Oberth vs Apoapsis burn to LKO

[tavert]

Pad mass is common over in challenges (otherwise how do you count solids on an even footing?). Cost, eventually. And with the discrete nature of components here, you have to set a specific payload to have a well-defined optimal design. Combinations of engines, fuel, and staging that work best to get a 10 ton payload into orbit won't necessarily be the same as for a 20 ton payload. And you have to consider whether your payload has any engines of its own, if you can use those during the ascent, etc.

Edited April 22, 2013 by tavert

[K^2]

Fuel and launch-pad mass are optimized by the same function. Slight variation for TWR of individual engines, but if you are using staging at all, that difference will be very much washed out by performance of lower stages. Definition of a payload is only relevant to a specific challenge, not to the optimization. And individual parts just determine how close you can get. So in the end, there is still just one good solution, dependent only on whether it's an SSTO or staged.

[tavert]

Fuel and launch-pad mass are optimized by the same function.

Wrong. One ignores engine mass completely, and can't properly count solids with the same single number. When you optimize for total mass including engines, you'll find the best choice of engines/staging/TWR varies depending on what the challenge is trying to accomplish.

If you strictly optimize for fuel, then you just end up maximizing ISP using aerospikes and LV-N's. Minimum mass gives more interesting combinations of solids and varied liquid engines, and TWR is far from constant in those designs.

... and we're way off topic. Whoops.

[EndOfTheEarth]

like this?

http://www.youtube.com/watch?v=VV4pP2_94Io

Yeah, that's the kind of technique that I'm doing currently, with the nose pointed below the horizon by the end of the burn. I'm gathering that it's less efficient?

[K^2]

Wrong. One ignores engine mass completely, and can't properly count solids with the same single number. When you optimize for total mass including engines, you'll find the best choice of engines/staging/TWR varies depending on what the challenge is trying to accomplish.

If you strictly optimize for fuel, then you just end up maximizing ISP using aerospikes and LV-N's. Minimum mass gives more interesting combinations of solids and varied liquid engines, and TWR is far from constant in those designs.

You are thinking of optimizing delta-V for fuel. That's not the same thing as optimizing fuel use for LKO launch. I'm actually integrating over the entire trajectory using a numeric solver and all of the physics that goes into KSP. Engine mass affects how the ship performs in gravity and therefore contributes to the fuel-optimized result.

Fact that I'm getting identical results for fuel consumption, height, and speed in game as I do in my computation kind of proves that I am looking after all the factors correctly. And the optimal operational range for first stage of a staged ship ends up being pretty specific regardless of design details.

[Eric S]

Yeah, that's the kind of technique that I'm doing currently, with the nose pointed below the horizon by the end of the burn. I'm gathering that it's less efficient?

It can be, but that doesn't necessarily mean that it is. If you're having to burn below the horizon by the end of the burn, you might want to get more aggressive with your "gravity turn." An earlier/faster gravity turn will impart more of your thrust into orbital velocity and less of it into vertical velocity so that you're still following the prograde marker when you reach apoapsis. With the right gravity turn, you could be hitting your desired orbital velocity just as you hit the desired apoapsis. Disclaimer: I'm not claiming that this "right" gravity turn is the most efficient, just saying that it's right for what's trying to be achieved here. On the other hand, if it's not the most efficient, it's probably not too far off.

You'll still want to be mostly vertical for the first 10Km or so, I usually start with about 2 degree turn early on so that I'm not dropping boosters on the launch platform, follow the prograde to about 10K, and then start pushing it over, harder at first, but then backing off so that I'm aimed 2-5 degrees below the prograde. Given that you want to do a continuous burn, you may want to do a bit more of a turn right away.

[Nao]

Fuel and launch-pad mass are optimized by the same function.

I would side with Tavert on this. Due to engines having finite TWR and non zero mass, the rocket with optimized launch mass will burn more fuel than the one optimized for fuel burned during the same mission.

From what i tested and flown even with semi-dense asparagus staging of about 15-30s between stages the difference in weight between the two optimization methods can be more than 5% (either in launch mass of fuel spent).

And more on topic, nowdays I always burn downwards a little 5deg usualy, 10deg max, near the end of ascent (when Ap goes above 50k) although i start my gravity run as low as 6000-8000m and keep not more than 5-10deg deviation from velocity vector.

If you're burning continuously to get to LKO, particularly if you're burning below the horizon, then you're expending extra fuel. Of course there will be a balance where this approach beats a too steep launch, but if you're burning downward at any point in the launch, then you're canceling out delta-v that you spent in order to get up to that altitude in the first place. With a clean gravity turn you should be able to hit a reasonable LKO apoapsis at 90% or more of the velocity you need in order to circularize at that altitude, and most of the time I make it into space with a periapsis. If that's not the case, then you should try making your turn earlier or more abruptly (which will also reduce your gravity drag.)

Again, the exact balance between the two approaches depends entirely on the specifics of the launch, but generally speaking it's better to coast along a fairly flat trajectory rather than burning up too much early and then burning down to correct it later.

I agree 100% with Jason, but just for the sake of completeness i would add that the idea of burning downwards during ascent isn't bad in itself but it's effect on ascent efficiency is quite small, and works only for certain ships.

The problem here is that the horizontal burn is best done at quite small altitude range: high enough to not get too much aero drag and low enough to get the most of the Oberth effect.

Since you want to ascent to that altitude as fast as possible (at terminal velocity) to minimize gravity drag, after getting to the horizontal burn altitude you carry too much vertical speed thus lowering the burn time needed to get Apopsis to LKO altitude, and increasing circularization burn.

If at this point you burn downwards a little you can stay longer in best altitude range getting more out of the Oberth effect. (even up to the point of seeing Pe above surface while still inside atmosphere). But the effect on final Dv to orbit is quite small thou.

This effect is even more noticeable when you do a gravity turn at 12km but you can save much more fuel by just turning earlier and flying closer to the velocity vector.

Edited April 22, 2013 by Nao

[Alistone]

I created a mainsail asparagus vessel designed to get a full orange tank plus an additional 3 man "tug" into to kerbin orbit. Launchpad weight of about 600 tons.

It took a lot of trial and error before I could successfully get it into orbit; but I can now routinely lift 70 tons to 100km LKO.

It's starting weight of 600 tons is launched with 7 Mainsail Engines; giving a thrust to weight ratio of just about 1.75. This was necessary becuase the massive vessel would succumb to "physicks" forces and break apart if it were experiencing more than about 2.5 times the force of gravity.

My experience has been to turn slightly at about 12km (maybe 10 degrees) then above 30km turning to about halfway between the orbital yellow circle and the horizon. Burning in a direction other than the horizon wastes thrust due to gravity losses. Burning in a direction other than the yellow circle wastes thrust due to steering losses. The minimum loss (in neglibible atmospheric resistance) is half way in between.

Optimally you could wait until apoapsis (where your yellow direction is on the horizon) and suffer "no" loss. But due to my low T/W ratio I would not be able to reach orbital velocity before I crashed back into Kerbin if I waited until apoapsis.

Also; little "pro-tip" I discovered on my final linear stage. The Mainsail kept thrusting up through the rest of the ship blowing everything up. But if I turn off the fuel tank directly above the mainsail then that tank remains very heavy and absorbs more of the force from the engine. By burning fuel from higher in the column it reduces the stresses on the joints as the top becomes lighter. You just have to make sure you remember to transfer the fuel back to the top before you jettison the mainsail & tank from the bottom.

[tavert]

That's not the same thing as optimizing fuel use for LKO launch. I'm actually integrating over the entire trajectory using a numeric solver and all of the physics that goes into KSP.

That's the right way to optimize the trajectory and throttle profile for a fixed design. I was thinking of writing something similar myself, I'd be curious to see your code and suggest better optimization solvers. There's a nonlinear interior-point solver I happen to be very familiar with that's extremely good. But if you're trying to optimize the design to achieve a specific task, there are far more variables and constraints and the solutions depend significantly on the choice of objective function. There are some now-gone challenges that demonstrated this, maybe partly salvageable from Google cache - the Smallest Eve Ascent Vehicle was a bit unique due to being Eve instead of Kerbin, but scoring by mass showed some interesting designs. The "best stock liquid fuel rocket efficiency" was scored only on fuel consumption and the designs were quite a bit different than they would have been if scored on mass to achieve the same goal (lifting an orange tank + large RCS).

Optimally you could wait until apoapsis (where your yellow direction is on the horizon) and suffer "no" loss.

Optimal in that you have no steering losses and horizontal thrust is best for reducing future gravity losses, but waiting incurs gravity losses of its own. Due to the Oberth effect you want as much of your thrust to be spent low in the gravity well and at high speed, but you have to balance that out with atmospheric drag at low altitudes as we all know.

Edited April 23, 2013 by tavert