The Quest For The Holy Trajectory

[LN400]

I am experimenting with various trajectories on different rocket designs, trying to find the¹ launch trajectory that will save me the most in fuel. I let kOS handle the throttle for a consistent TWR during the first part as well as the initial rotation for the g-turn. Target orbit is due east inclination 0 +/- 0.01*, apo between 71000m and 72000m, peri no more than 500 m below apo.

So far, the best result for a selection of single engine 2 stage designs have been:

TWR from launch until 30000m: 1.50

Rotation angle: 3*

Altitude when g-turn hits 45* above horizon: 30000m

Time to apo during ascent: 45-50s (not enough tests to narrow it down) reached no later than 24000m, no sooner than 20000m

Altitude for attitude 0* above horizon: 60,000m-61.000m

Time to apo before the final circularisation burn: No more than 2 minutes, no less than 1 minute.

 

Now: Any suggestions on how to improve the trajectory? What are you looking for in a good launch trajectory?

 

 

¹ if such a thing exists.

Edited August 17, 2016 by LN400
corrected error

[Signo]

I try to look for the speed related to pitch instead of the altitude.

I.E. 300 m/s @ 67° pitch followed by 700 m/s @ 45° pitch.

 

[LN400]

4 minutes ago, Signo said:

I try to look for the speed related to pitch instead of the altitude.

I.E. 300 m/s @ 67° pitch followed by 700 m/s @ 45° pitch.

 

I take it that would be surface speed, correct?

[Signo]

Yes, correct. 

It seems to me that horizontal speed rules when you need to get to orbit. (due to my experience with winged crafts, you can attain orbit with almost 0° pitch)

[LN400]

30 minutes ago, Signo said:

Yes, correct. 

It seems to me that horizontal speed rules when you need to get to orbit. (due to my experience with winged crafts, you can attain orbit with almost 0° pitch)

What about losses due to traveling through dense atmosphere for a longer distance? I am curious about the sweet spot between that distance and speed to clear that area asap.

[Signo]

Well, this may happen with planes due to their lower TWR and the need to maximize "open cycle" speed.

Following your "altitude rule of thumbs" or my speed based one, if you execute properly you will find you are well above the red zone when going at 0°. 

Rockets flying between 23000m and 50000m do not suffer so much from drag. Above 50000m I am generally already coasting to the Ap (I tend to have overpowered second stages I try to use as transfer stages too - I like to use "powerburns" for my interplanetary transfers).

[Spricigo]

3 hours ago, LN400 said:

What about losses due to traveling through dense atmosphere for a longer distance? I am curious about the sweet spot between that distance and speed to clear that area asap.

An idea here it's to have less thrust and more vertical trajectory while in dense atmosphere and as air density decrease push and turn harder. 

This seems strange since it may result in longed time and increases gravity losses  but the idea it's to find the sweet spot between too much gravity and too much drag losses. Since drag will increase with velocity and decrease with height going fast and low it's what we want to avoid.

[Laie]

8 hours ago, LN400 said:

What about losses due to traveling through dense atmosphere for a longer distance? I am curious about the sweet spot between that distance and speed to clear that area asap.

Last time I tried this, I found that you gain a lot by "circularising" at 20-30km. Air isn't all that dense anymore, though heat still is an issue.

The most dV-efficient launchers I ever used had starting TWRs >2.5 and were placed on the pad at like a 60° angle -- no way to perform a gravity turn fast enough, at least not from the vertical. I had to launch sideways.

For best results, you do most of the work at 20-30km of altitude, but see to it that you're still going definitely upward, all the time. The apoapsis shouldn't be on the far side of Kerbin, but only 1/4 to 1/3rd rotation away. Accelerating to insane speeds in the atmosphere is still a net gain, but an extended coast will devour it all.

The basic principle holds up in RSS/RO as well: gravity losses tend to be worse than drag, worse even than steering losses. You want to gain a lot of sideways velocity pretty early, even if you later have to compensate by pitching upwards. 10 degrees above prograde is *cheap*.

[Snark]

On 8/17/2016 at 2:39 AM, LN400 said:

Altitude when g-turn hits 45* above horizon: 30000m

30 km altitude for 45 degrees seems really high-- I'd say you need a much more aggressive turn, something more like 10 km would be more appropriate.

Lots of good discussion on the topic has been going on here:

...in particular, @Norcalplanner has done some interesting analytical comparisons of the benefits of various trajectories, which seems perfectly in line with what you're asking here.

[LN400]

1 hour ago, Snark said:

30 km altitude for 45 degrees seems really high-- I'd say you need a much more aggressive turn, something more like 10 km would be more appropriate.

Lots of good discussion on the topic has been going on here:

...in particular, @Norcalplanner has done some interesting analytical comparisons of the benefits of various trajectories, which seems perfectly in line with what you're asking here.

I have been following that, and reading the links posted there for a couple of days now and it does indeed appear to be correct, albeit a bit counter intuitive that a shallower trajectory going "too fast" will actually save dv.

Last test I did was, using kOS to handle the rotation, leaving the throttle at full, starting TWR 1.65, end twr over 3.75_{changed from 3+}

rotation angle 3.5 degrees starting when the surface speed reached 20 m/s and held until the speed reached 45 m/s where the steering unlocked and aerodynamics took over.

Remaining dv in orbit was 415 m/s. Now that doesn't mean much in itself but the previous test with locked twr and steeper climb had me in orbit with 195-205 remaining dv.

I am still going to see if there is a way to get more savings by limiting the thrust somewhat. The engines were kicking but the acceleration didn't match in the lower atmosphere (up to around 25000 m). After that, the acceleration went nuts and I ended up with an apo too high 8 minutes ahead and still I ended up with more than twice the amount of dv remaining.

Edited August 18, 2016 by LN400
Fixed error

[Norcalplanner]

I think that it's time to do some more testing in stock to get some non-RSS info on aero vs gravity losses. I'll put it in a new thread as soon as it's done, either later today or tomorrow.

[LN400]

One issue I can't seem to sort out is that with max thrust at close to 4, the apo is several minutes ahead at altitudes as low as 25000m whether the prograde is at 45* or 15* above horizon. More testing needed here indeed.

[Norcalplanner]

35 minutes ago, LN400 said:

One issue I can't seem to sort out is that with max thrust at close to 4, the apo is several minutes ahead at altitudes as low as 25000m whether the prograde is at 45* or 15* above horizon. More testing needed here indeed.

What does your rocket look like? Could you describe it or post a picture?

[LN400]

1 hour ago, Norcalplanner said:

What does your rocket look like? Could you describe it or post a picture?

from top to bottom:

Mk 16 parachute

M-315 Probe Core (mod: kOS)

Kerbal Engineering System (mod: KER, flat and centered on the nose of the pod)

Mk I Pod (no monopropellant) + 2x OX-STAT solar panels in symmetry.

TR-18A Decoupler

Procedural Liquid Tank (mod: procedural parts, 1.25m x 0.75 m)

0.625m "Sparkler" engine (mod: Modular Rocket System)

Interstage Fairing Adapter (mod: procedural fairings, 2 nodes) + fuselage fairing

SA-4 Fuel Tank (mod: KW Rocketry)

4x Basic Fin

LV-T30 Engine

Edited August 18, 2016 by LN400
adding component and pic

[Norcalplanner]

2 hours ago, LN400 said:

from top to bottom:

Mk 16 parachute

M-315 Probe Core (mod: kOS)

Kerbal Engineering System (mod: KER, flat and centered on the nose of the pod)

Mk I Pod (no monopropellant) + 2x OX-STAT solar panels in symmetry.

TR-18A Decoupler

Procedural Liquid Tank (mod: procedural parts, 1.25m x 0.75 m)

0.625m "Sparkler" engine (mod: Modular Rocket System)

Interstage Fairing Adapter (mod: procedural fairings, 2 nodes) + fuselage fairing

SA-4 Fuel Tank (mod: KW Rocketry)

4x Basic Fin

LV-T30 Engine

Interesting - very little control authority and a high TWR. Probably makes it challenging to pilot. What's the initial atmospheric TWR?

Off the top of my head, my inclination would be to put in a Swivel instead of the Reliant, ditch the fins, and increase the first stage fuel tankage by 25 or 50 percent.

And is there a reason you're focusing on fuel used? Many people are more concerned about total cost, payload mass fraction, or cost per ton to orbit.

[LN400]

14 minutes ago, Norcalplanner said:

Interesting - very little control authority and a high TWR. Probably makes it challenging to pilot. What's the initial atmospheric TWR?

Off the top of my head, my inclination would be to put in a Swivel instead of the Reliant, ditch the fins, and increase the first stage fuel tankage by 25 or 50 percent.

And is there a reason you're focusing on fuel used? Many people are more concerned about total cost, payload mass fraction, or cost per ton to orbit.

Initial TWR at the moment is 1.61(3.55) atm, 1.73(3.80) vac.

As for control authority, it is surprisingly easy to control as long as you don't allow the nose to wander off the prograde marker more than 1-2 degrees. I assume the aerodynamics and the reaction wheel together get the job done. Nose wander off more than 2 degrees and it gets hairy, 3 degrees and you're done for but keeping it on target is really no issue. I chose the T30 for the lower mass/greater thrust and since it's this easy to keep straight, I am reluctant to replace the engine on this particular design for one heavier with less thrust.

I wonder, why ditch the fins? Drag or mass?

As for the upper stage and increasing the capacity: You mean do that and reduce the capacity of the first tank?

 

Reason I look at fuel and dv is I have a curious mind  I am looking for a way to script the launch for kOS but I want to have a more thorough picture of what is going on. Even if it turns out to be less important I still would like to know. Beside, if I can get away with less fuel, then that means smaller tank and less mass which in turn can be traded for either more dv or a cheaper design. At least that's what I figure.

 

EDIT: I must also mention that the initlai rotation happens at so low speed (20 m/s) the reaction wheel has no problem holding it there for the few seconds it needs to stabilise.

Edited August 18, 2016 by LN400

[Norcalplanner]

10 minutes ago, LN400 said:

Initial TWR at the moment is 1.61(3.55) atm, 1.73(3.80) vac.

As for control authority, it is surprisingly easy to control as long as you don't allow the nose to wander off the prograde marker more than 1-2 degrees. I assume the aerodynamics and the reaction wheel together get the job done. Nose wander off more than 2 degrees and it gets hairy, 3 degrees and you're done for but keeping it on target is really no issue. I chose the T30 for the lower mass/greater thrust and since it's this easy to keep straight, I am reluctant to replace the engine on this particular design for one heavier with less thrust.

I wonder, why ditch the fins? Drag or mass?

As for the upper stage and increasing the capacity: You mean do that and reduce the capacity of the first tank?

 

Reason I look at fuel and dv is I have a curious mind  I am looking for a way to script the launch for kOS but I want to have a more thorough picture of what is going on. Even if it turns out to be less important I still would like to know. Beside, if I can get away with less fuel, then that means smaller tank and less mass which in turn can be traded for either more dv or a cheaper design. At least that's what I figure.

I'll have to play with this when I get home tonight.

So if I'm hearing you right, things get unstable starting around 3 degrees off prograde. At what speed? Is it possible to crank the rocket over 5 or 6 degrees starting at 10 m/s without losing control, and then lock to prograde after it recovers from the turn?

It will probably be a few hours before I can give this my full attention, but it's certainly interesting.

And to answer your other questions, I'd remove the fins to save mass, drag, and cost. And I would increase fuel in the bottom stage for the Reliant, not the upper stage - sorry if that wasn't clear.

Edited August 18, 2016 by Norcalplanner
Answered questions

[LN400]

5 minutes ago, Norcalplanner said:

I'll have to play with this when I get home tonight.

So if I'm hearing you right, things get unstable starting around 3 degrees off prograde. At what speed? Is it possible to crank the rocket over 5 or 6 degrees starting at 10 m/s without losing control, and then lock to prograde after it recovers from the turn?

It will probably be a few hours before I can give this my full attention, but it's certainly interesting.

Sorry, I should have mentioned. Speed gets real high at around 10000m (past 300 m/s) and around 500 at 14000. That's when it gets real crucial to be on the prograde but again, just the tiniest, quickest taps on the steering keys and it's sorted. The kOS script also auto engage SAS at 20,000m just before decoupling the lifter and from there on out steering is no issue at all. Temperature is though, KER showing close to 87-90% to critical temp at around 25000m and up to around 45000-50000.

 

I tested 3.5 degrees but at 20m/s and only around 1.63 TWR, the rocket is about 10* above horizon at 20,000 m and first stage doesn't last more than up to around 18000.

Edited August 18, 2016 by LN400

[Norcalplanner]

4 minutes ago, LN400 said:

Sorry, I should have mentioned. Speed gets real high at around 10000m (past 300 m/s) and around 500 at 14000. That's when it gets real crucial to be on the prograde but again, just the tiniest, quickest taps on the steering keys and it's sorted. The kOS script also auto engage SAS at 20,000m just before decoupling the lifter and from there on out steering is no issue at all. Temperature is though, KER showing close to 87-90% to critical temp at around 25000m and up to around 45000-50000.

Those speeds actually seem low to me. I'm frequently at 700 m/s at 10 km.

Just to confirm, you're running 1.1.3?

[LN400]

5 minutes ago, Norcalplanner said:

Just to confirm, you're running 1.1.3?

That's affirmative.

So, low speed? Hm I need to have a good look at this. Would very much be interested in any findings of yours.

 

EDIT: In case you use kOS and want the same script as I use

set rot to -2.75.
set rot_vel to 15.
set rot_vel_end to 35.
set run_prg to true.

declare steer to up + r(0,0,180).
lock steering to steer.

SAS OFF.

when airspeed > rot_vel then
{
	set steer to up + r(0,rot,180).
	when airspeed > rot_vel_end then
	{
		unlock steering.
		set run_prg to false.
	}
}

wait until run_prg = false.

wait until altitude > 20000.

SAS ON.

I let it run and only tap the keys to keep the nose on target. It says speed for initial rotation is 15 but it takes a second or two before things happen so it reaches 20 before it tilts over.

Edited August 18, 2016 by LN400

[Norcalplanner]

I'll be sure to run some with a 1.25m rocket - all of my tests so far have been 2.5m, so that may be part of the discrepancy.

[LN400]

1 hour ago, Norcalplanner said:

And to answer your other questions, I'd remove the fins to save mass, drag, and cost. And I would increase fuel in the bottom stage for the Reliant, not the upper stage - sorry if that wasn't clear.

Sorry, just saw this. As I understand it, having a bit of drag near the bottom together with a streamlined nose helps keeping the rocket on track. I removed the fins for a modified version of the rocket and will see how it behaves, same settings as before but 40kg lighter and no drag at the rear.

 

EDIT: I can confirm that without the fins, the design is a nightmare to control. Fins going back on for 40kg.

Edited August 18, 2016 by LN400

[Norcalplanner]

16 minutes ago, LN400 said:

Sorry, just saw this. As I understand it, having a bit of drag near the bottom together with a streamlined nose helps keeping the rocket on track. I removed the fins for a modified version of the rocket and will see how it behaves, same settings as before but 40kg lighter and no drag at the rear.

 

EDIT: I can confirm that without the fins, the design is a nightmare to control. Fins going back on for 40kg.

Fins should only come off if the Reliant is replaced with a Swivel. I can only imagine how the Reliant was without them...

[LN400]

36 minutes ago, Norcalplanner said:

Fins should only come off if the Reliant is replaced with a Swivel. I can only imagine how the Reliant was without them...

I hear you, loud and clear

 

EDIT:

Set initial TWR to 1.50 atm (from 1.61), and adjusted the script to rotate 1* (set rot to -1, old value -2.75) and got remaining dv 425 m/s which is a jump up from the previous 375-ish from the previous script. Drag seems to be a factor to consider. One launch only but the jump was noticable.

Edited August 18, 2016 by LN400

[Norcalplanner]

25+ runs complete, including some with 1.25m rockets.  Spent more time varying initial velocity for the gravity turn this time.

Initial impressions:

No big surprises with the 2.5m rocket.  Very possible to get to orbit for less than 3,100 m/s with an initial TWR of 1.57.  Not worth cranking it over more than 10 degrees under any reasonable scenario.

1.25m rockets are a whole other kettle of fish.  Really difficult to get below 3,300 m/s to get to orbit with a serially-staged rocket.  A bit easier using 0.625m SRBs from SpaceY, making a smaller version of the 2.5m rocket configuration.  One thing that does appear to make a difference- you can save a bit of delta V by moving those two solar panels off of the capsule and on to the top fuel tank.  Less likely to explode this way too.

It's definitely going to be tomorrow before I put together an imgur album and tally the numbers.