Best DG Mark 4 launch method

[Andy44]

This is an old thread topic which comes back from time to time, I though I'd open it back up.

The question: Flying a stock Delta Glider Mark 4 (NOT THE DG4!), the one that comes with Orbiter, and starting from the "Quick Start" scenario on SLF Runway 33, what's the least amount of propellant you can use to get to a 200 km orbit taking off direct east? And by what method?

I have tried 3 launch methods.

1. A conventional rocket launch, in which I take off and point vertical, gradually pitching downrange. I have to set the trim to negate the wing lift during the initial ascent in order to stay vertical.

2. A wingborne launch. After taking off, I bank towards downrange, compass heading 90 degrees, and just climb like a conventional aircraft, using the wing for lift and the engine mostly for downrange acceleration. I break Mach 1 around 10 km altitude and most of my acceleration happens around 50 km or so, with engine cutoff at around 80-90 km, followed by coast to apogee and circularizing.

3. A combination of the first two, in which I climb initially at about a 50 degree angle before pitching down to accelerate downrange. This gets me out of the drag region quicker, but uses less wing lift. I have to gimbal the engines to maintain pitch angle at high altitude.

Method 2 seems to work well and leaves me with just over 58% fuel left after circularizing, but I have heard of people making it with like 60% and wonder what and how you guys are doing your DG launches. All 3 methods seem to leave about 55-58% to me, but method 2 is the most fun as you are really flying an airplane to orbit.

 

[flytandem]

Another option is to change the throttle of the engines. I have done mostly your third method (50 degree initial nose up) but keep engines at lower level so I don't create so much parasitic drag. Then "go with throttle up" when above 12-15 Km agl.

 

[Joss]

Ok.. let's look at some facts before making any explanation:

Assuming atmospheric pressure at sea level = 1 atm then:



fraction of 1 atm average altitude (m) (ft)
1 0 0
1/2 5,486 18,000
1/3 8,376 27,480
1/10 16,132 52,926
1/100 30,901 101,381
1/1000 48,467 159,013
1/10000 69,464 227,899
1/100000 96,282 283,076

Since drag is proportional to atmospheric pressure and more drag means more fuel wasted the faster you go.. It makes sense to wait to 5500 meters to have half the drag before pushing the pedal to the metal, and even makes sense to wait to 8400 meter to reduce it to 1/3.

How much pitch up to use? If you really want to make it with minimum fuel you need to know this: Parasite drag is pretty much proportional up to 12º AoA.

There is another source of drag that depends on your mach number and that appears above M 0.75 so you dont want to go above that unless you have to.

Therefore and putting this all together :
1) Take off and turn to prograde heading at or preferably below 250 m/s (M.5 aprox)
2) Trim your pitch to maintain 12º AoA
3) Adjust throttle to maintain 250m/s IAS
4) Once 250m/s = M0.7 Throttle to maintain M0.7
5) The rate of climb will fall as altitude increases.. Therefore once climb rate is too small increase thrust (even if requires speeds above M0.7) to maintain a nice positive rate of climb

Eventually you will reach 100% thrust having increased power proportionally to the decrease in drag and therefore using the smallest amount of fuel.

Now there is another point where you will need a lot of fuel if you are not careful and that is orbit circularization. In order to save fuel there you have to trim Ap to exactly the altitude you want and you have to get there with the smallest climb angle practicable.

It takes some practice and I recommend you observe Dan Steph's DG IV autopilot performing the last minutes of LEO climb and you'll see what I mean when you realize by the time the maneuver is about to finish the AoA gets closer to 0º.. You can see that visually in the circle with the cross in it in the HUD getting closer to the 0º mark.

It takes time and discipline.. IT is not by all means the fastest or the funniest way to get to LEO but it definitely is the cheapest.

Some people may argue than a vertical launch is the cheapest way (granted you have a power ratio > 1:1) but that's true for misiles or rockets which since they dont have wings dont have the induced drag either.

Got to go to work now!

 

[C3PO]

I think you have to include ISP in that equation. In complex mode the DG's ISP goes from 3.2e4 at the surface to 4.0e4 in vacum. This makes the ballistic ascent look even better.

 

[penlu]

Erm. I've done the 1st option, rocket takeoff, but I find you're too heavy to accelerate much with full fuel. Started with 70 percent, ended with 45 or so.

 

[Andy44]

I tried Joss's method of subsonic climb, but it didn't seem to use any less fuel.