[Spaceships] with wings

[MatthewVilter]

I was designing a transatmospheric shuttle so I looked at "Getting into Space" on page SS37, and now I'm confused.

Look at this: "The spacecraft’s acceleration must also exceed gravity (1G, for Earth), or it must be winged (in atmosphere) or have contragravity lifters."

Does this mean contragravity lifters are only good for unstreamlined craft?

Does this mean I can put 1/3 of an Ion Drive (0.000167 Gs of acceleration) on my winged ship and fly to orbit in (5.6*.045/.000167)~=1509 hours??!!??!!

EDIT: Also, if I use a Ram-Rocket engine how long does it run on atmosphere? Is it assumed that I can use it to get the full delta-V saving from air-speed on page SS37 -AND- use it to burn the fuel needed for the remaining delta-V?

[Anthony]

Originally Posted by MatthewVilter

Does this mean contragravity lifters are only good for unstreamlined craft?

No, it means contragravity lifters let you get into orbit with low acceleration.

Originally Posted by MatthewVilter

Does this mean I can put 1/3 of an Ion Drive (0.000167 Gs of acceleration) on my winged ship and fly to orbit in (5.6*.045/.000167)~=1509 hours??!!??!!

No, because your maximum speed is lower than your stall speed. Assume you need 0.1 Gs.

[MatthewVilter]

Originally Posted by Anthony

No, it means contragravity lifters let you get into orbit with low acceleration.

That's my point, as far as I can tell so do wings.

Originally Posted by Anthony

No, because your maximum speed is lower than your stall speed. Assume you need 0.1 Gs.

That sounds fine to me. Unless I get more info I will go with this. But where are you getting this number?

In this case I add 1/3 of a Jet Engine and 2/3 of a jet fuel tank to get 2 hours of ~.333 G acceleration in atmosphere. So now I need to know how long the Jet Engine is running during takeoff. I guess you find out how long it takes to get to max air-speed and turn the engine off after that. Okay:

Time to max speed (in sec) = Max speed (in mph)/Acceleration (in mph/sec)

Max speed (in mph) = 2,500 * SqRoot(Acceleration in Gs)

Acceleration (in mph/sec) = Acceleration in Gs * 20

So in my case:

Max speed (in mph) = 2,500 * SqRoot(.333) = 1443.38

Acceleration (in mph/sec) = .333 * 20 = 6.67

Time to max speed (in sec) = 1443.38/6.67 = 216.4 sec = 3.6 min = .06011 hours

That's 33 trips to orbit! (I will still need 5.2 delta-V in Hydrogen for each trip)

^ does this look right?

EDIT: I was ninja'd. :P

So yeah .1 Gs seems about right.

About Ram-Rockets, because air-speed is logarithmic adding the Ram-Rocket option to space drives if you already have .1+ Gs in atmo is not cost effective. But (not that I see this in the rules) don't ram-rockets need high air-speed to work in the first place? Why/how would you use a ram-rocket?

[David Johnston2]

Originally Posted by MatthewVilter

I was designing a transatmospheric shuttle so I looked at "Getting into Space" on page SS37, and now I'm confused.

Look at this: "The spacecraft’s acceleration must also exceed gravity (1G, for Earth), or it must be winged (in atmosphere) or have contragravity lifters."

Does this mean contragravity lifters are only good for unstreamlined craft?

A lack of streamlining would still degrade atmospheric performance. (Spaceships p.35)

Does this mean I can put 1/3 of an Ion Drive (0.000167 Gs of acceleration) on my winged ship and fly to orbit in (5.6*.045/.000167)~=1509 hours??!!??!!

And a runway that circles the planet? Obviously not. That acceleration would be insufficient to overcome atmospheric drag. While Spaceships doesn't give a minimum thrust to get airborne, that doesn't mean there isn't one. I arbitrarily set the minimum at 0.1 gs.

EDIT: Also, if I use a Ram-Rocket engine how long does it run on atmosphere? Is it assumed that I can use it to get the full delta-V saving from air-speed on page SS37 -AND- use it to burn the fuel needed for the remaining delta-V?

Sure.

[munin]

Originally Posted by David Johnston2

... While Spaceships doesn't give a minimum thrust to get airborne, that doesn't mean there isn't one. I arbitrarily set the minimum at 0.1 gs. ...

Here's a semi-non-arbitrary GURPS Spaceships way to do it: When calculating a spacecraft's air speed you round to the nearest 100 mph (p. SS35), so you need to have enough Gs to get at least 50 mph before rounding, otherwise you'll get rounded down to zero. 50 mph can be achieved with a streamlined 0.0004 G (0.04 G unstreamlined), so anything less than that gives you an air speed of zero.

Aircraft have gotten off the ground with lesser air speeds than that, in the neighborhood of 15 mph for some human-powered craft, but they had higher acceleration to overcome the drag.