Is achievingorbit that easy?

[LuciusSummers]

Originally Posted by kdtipa

I haven't read what you refer to (no clue what SM+3 design for reentry refers to), but it's sounding like a winged design. You can't use wings to reach space. The higher you go, the lower the air pressure, and therefore the less lift wings can produce. If the air is thin enough, it just won't support the winged vehicle anymore.

When either or not a spaceship has wings, fins or about any other design feature makes no difference to achieving orbit. (Baring weight/air resistance).

The main reason an atmospheric plane etc cannot achieve orbit is the fact that jet engines and propellers do not work in a vacuum.

Else its simply a matter of thrust vs gravity.

There are space ships today that use jet engines to get as high as they can like a plane then hit the rocket engines to go the last mile as it were.

[Anthony]

Originally Posted by LuciusSummers

When either or not a spaceship has wings, fins or about any other design feature makes no difference to achieving orbit. (Baring weight/air resistance).

Actually, it does -- a spaceship with wings can in principle achieve orbit without every having thrust exceeding 1G, a spaceship without wings cannot.

[LuciusSummers]

Originally Posted by Anthony

Actually, it does -- a spaceship with wings can in principle achieve orbit without every having thrust exceeding 1G, a spaceship without wings cannot.

And my second part of the thing was it comes down to thrust vs gravity. So yes if your using the wings to generate extra thrust.

My simple point was the miss science that seemed to suggest that winged rockets could not achieve space flight was wrong. Or at least the way he wrote his reply seemed to suggest it was the wings that stopped space flight.

[Agemegos]

Originally Posted by Anthony

Actually, it does -- a spaceship with wings can in principle achieve orbit without every having thrust exceeding 1G

Only if its maximum airspeed exceeds circular-orbit velocity. Once the air gets thin enough that you can exceed Air Speed then the wings stop providing lift.

According to Spaceships p. 35 maximum airspeed is 2,500 m.p.h times the square root of acceleration in gees (for a streamlined spacecraft). Orbital speed is 5.6 miles per second, or 20,160 m.p.h. So you need 65 gees of thrust to fly at orbital speed.

[lwcamp]

Originally Posted by Brett

Only if its maximum airspeed exceeds circular-orbit velocity. Once the air gets thin enough that you can exceed Air Speed then the wings stop providing lift.

In GURPS maybe (I haven't really absorbed the Spaceships rules yet). The aerodynamics works a bit differently, though.

Drag: D = C_D A_F rho v^2
Lift: L = C_L A_W rho v^2
Thrust: T = constant
Weight: W = M g ~ constant

where C_D is the drag coefficient, C_L is the lift coefficient, A_F is the frontal area, A_W is the "wing" area, rho is the density of the air, v is the velocity, M is the mass, and g is the acceleration due to gravity.

For level flight, you need T = D and L = W. The spaceplane areas and aerodynamic coefficients do not change (caveat - they do not change for a given angle of attack - the angle of the spaceplane with respect to the local airflow - but you do have different C_L and C_D for different angles of attack), so as rho decreases (higher altitudes) the maximum and minimum v will increase (maximum at full throttle, T is at maximum, and the angle of attack gives the minimum C_L and C_D; minimum when the angle of attack is at the stall angle, giving maximum C_L).

These relations fail when the mean free path of a gas molecule in the air before undergoing a collision with another air molecule gets to be similar to the linear dimensions of the spaceplane. At room temperature and pressure, the mean free path of air molecules is about 65 nm, and is inversely proportional to pressure, and thus inversely proportional to density. This gives a 15 million-fold decrease in density before the mean free path gets to about a meter in length (presumably the spaceplane is on the order of a meter or ten or a hundred in length, so aerodynamics should apply up to this point). This gives a sqrt(15,000,000)=4000 fold increase in the maximum velocity. If the spaceplane can reach Mach 1 at sea level (0.34 km/s), it can reach Mach 4000 at the limits of aerodynamic performance (about 1,300 km/s, well in excess of low earth orbit speed).

Of course, these relations also break down with air breathing engines, where thrust will most certainly not be constant with altitude. And of course you need enough delta-V for your rocket to get to orbital speed as well.

Luke

[Anthony]

Originally Posted by Brett

Only if its maximum airspeed exceeds circular-orbit velocity.

Actually, that's not really necessary -- both maximum airspeed and stall speed are inversely proportional to the square root of air density, so you can increase air speed indefinitely by moving upwards. In practice you may need to break out of atmosphere a bit earlier, to avoid being cooked by plasma formation from passing through atmosphere at 3+ kps.

[blacksmith]

Originally Posted by Anthony

Actually, that's not really necessary -- both maximum airspeed and stall speed are inversely proportional to the square root of air density, so you can increase air speed indefinitely by moving upwards. In practice you may need to break out of atmosphere a bit earlier, to avoid being cooked by plasma formation from passing through atmosphere at 3+ kps.

What I wonder is could you use the skipping on the upper atmosphere idea that some hypersonic transports have suggested to gain the lift when airspeed get high enough.

[jacobmuller]

Originally Posted by Brett

Only if its maximum airspeed exceeds circular-orbit velocity. Once the air gets thin enough that you can exceed Air Speed then the wings stop providing lift.

According to Spaceships p. 35 maximum airspeed is 2,500 m.p.h times the square root of acceleration in gees (for a streamlined spacecraft). Orbital speed is 5.6 miles per second, or 20,160 m.p.h. So you need 65 gees of thrust to fly at orbital speed.

Would making local gravity the cutoff point for a relaunchable vehicle, and delta-v for escape velocity the fuel minimum, be a solution ie can you leave Earth orbit with 1g thrust and 7+mps of delta-v? Although that would contradict pg37 "Getting into Space" 2nd paragraph, last line: "or it
must be winged (in atmosphere)". But, if the book is wrong, it's wrong.

Is this part of Spaceships wrong or merely oversimplified?

[Anthony]

Originally Posted by jacobmuller

Is this part of Spaceships wrong or merely oversimplified?

It's partially both. The maximum speed formula is flat wrong, since it's invariant in vehicle size (due to the square/cube law, larger vehicles should be faster) and air pressure (the lower the air pressure, the faster a vehicle with a non-airbreathing engine can go), the comment about wings is an oversimplification (wings would let you get into orbit with somewhat below local surface gravity, but you're not getting into orbit with 0.01G thrust, wings or no wings).

[Ulzgoroth]

Originally Posted by LuciusSummers

When either or not a spaceship has wings, fins or about any other design feature makes no difference to achieving orbit. (Baring weight/air resistance).

The main reason an atmospheric plane etc cannot achieve orbit is the fact that jet engines and propellers do not work in a vacuum.

Else its simply a matter of thrust vs gravity.

There are space ships today that use jet engines to get as high as they can like a plane then hit the rocket engines to go the last mile as it were.

And in Spaceships, it's often a motive for ramrocket capable thrusters, though the sheer cost of dual-mode rockets is problematic.