zero said:
A question linking to Rikki Tikki's point about L/D ratio...
If I was writing a hard scifi story and for some reason I had a ship that accelerated at lets say for simplicity, 0.5 Gs... then it would probably need staging for lift off from Earth, correct?
However, could that be circumvented in a way by the added D/L ratio by adding a wing-wedge shape to the ship (thus also giving me a good place to put my heat shields on)?
If it took about a D/L ratio of 2 to succeed at lift off, how much D/L would a 100dton (streamlined) ship that has a Traveller Thrust of 0.5 G and how could it be modded to achieve a lift-off if necessary (actual science to me seems that even if D/L could be heightened I would still probably need staging)?
Actually, staging is related to how much fuel you can carry and how quickly you use it up. It is a way to lighten the ship as you go.
A lower acceleration (in atmosphere where lift can be provided) will simply require more TIME to reach the required speed. Specifically, for your purposes, a lower L/D ratio will increase the speed needed to for the ‘wings’ to support the ship, so the ship will have a higher stall speed than a ship with a better L/D ratio. The lower acceleration will require more time to reach takeoff speed so the ship will need a longer runway for takeoff (or a catapult launch like aircraft carriers use).
Once airborne, the ship will need more time to reach orbital velocity than a 1G ship would.
Some very rough numbers:
To reach low earth orbit (LEO) takes about 8 km/sec of velocity plus about 2 km/sec of velocity lost accelerating through the atmosphere at 4G.
So a 4G rocket requires about 4.2 minutes to reach LEO.
A 1 G spaceplane requires about 8 km/sec of velocity plus about 8 km/sec of velocity lost accelerating through the atmosphere, and takes about 27 minutes getting to LEO.
A 0.5 G spaceplane requires about 8 km/sec of velocity plus about 18 km/sec of velocity lost accelerating through the atmosphere, and takes about 40 minutes getting to LEO.
So here is where your tradeoffs come in. A 4G engine will burn more fuel per second than a 0.5G engine, but requires fewer seconds of operational time to ‘get the job done’. All things being equal, the more efficient (0.5 G) engine should use less fuel to accelerate from 0 to 8 kps (orbital velocity) than the more powerful (4 G) engine, but as you may have guessed al things are not equal. Notice that the longer that it takes to get out of the atmosphere, the more acceleration is lost to atmospheric drag. All three spacecraft end up in LEO travelling at 8 kilometers per second, but the 4G rocket only looses 2 kps of acceleration pushing through the atmosphere while the 0.5 G rocket looses 18 kps accelerating through the atmosphere. So the slower the ascent, the more fuel is wasted pushing against the atmosphere.
At current technologies, it is more efficient to hurry through the air and suffer the minimum atmospheric drag penalty. Different (future) technologies may yield different results. For example a nuclear engine can heat the air and use that to achieve thrust – thus ‘fuel’ (actually reaction mass) is free and infinite as long as the ship is in the atmosphere, so who cares how much is lost fighting drag. Another advantage to a slow ride is generally less wear on the craft and systems. So a 4G rocket will currently use less fuel, but require more maintenance (or be disposable) than a reusable space plane. Since fuel is everything in modern rocket design, we use rockets. A breakthrough technology (like Traveller) could easily swing the balance in the direction of aircraft like launches to space.
