Flight Question Good launch trajectory to reach orbit

[SpaceShipOne]

Hi,
I am trying to reach orbit with spaceships like CEV-Orion or SpaceX Dragons but its much more harder than with the DG IV. I tried a few searches on this site to find a way to make a good launch trajectory to succesfully reach orbit (and still have fuel) but I did not found what I need. Can someone help? I have an heading of 90 degres.

Thx

 

[Urwumpe]

You can't get a simple trajectory to follow, because this depends a lot on the abilities of your rocket.

Practically speaking, you want to achieve the following:

• Have cutoff as soon as possible
• Have as little vibrations and accelerations as possible
• Pitch down as soon as possible and burn horizontal most of the time
• Avoid plane changes
• Avoid denser parts of the atmosphere
• Have only little thrust deflected for controlling your attitude.

For the Ariane 5 or the Space Shuttle, this can for example mean that you fly a strong S-shaped trajectory, leaving the gravity losses at the ineffective booster rockets and let the hydrogen powered core stage/SSMEs do most of the horizontal velocity change.

if you only do slow and smooth maneuvers, you need only little fuel and thrust for attitude changes - so you prefer this.

There is a mathematical formula for this, the ascent equation:

Total dV = Orbit Velocity + Gravity Losses + Aerodynamic Losses + Control Losses + Plane Changes - Earths Rotation at launch site

Note that all factors interact - if you minimize gravity losses by flying as shallow as possible, you get more control and aerodynamic losses.

For LEO, the Total dV is BTW as conservative upper-limit estimate 9200 m/s. What your rocket has more, is reserve for playing around.

 

[N_Molson]

There's a very basic thumb rule that works quite well :

00-10 km : pitch 90 -> 80
10-20 km : pitch 80 -> 70
...
80-90 km : pitch 10 -> 5

After that, try to get an Apoapsis over 120 km. When you reach that altitude, wait for the Vertical Velocity to reach 0, then pitch up to keep Vertical Velocity & Vertical Acceleration near 0.

Usually, that works, depending of your rocket/spacecraft, of course.

Also keep in mind that when the (empty) Stage1 shuts down, you have to anticipate the fact that the (full) Stage2, at ignition, will provide less accelaration, so bend up your trajectory in anticipation. Same applies for Stage3 if any.

Hint : try to launch from Kourou, it's near the equator and you'll gain precious DeltaV.

 

[kwan3217]

For a realistic rocket,try this: Use the surface MFD. Take off vertically of course, then 10 seconds after launch start pitching down slowly.You should get to about 45deg when you pass 15km, and about 30deg when you drop the first stage. From here, also watch apogee on the Orbit MFD. I like to switch the distance units to get altitude rather than radius.

During the second stage, you are doing a balancing act. Think about the up and horizontal components of your thrust. You need enough up to lift your apogee to the target, eventually, just such that your vertical speed reaches zero when you hit circular speed at apogee and cut off. So watch the vertical speed and acceleration on the surface MFD also. Vertical acceleration will probably be negative, but not too negative. You should know how long it takes the real rocket to reach orbit. For instance, the Shuttle is about 8 and 1/2 minutes, Falcon about 7, and a whopping 12 for the Saturn V. So let's say you have 200 seconds of powered flight left. Your vertical speed is 500m/s up, and your vertical acceleration is 5m/s down. This says you will reach apogee (zero vertical speed) in about 100 seconds. Pitch to change the vertical acceleration to 2.5m/s and you will reach apogee in 200 seconds. Keep in mind that as you pick up horizontal speed, you will gain more and more "centrifugal force" and your vertical acceleration will change. You will need to keep pitching down to keep vacc where you want it.

After enough practice, you will get used to how much pitch up to use during the second stage. Don't worry if you feel like you have to pitch up a lot to keep your vertical speed up. Pointing up at 30deg gains you 50% of your full thrust in the vertical direction, but only loses 13% of your thrust in the horizontal direction.

So, how do you get to orbit? The same way you get to Carnegie Hall: Practice!

Or, you can use the PEG MFD on Orbit Hangar.

 

[SpaceShipOne]

Thanks for all the hints!:tiphat: