[Spaceships] Designers' Championship I: ground to LEO

[Chansith]

Originally Posted by thtraveller

Is that meant to be all divided by A?

So it is. My mistake.

[joelbf]

Originally Posted by thtraveller

As most of the ticket cost is variable cost the ticket price should scale with ship size fairly linearly.

Funny, i did an almost exactly equivalent design, including going from jet assisted to ramjets when spaceships was released, but I have since thrown out my notes. Cool that you actually did a design, myself I really don't grook the eco-speech in the ground rules.

Since cost mostly is from fuel, it might be cheaper to have more engines as ram-jets. You would be saving 0,58 mps worth of hydrogen each trip if you used three ram-jets, costing M2,4 extra. Also do you really need 3G's accel? Reducing to 1,5G gives you twice the amounts of passengers though I don't know what that will do with the timing-calculations from the ground-rules, but since T is k/A for some k (p. 37) we should be roughly doubling the to-orbit time, and I don't think 6 or 12 minutes will matter (but hey, I really don't understand much of the ground-rules).

So shamelessly building upon your design I would do the Alida mk Boobis with three ram-jets and 6 passenger modules. Hoping that almost tripling the price of the shuttle and increasing to-orbit time with 6 min would pay of.

[joelbf]

Note: I'm to lazy to do an actual complete design, but I'm tossing out ideas for grabs.

Originally Posted by thtraveller

1 Armor (Advanced Laminate)

Also while I'm at it you don't really need "Advanced Laminate", and while it won't matter much it will be strictly cheaper if you go with Steel, and you don't need the DR iirc.

Next is the cost of the pilot. While it's a small part you could employ a faux multistage design, doing an sm+5 upper stage without engines that never leaves the sm+6 lower stage. This lets you keep the sm+5 control-room but roughly six times your previous # passengers, giving you more seats/pilot (though I don't know what will happen with seats/shutle-cost). David stated in some post that this should really lower sr/hnd but that might be ok.

[elustran]

Well, you want at least 3G of accel to get out of earth orbit because 1G of that is going to be wasted negating earth gravity, so you can't really afford to drop engines, and, by the rules as written, ram-rockets are pretty expensive. It might make sense to allocate some tankage to water because it's cheaper, but water is going to come out a bit more radioactive, and might kill some of the dV, but it could allow for less engine because of the higher thrust.

Either way, using a standard, hydrogen-fueled jet engine, similar to the original design, allows the lighter to gain more altitude before it engages its fission thermal rockets, which might be more amenable to an eco-sensitive populace.

I've been thinking, too, that it might make sense to use a laser-launch design that uses water as its primary reaction mass rather than ablative plastic. Water would be far less toxic, and is widely available. If, by RAW, water won't give enough isp to attain good dV while carrying enough passengers, hydrogen is probably a pretty good idea too.

[joelbf]

Originally Posted by elustran

Well, you want at least 3G of accel to get out of earth orbit because 1G of that is going to be wasted negating earth gravity, so you can't really afford to drop engines, and, by the rules as written, ram-rockets are pretty expensive. It might make sense to allocate some tankage to water because it's cheaper, but water is going to come out a bit more radioactive, and might kill some of the dV, but it could allow for less engine because of the higher thrust.

What? IIRC you just need a tiny bit more than 1 G to reach orbit, but it might take a looooong time. BUT you could for example use three engines for 1,5G and still make it out to space quite easily. And with a winged design you can probably argue that you can glide-land on 0,5 G so that gives you the possibility to have 2/3 of your engines faulting. As for tankage, since you can make it out to space on 3 engines but need 9 tanks, reducing dV by increasing thrust seems pretty useless.

Originally Posted by elustran

Either way, using a standard, hydrogen-fueled jet engine, similar to the original design, allows the lighter to gain more altitude before it engages its fission thermal rockets, which might be more amenable to an eco-sensitive populace.

Jet engines are not hydrogen fueled. Jet engines are jetfuel-fueled.

[Arazael]

Originally Posted by Boobis

What? IIRC you just need a tiny bit more than 1 G to reach orbit, but it might take a looooong time. BUT you could for example use three engines for 1,5G and still make it out to space quite easily. And with a winged design you can probably argue that you can glide-land on 0,5 G so that gives you the possibility to have 2/3 of your engines faulting. As for tankage, since you can make it out to space on 3 engines but need 9 tanks, reducing dV by increasing thrust seems pretty useless.

Yes, 1.5G is plenty to reach orbit, as long as you have enough dV. As to glide-landing, with a well designed orbiter its theoretically possible with no engines at all working. With 0.5G and a long enough runway it should be easy. A winged design can also increase your fuel efficiency on the way up, unless you launch vertically.

Originally Posted by Boobis

Jet engines are not hydrogen fueled. Jet engines are jetfuel-fueled.

Jet engines are definitely jet-engine fueled. A hydrogen-fueled variant is possible, but the version in GURPS spaceships isn't one.

[thtraveller]

I think my economics might need revising slightly as the fission air ram is likely to need new fission cores every couple of years. Though ship cost isn't a major factor in the ticket price so likely only a few hundred dollars per ticket increase.

[thtraveller]

Originally Posted by Boobis

What? IIRC you just need a tiny bit more than 1 G to reach orbit, but it might take a looooong time. BUT you could for example use three engines for 1,5G and still make it out to space quite easily.

But it takes a lot longer so you need a lot more fuel. You have gravity pulling down on you the whole time. Worst case is you subtract 1G from your thrust and discount all the delta-V from the fuel that 1G of engine uses for the time it takes you to reach orbit.

[elustran]

Think of it this way - if you have a 1.5 G engine, you're only accelerating at .5G, but still burning 1.5 G of fuel. You can improve efficiency by angling your engine, giving you a maximum of 1G of vertical thrust to cancel out gravity and 1.12G of horizontal acceleration with 1.5 G thrusters, but your dV to LEO would be pretty compromised with only about 75% fuel efficiency... in other words, you'd need to burn about 30% more dV than your orbital velocity. With 3G of acceleration angled properly, you can have 2.83G of horizontal acceleration while canceling out gravity, for an efficiency of 94% - you'd only be burning about 6% more dV than your orbital velocity. Of course, considerations are a little more complicated because you still need to get out of atmo and you suffer from atmospheric drag, but those complications are greatly reduced with jets or ram-rockets to bring you to the top of the atmosphere.

As far as powering jets with hydrogen, a lot of scramjet designs burn H, and I imagine hydrogen will be a more common fuel in a hydrocarbon poor future than standard Jet-A.

[joelbf]

Originally Posted by thtraveller

I think my economics might need revising slightly as the fission air ram is likely to need new fission cores every couple of years. Though ship cost isn't a major factor in the ticket price so likely only a few hundred dollars per ticket increase.

Wouldn't they operate 50 years on internal fuel like the TL9 fission powerplants?