Launch profile stinks

[Ron Devu]

        I have tried several ways of managing launch profiles to attain LEO at about 100km circular.

 

1. Lean over to 20 degrees asap and hold that attitude. Typical result is a massive altitude overshoot, and a suborbital trajectory.

2. Lean over to 20 degrees then follow prograde. This also leads to a massive altitude overshoot and a perigee of 45 km, but the peak velocity is better.

3. Lean over to 20 degrees, follow prograde until apogee is 100 km, then fly horizontally. This results in flying horizontally while still in the atmosphere and the drag is pretty ferocious, and the Terrier spends a lot of time burning in suboptimal conditions.

 

There has to be a better way, anyone know it?

The rocket used was a basic capsule, a T400 of fuel and a Terrier, boosted by 3 T400 cans on a Swivel.

[Vanamonde]

If you're overshooting the desired altitude, you are not shutting the engine down soon enough. 

Here's a guide I made a while back. I haven't updated it lately but it should still be viable. 

 

[radonek]

I don't think there is a practical way to get up to circular orbit in single burn. My usual way up:

1. Straight up to get decent speed (depends on TWR)
2. Lean over slowly to full horizontal at ~60km altitude.
3. Burn until slightly under target altitude.
4. MECO, coast to AP minus ~30s.
5. Circularize.

[Kryxal]

The usual goal is to be somewhere about 45 degrees to the horizon when passing 10 km, or maybe a bit higher.  That sort of depends on your TWR, though ... I can't judge your actual TWR and angle from what you said above, but I'd never succeed with the profile you seem to describe.

[FloppyRocket]

The details for getting an efficient launch (how much to tilt after launch at at what altitude to start the tilt) depends on each specific rocket. There is no single formula. You rather have to iterate by "revert to launch" to figure out what works for a specific rocket and payload.

To make it simple and only adjust one launch parameter, you could try only varying the altitude where you start to tilt over. It might be anywhere between 50 and 1000 meters. It depends on the rocket.

• Once you start to tilt, always set it to 5 degrees, wait for the rocket to stabilize there, then switch to SAS Prograde.
• Cut off the throttle when you get the Ap you want.
• Set a maneuver node at the Ap to circularize.
• Re-start the engine at 1/2 the burn time before the Ap.

A useful checkpoint is the altitude when the pitch reaches 45 degrees. Something around 5K to 10K is reasonable, depending on the rocket.

• If the rocket gets too high too early, revert to launch and pitch over at a lower altitude.
• If the rocket generates too much flaming heat and never rises to orbital altitude, pitch over at a higher altitude.

 

Edited April 9, 2019 by FloppyRocket

[Dropbear67]

if you want to do a "hands off" gravity turn, then you need an idea of what your initial TWR ratio is .. the higher the TWR, then the earlier you need to tilt to the east...  There's no "right" answer, but for "high TWR" launches I usually kick over at around 30-40 m^s. Once you've got your lean to about 10 degrees, hit the prograde SAS button and ride it hands off to orbit... You can play with the throttle to "help" adjust the rate of the lean, but that only has marginal effect usually.. reduce throttle to lean more, increase throttle to lean less.

For lower TWR launches (which I try and avoid) you can wait until around 100m^s to start the lean.

If your TWR is too low and you lean too early then your gravity turn will be too steep and you will spend too much fuel trying to punch horizontal through the atmosphere.

If your TWR is too high and you lean too late then your ship will want to "stand up" vertically and you end up reaching your desired apoapsis with very little horizontal velocity, making your circularisation burn huge.

As others have said, aim to pass the 45 degree marker somewhere between 10K and 15K altitude and aim to be above 1400 m^s  or so when you do the "coast" when you kill your burn.

(I'm by no means a KSP expert, but I've found this works for me and is reliable).

[Aegolius13]

There's nothing wrong with launching into a suborbital trajectory.  Due to the laws of orbital mechanics, it is IMPOSSIBLE to get into orbit without doing some portion of your burn in space.  In rare cases, you can get there with one continuous burn, but to do so would require a very specific launch profile and TWR curve over the course of your ascent.  It's much more common to do a big initial burn to get into the suborbital trajectory, cut off the engines, and then do a circularization burn at apoapsis.  

Given that, your second option (perigee of 45km) seems like it's on the right track, though you could cut off sooner to avoid the apo overshoot.  I often end up with a substantially lower (or even  negative) periapsis at the end of my initial burn.

One rough metric I like to use -- how much delta-v is required for the circulation burn?  If it's like 1,000 m/s or something, yeah, the profile probably needs work.  But if it's a hundred or two, you're likely in pretty good shape. 

 

[Superfluous J]

I personally keep my twr around 1.3-1.6 on launch, and completely ignore altitude and instead watch my apoapsis. I like to be at 40 degrees when it's at 20km, and horizontal when it's at 50km. I stop burning - frequently with a periapsis above the ground - when it's at my desired altitude of (usually) 100km.

And of course I never sharply turn. I try to slowly and smoothly turn the whole way.

[Ron Devu]

Wow so many answers, which one is best? This is a gold mine of ideas, thanks guys. I see there are certain things everyone is doing in a similar way, I'll have to adopt those for sure.

[FleshJeb]

In addition to the other advice, regarding your #3: There is almost no drag and no heating above 36km, burn as flat as you like. Likewise, all vacuum-optimized engines are pretty efficient at 7km, and very efficient by 10km, so that's not an issue either.

For efficiency, try to keep your circularization burn under 300m/s. Personally, I aim for under 100m/s.

The Scott Manley Rule: If you can easily keep track of your time to apoapsis, try to hold it to 1 minute when you're above 20km or so. I prefer 45-50 seconds, but 1 minute is a good, safe benchmark.

 

[KerikBalm]

There is a reason the shuttle had an OMS:

https://en.wikipedia.org/wiki/Space_Shuttle_Orbital_Maneuvering_System

Quote

the system allowed the orbiter to perform various orbital maneuvers according to requirements of each mission profile: orbital injection after main engine cutoff, orbital corrections during flight, and the final deorbit burn for reentry.^[2] The OMS consists of two pods mounted on the orbiter's aft fuselage, on either side of the vertical stabilizer.^[2] Each pod contains a single AJ10-190 engine,^[3] based on the Apollo Service Module's Service Propulsion System engine,^{[citation needed]} which produces 26.7 kilonewtons (6,000 lb_f) of thrust with a specific impulse (I_sp) of 316 seconds.

The space shuttle SSMEs didn't put the shuttle into a circular orbit, they got it close, but the shuttle OMS needed to fire at apoapsis to raise the PE and complete orbital insertion.

So... cut your engines when you get your Ap high enough, and then fire engines at Ap... that or very deep throttling of your engines to do it all in 1 burn