weird occasions where somebody tries to defend Spaceships by painting it as doing something much worse than what it actually does.

The text of Spaceships indicates to me a really clear understanding of the breakpoints and other factors that make a 'mission average' acceleration a largely useless number. Furthermore, if that's what it was trying to do it would be obviously doing it wrong since the mission average mass obviously depends on the fuel fraction while the Spaceships' acceleration values do not.

Deciding that key statistics will not be subject to frequent recalculation even if logic suggests maybe they 'should' is an understandable and familiar GURPS game design choice. It doesn't need to 'justified' as an ill-concieved 'abstraction' of things the game does not in any way treat as abstract.

[Agemegos]

Originally Posted by Ulzgoroth

The text of Spaceships indicates to me a really clear understanding of the breakpoints and other factors that make a 'mission average' acceleration a largely useless number. Furthermore, if that's what it was trying to do it would be obviously doing it wrong since the mission average mass obviously depends on the fuel fraction while the Spaceships' acceleration values do not.

When I've compared the figures of the engines from Spaceships with those of real engines from the Atomic Rockets website, Wikipedia, and my copy of Humble, Henry, & Larsen Space Propulsion Analysis and Design I've found that the figures in Spaceships are pretty accurate representations of some real engines, prototype, or detailed proposal. mps per tank is close to 1/20 times exhaust velocity; acceleration per system is close to 1/20 times thrust-to-weight. And that table for adjusting delta-vee for large propellant fractions has correct values from Tsiolkovsky's equation. That's all as it ought to be for the system to represent the performance of the spacecraft fully loaded. Delta-vee is adjusted for vessels with large propellant fractions, but acceleration is not adjusted for partly-unloaded vessels — probably because that would be fussy and complicated.

I've seen a couple of issues arise because of the choices of what engines to represent. In this case it seems that the figures are correct for a hydrogen-burning orbital engine, and so don't match a kerosene-burning launch engine (which might be represented as a sort of "high thrust" option). In a recent thread I found that the stats for the TL7 fission rocket match NERVA or Pewee (which reached test-firing prototype stage) but underestimate what was expected from DUMBO (which was cancelled because of a political decision).

[Pectus Solentis]

Originally Posted by Agemegos

In this case it seems that the figures are correct for a hydrogen-burning orbital engine, and so don't match a kerosene-burning launch engine (which might be represented as a sort of "high thrust" option).

hmm... well, I got it.