Efficiency . . . I am Lacking

[closette]

Fore the 2D ascent problem, I think doing a 'backward' calculation would be the best approach to it (starting from orbit burning in a retrograde direction with increasing fuel quantity).

That might work for a Munar ascent, but for Kerbin you have drag breaking the symmetry as well as fuel use, so the ascent to orbit guidance is a unique problem on its own. And staging might be involved. For those reasons I\'ve been trying to find a workable SSTO optimization for a Munar ascent. Using a 'shooting' forward-propagation method with a linear tangent steering law and full thrust to see where one ends up after a set time with a given starting pitch and pitchover rate seems to be one way to go.

That would generate a table of final orbital states given those initial conditions, and one could pick the closest desired. I\'m admittedly a bit out of my depth on the literature though...

[Ivan Ivanovich]

If I\'m not mistaken your table calls for a TWR of about 2, maybe a bit lower at the start (provided some dead weight can be jettisoned during ascent). A TWR of 2 (or close to it) is pretty hard to achieve with a Mun-capable rocket. My Mun rocket has an initial TWR of 1.2:1, actually.

My Mun return craft weighs 7.6 ton, which is dead weight during ascent. Given that I have 6 + 1 engines thrusting initially, giving me 1375 thrust, for a TWR of 2:1 my ascent stage may only weigh about 61 ton, which is basically 14 tons for the engines and 18 fuel tanks. That\'s not enough for a Mun shot.

[closette]

Hi Ivan,

Correct that to keep up with terminal speed on ascent, you need at TWR of at least 2 at launch - in practice more like 2.5-3 so that you can get up above 100 m/s quickly and keep accelerating from there - that\'s what I use SRBs for.

I appreciate that with the 'dead weight' of a Mun lander/return vehicle this is difficult to achieve a launch TWR > 2, but it\'s not impossible. Some of the Mun vehicles listed under Darkshadow\'s Stock Repoistory thread have some good examples. and recently I recall that Kosmo-not won a challenge with minimalist Mun return vehicle (which takes a lot of careful flying, but which HAS to be fuel efficient on ascent).

As I mentioned, my craft tend to fall behind the optimal speed vs. altitude curve after around 12000 m, but as long as one isn\'t too far off the optimum values at low altitude, the hit on fuel consumption isn\'t too bad. But my early over-built Mun rockets which struggled upwards at ~60 m/s for the first 10000 m were horrible in terms of fuel efficiency.

It sounds like your Mun return craft is a bit on the massive side, by the way, given that a pod + parachute + 2 RCS tanks can make it all the way back from the Munar surface to Kerbin, so more SRBs at launch would be called for. And perhaps convert 3 of your first stage engines to tricoupler + 3 LFEs to triple their thrust (just make sure you jettison these gas guzzlers early).

Hope this helps...

[Randox]

So reading through this, apparently all my rockets might as well have bar doors attached to them.

My normal rocket for my trips to Mun don\'t orbit at all. Trying to tilt it before the first stage is entirely gone would probably result in a one way ticket back to kerbin (because it gets fairly top heavy as it run out of first stage fuel, and can\'t support its front end). Also, none of my rockets can get anything close to those speeds. I think I hit 2200 with my Mun rocket at like 400,000m.

My orbiter rocket starts its turn over at exactly 35,000m, and hits the horizon between 60,000 and 70,000 aiming to hit a 200,000 orbit.

I\'ve only just started going backwards and making lighter rockets. Mostly I\'ve been taking fuel tanks off the Mun rocket, and the reduced weight has made it easier to control, and its reduced weight roughly compensates it for the lost fuel capacity (it only lost a tank and a half though). My general goal is to have as much extra fuel with me as I can once I leave Kerbin, though I should probably focus more on the TWR.

I sense some math in my future.

[closette]

I doubt if my Mun rockets would do much better if launched straight at the Mun, but here is an excellent video by astrophysicist Scott Manley which demonstrates the advantages of pitching over, even when on an escape path:

In short, using thrust to increase orbital speed, as opposed to altitude, saves you from having to lift a lot of fuel 'uphill' against gravity.Hope this helps.

[Zephram Kerman]

My Munships tend to fall behind the ascent profile around 10-12 km. So at that point I\'m not worried about air friction, and start the gravity turn. Not much, just a few degrees at a time until the air gets thin. I know it sounds too early to most of us, but doing this seems to pay off. The trick is to start the turn slowly at first.

[SiliconPyro]

I have started the slow turn method and it seems to be working rather well. I start my turn just as I\'m leaving the lower atmosphere and turn at a rate of about 10 degrees per 10km. I doubt it\'s perfect, but I\'m making orbit reliably and consistently.

I\'m curious though, is there an ideal altitude over Kerbin to burn toward the Mun? Assuming youu are in a circular orbit with no inclination.

[Randox]

Shouldn\'t matter what your altitude is, as long as you\'ve cleared the atmosphere. The higher you are the easier it will be (shorter burn), but its the same total energy required. You can accelerate early to get a high orbit, or save it for the TLI burn.

[Kosmo-not]

The ideal altitude for a TMI burn, or any burn, is as low as possible. This takes advantage of the Oberth effect. Doing it from higher altitudes means expending more energy.

[Randox]

The ideal altitude for a TMI burn, or any burn, is as low as possible. This takes advantage of the Oberth effect. Doing it from higher altitudes means expending more energy.

Interesting. Just looking at that now. Makes sense now that its been pointed out.

[VincentMcConnell]

I have started the slow turn method and it seems to be working rather well. I start my turn just as I\'m leaving the lower atmosphere and turn at a rate of about 10 degrees per 10km. I doubt it\'s perfect, but I\'m making orbit reliably and consistently.

I\'m curious though, is there an ideal altitude over Kerbin to burn toward the Mun? Assuming youu are in a circular orbit with no inclination.

100KM is what I\'ve always done. Orbit at 100KM and then make TLI when the mun rises over the horizon.

[Foo52]

I can\'t even get my rockets to repeat the same launch path as its predecessor even though its the same rocket, is there any tips to having uniform launches?

[Zephram Kerman]

I can\'t even get my rockets to repeat the same launch path as its predecessor even though its the same rocket, is there any tips to having uniform launches?

Remove and replace the pilot.

Seriously though, all I can think of for consistent performance is to make your rocket stable, and keep your ascent profile consistent.