[Spaceships] A Few Questions

[scc]

Originally Posted by Fred Brackin

That 3G limit was true for the Shuttle but that has to do with weight savings. The crew could have taken more and rockets have been designed that gave more. Gemini launches topped out just short of 7 Gs. So did some X-15 flights.

3Gs isn't some magic number for either hardware or wetware.

I crunched the numbers a little while back for Merlin engines SpaceX makes, they'd provide 9G in Spaceships

And something I should have asked earlier:

5. What are the effects in SS terms of using a full-flow staged combustion engine?

6. Similar to the above, what are the effects of using an aerospike engine?

[Fred Brackin]

Originally Posted by scc

I
And something I should have asked earlier:

5. What are the effects in SS terms of using a full-flow staged combustion engine?

6. Similar to the above, what are the effects of using an aerospike engine?

Probably below the level of resolution in both cases.

The aerospike would just give you a slightly higher average effciency over a typical burn time in atmosphere.

I've ever heard of the other one.

The important thing in Spaceships is the Delta-V.

[scc]

Originally Posted by Fred Brackin

The aerospike would just give you a slightly higher average effciency over a typical burn time in atmosphere.

I've ever heard of the other one.

The important thing in Spaceships is the Delta-V.

Full-flow staged combustion, what "Full gasification of components leads to faster chemical reactions in the combustion chamber, which improves performance." means in game terms is rather undefined.

As for aerospike, Wikipedia says reduction in fuel usage of 25%-30%, divide stock rocket engine Delta-V by those number and you get performance between .2 and .21 mps.

[AlexanderHowl]

At least from my readings on the subject, 5g tends to be the golden number. Anything below that, and you start suffering significant gravitational drag. Anything above that, and you start having to limit your crew and passengers to very healthy people.

[Michael Thayne]

For armor for civilian ships, I like to think in terms of protecting against space debris. Typically, space debris will hit either the front or the rear (the latter while doing a breaking burn). So in some of my civilian ship designs, I armor the front and rear but not the central hull. 6 dDR protects you in orbit or for low-energy planetary transfers. For fast ships, you might want up to 6 dDR per 20 mps delta-V, though at very high speeds you might rely on lasers to destroy debris rather than armoring against it.

Also, some published designs seem to think winged landers should have armor on the entire hull, to survive re-entry I guess. There's no official rule on that but it seems plausible?

Originally Posted by AlexanderHowl

At least from my readings on the subject, 5g tends to be the golden number. Anything below that, and you start suffering significant gravitational drag. Anything above that, and you start having to limit your crew and passengers to very healthy people.

Is gravitational drag that much of an issue for typical mission profiles? It's been awhile since I've played Kerbal Space Program, but I remember getting the sense that gravitational drag is mostly NBD since you spend most of your time thrusting sideways.

EDIT: According to Wikipedia, the Saturn V lifted off with less than 1.6g, so my Kerbal Space Program memories seem correct.

[AlexanderHowl]

It is an issue if you want to get more than 10% of your initial mass to LEO (remember, an extra 1 km/s of delta-v can double reaction mass requirements). The faster you get to LEO, the lower the gravitational drag, so the lower the required reaction mass.

[Michael Thayne]

Originally Posted by AlexanderHowl

It is an issue if you want to get more than 10% of your initial mass to LEO (remember, an extra 1 km/s of delta-v can double reaction mass requirements). The faster you get to LEO, the lower the gravitational drag, so the lower the required reaction mass.

I'm not sure this is true. The Saturn V was limited to delivering less than 5% of its mass on the launch pad to LEO, but that was partly because its lower stage used kerosene-oxygen fuel, which has lower specific impulse than hydrogen-oxygen. My understanding is that the Apollo missions' delta-V budget to get to LEO was only ~2 km/s more than Earth's orbital velocity, and much (most?) of that would have gone to fighting air resistance. Based on the numbers I've seen, variations in rocket thrust and aerodynamics might let you deliver 11% of mass to LEO rather than 9% (if you use hydrogen-oxygen all the way), but that's not the only consideration that matters in rocket design (otherwise the Saturn V wouldn't have used kerosene for the first stage).

Also, remember that while GURPS ignores this, realistically thrust-to-weight ratio increases as you burn fuel. The Saturn V's first stage started off at a mere 1.6g, but this could gradually increase to almost 4g as the fuel tanks emptied.

[Ulzgoroth]

Originally Posted by scc

As for aerospike, Wikipedia says reduction in fuel usage of 25%-30%, divide stock rocket engine Delta-V by those number and you get performance between .2 and .21 mps.

I think the point of aerospike is to mitigate the performance loss rockets experience when operating in an atmosphere. Which GURPS Spaceships doesn't model at all.

Originally Posted by Michael Thayne

Also, some published designs seem to think winged landers should have armor on the entire hull, to survive re-entry I guess. There's no official rule on that but it seems plausible?

Most published designs think essentially all spacecraft should have armor on the entire hull. Spaceships whose pressure hull is as flimsy as a modern car door are not very popular with the authors.

Winged or not, reentry shielding only needs to be on one face. How you model that in basic Spaceships design if the face is the 'bottom' rather than the nose or tail is of course a thing.

[David Johnston2]

[QUOTE=Ulzgoroth;2226623]I think the point of aerospike is to mitigate the performance loss rockets experience when operating in an atmosphere. Which GURPS Spaceships doesn't model at all.

Most published designs think essentially all spacecraft should have armor on the entire hull. /QUOTE]

Well if you don't then soon or later you're going to get a puncture. That may not be a big deal with freighters that are mostly evacuated cargo hold though.

[Anthony]

Originally Posted by scc

Full-flow staged combustion, what "Full gasification of components leads to faster chemical reactions in the combustion chamber, which improves performance." means in game terms is rather undefined.

It means 'performance comes closer to the theoretical limit to some undefined degree'.

Originally Posted by scc

As for aerospike, Wikipedia says reduction in fuel usage of 25%-30%.

It says '25–30% less fuel at low altitudes'. What this means in practice is probably that you can just use the vacuum ISp, which Spaceships already does because dealing with computations about atmospheric performance is way too complicated.