Transfering to Mun: First to LKO or Directly to Mun?

[arkie87]

So, I've been playing career mode again, and in the early tech tree, its hard to do a gradual (i have FAR installed) gravity turn before you have proper control surfaces and/or reaction wheels-- i either fail to turn enough and end up being flung up high out of the atmosphere or turn too much and spin out and die; every once in a while, ill get it right...

So what i decided is that while doing a proper gravity turn is the most efficient way of getting into LKO, it's not so bad to transfer directly to the Mun by burning vertically upward from KSC (during correct transfer window). Plus, burning vertically upward is relatively easy to control (less risk of spinning out etc)....

AND..... BEST OF ALL....

It allows you to obey the #1 rule of KSP:

MOAR BOOSTERS!!!!!

EDIT: Other advantages of Vertical Ascent vs. LKO-to-Mun are:

(1) Utilization of Oberth effect the entire time (vs. LKO, you waste the initial climbing part when you turn off pro-grade); nevertheless, even theoretically, vertical requires more deltaV than LKO to Mun for this particular case. For other planets/moons, this might not be the case.

(2) With FAR, does not require control surfaces or reaction wheels or expensive thrust-vectoring engines to steer the craft into a gravity turn. Thus, strapping loads of SRB's and going vertical might be cheaper (in terms of Kerbucks) than using a mainsail/skipper and using thrust vectoring to get into LKO first.

(3) Eyeballing is easy-- just aim 90 degrees in front of the Mun or Minmus; or burn at sunrise/sunset if you are going interplanetary at optimum launch window

(4) With FAR, it is possible if you are too aggressive with gravity turn, you craft will spin out of control, thus, ruining the mission.

(5) Optimum ascent path to minimize deltaV is very sensitive and its not necessarily so easy to hit every time going to LKO first vs. vertical ascent is easy to hit optimally every time. GoSlash27 predicted approximately 340 m/s wasted deltaV using vertical ascent method, but its possible to waste that going into LKO first, if you accidentally raise your apoapsis too high, stay too low for too long, turn craft too aggressively, or do something else sub-optimally...

Edited December 14, 2014 by arkie87

[zekes]

I tend to go LKO, that way you can do systems check, and breathe a bit before Munar insertion.

[arkie87]

I tend to go LKO, that way you can do systems check, and breathe a bit before Munar insertion.

I think it's against Kerbalism (Kerbal religion) to do a systems check

[Dizzle]

I usually wait for a direct insertion window, but sometimes I overestimate my craft and see the Mun coming over the horizon at only 1km/s.

[Sanguine]

Direct mun-shot is always more efficient, less chance of failure due to a flawed gravity turn, shorter warp time, less delta-v consumed...

Wait till the mun is about 45 degrees, I believe, from KSC - side of kerbin, and you'll get a nice intercept.

[Luis]

A vertical ascent to space is the least efficient trajectory. You are essentially subtracting gravity from your thrust all the way up.

[Hannu]

Only benefit of direct straight up Mun injection burn is avoiding difficulties in LKO insertion. But it is energetically better to go LKO first. Moreover, on LKO you can use maneuvering nodes and can achieve much more accurate (and therefore fuel saving) burn to correct Pa and latitude over Mun. Even if you make one burn, it is better to make gravity turn and burn most to prograde direction. Some real planet probes have done that. Most are inserted first on LEO and after some checks they make transfer injection burn.

[Dizzle]

A vertical ascent to space is the least efficient trajectory. You are essentially subtracting gravity from your thrust all the way up.

I don't think the thread is about going straight up, but about launching so that by the time you reach space, you're already where you need to be to make your injection burn, so you never have to wait or stop your engines.

[Ippo]

Vertical ascent =/= direct injection.

direct injection is more fuel efficient because you don't use energy to raise your Pe, but it's less forgiving of errors...

[Superfluous J]

Space flight is all about trading fuel for other things. Frequently, we trade it for time but in this case, you are trading more for convenience.

And it's a pretty lopsided trade. Perfect timing, flawless ascent, lack of maneuver node assistance, and annoying multi-minute retry loop (or if you're not the type to revert flights, danger of your Kerbals getting tossed into space) versus a very small amount of extra fuel.

Yeah I'll use the fuel, if its all the same to you

[MalfunctionM1Ke]

This is what come to my mind immdiately

[katateochi]

I tend to go LKO, that way you can do systems check, and breathe a bit before Munar insertion.

I do the same. Plus this way things are more predictable. If going to space is an equation, I find this approach has more constants.

[Wanderfound]

Vertical ascent = bad.

Very flat ascent that leads directly into a Munar transit burn = best.

OTOH, the Mun and Minmus are cheap enough to get to that hyper-efficiency is generally unnecessary. So, I'd normally circularise into a 70x70 orbit before heading Munwards, unless I happen to notice the Mun breaking the horizon just as my apoapsis hits 70km, in which case I may just leave the throttle on and go direct.

The real fuel saving to be made on a Mun trip is in learning how to land efficiently. Come in as flat as you can; try to hold your altitude at a constant small distance (i.e. <2,000m) above the surface while you completely negate your lateral velocity, then burn as little as possible during final descent. Every unit of fuel spent fighting gravity is a unit wasted.

[The_Rocketeer]

A vertical ascent to space is the least efficient trajectory. You are essentially subtracting gravity from your thrust all the way up.

This is an argument I've had a handful of times with a friend of mine. He's always insisted that vertical ascent to Mun was more efficient than LKO > Hohmann transfer, and despite a couple of ding-dong debates about it, I was actually (grudgingly) coming around to his point of view.

I tried his approach in my Saturn V analog. I think it ran out of delta-V at about 8Mm. However, he also claimed that his rocket didn't have the fuel to perform the string of manoeuvres that I tend to use, so I had put this down to a lack of staging optimisation.

I'd be really interested to see the hard maths that refutes his point, partly because I'd like to trump him once and for all, and partly because he's got me doubting my own instincts on this.

Edited December 11, 2014 by The_Rocketeer

[Wanderfound]

This is an argument I've had a handful of times with a friend of mine. He's always insisted that vertical ascent to Mun was more efficient than LKO > Hohmann transfer, and despite a couple of ding-dong debates about it, I was actually (grudgingly) coming around to his point of view.

I'd be really interested to see the hard maths that refutes his point, partly because I'd like to trump him once and for all, and partly because he's got me doubting my own instincts on this.

You're both half-wrong and half-right.

Vertical ascent = massive gravity losses. Near horizontal ascent that leads directly into a transfer burn = minimal gravity losses.

The most efficient way to do it is with a gravity turn that ends almost flat, but pops you into space at about the same time that the Mun comes over the horizon, so that you can go straight into the transfer without a circularisation burn. Maximum Oberth, minimum gravity loss.

[Superfluous J]

This is an argument I've had a handful of times with a friend of mine. He's always insisted that vertical ascent to Mun was more efficient than LKO > Hohmann transfer, and despite a couple of ding-dong debates about it, I was actually (grudgingly) coming around to his point of view.

I tried his approach in my Saturn V analog. I think it ran out of delta-V at about 8Mm. However, he also claimed that his rocket didn't have the fuel to perform the string of manoeuvres that I tend to use, so I had put this down to a lack of staging optimisation.

I'd be really interested to see the hard maths that refutes his point, partly because I'd like to trump him once and for all, and partly because he's got me doubting my own instincts on this.

Forget the maths, send me the craft files and tell me what mods you use and I'll fly them both using both methods (straight up vs orbit first). I'll post it on YouTube. All your friend needs to do to refute me is to get his craft to Mun via direct ascent and have more fuel in his tank than I do after getting to orbit first.

[arkie87]

I don't think the thread is about going straight up, but about launching so that by the time you reach space, you're already where you need to be to make your injection burn, so you never have to wait or stop your engines.

I said "burning vertically upward" so actually that is what i meant

I dont see how Oberth effect has anything to do with this -- you utilize oberth effect by doing gravity turn and by burning vertically upward the whole time since both instances you are burning prograde.

The only difference is how you raise your apoapsis, and i've heard, it's more effiicent to burn sideways than vertically.

This is an argument I've had a handful of times with a friend of mine. He's always insisted that vertical ascent to Mun was more efficient than LKO > Hohmann transfer, and despite a couple of ding-dong debates about it, I was actually (grudgingly) coming around to his point of view.

I tried his approach in my Saturn V analog. I think it ran out of delta-V at about 8Mm. However, he also claimed that his rocket didn't have the fuel to perform the string of manoeuvres that I tend to use, so I had put this down to a lack of staging optimisation.

I'd be really interested to see the hard maths that refutes his point, partly because I'd like to trump him once and for all, and partly because he's got me doubting my own instincts on this.

I am running some numbers now and i'll get back to you.

[numerobis]

Off the top of my head I remember it being about 2km/s difference, plus there's the additional benefit that your TWR doesn't need to be as high to get all the benefit of burning low in the gravity well -- so you don't need as much engine mass. I think it's from zarakon's challenge on hitting the moon as fast as possible? Or maybe I'm conflating two challenges.

[arkie87]

I am running some numbers now and i'll get back to you.

Assuming Kerbin has no atmosphere and is perfectly spherical so LKO is at 0 m altitude and zero rotation:

Burning vertically upward to Mun's altitude takes a total of 3348 m/s. If you include circularization burn once you get to to altitude, it takes an additional 529.4 m/s, totaling 3878 m/s.

Burning horizontally upward to Mun's altitude from surface of Kerbin takes 3352 m/s (4 m/s more!). If you include the circularization burn one you get to altitude, it takes an additional 369.8 m/s, totaling 3722 m/s.

Thus, horizontal beets vertical by 155.8 m/s, but only if we assume a transfer from 0 m altitude. Since even horizontal approach must burn vertically upward for a bit (I'd venture to say way more than 155.8 m/s), I'd say vertical ascent method is probably more fuel efficient...

Edited December 11, 2014 by arkie87

[numerobis]

I get roughly the same numbers as you for a spacecraft that does an impulse at 0m altitude on Kerbin to reach orbital velocity, and simultaneously another impulse to do a Hohmann to 11.4e6 m altitude (Mun's orbit), followed by the second burn up there. It's not accurate for a lunar orbit injection, since that throws away Oberth effects on arrival along with possibilities for gravity assists, but it's close enough.

How do you get the all-vertical numbers?

Oh, and I *was* conflating two challenges: zarakon's was 15 minutes, get as far away as possible using at most 50k units of fuel, another was hit the Mun as fast as possible.

Edited December 11, 2014 by numerobis

[rkman]

a proper gravity turn is the most efficient way of getting into LKO, it's more efficient to transfer directly to the Mun

More efficient than the most efficient?

[arkie87]

More efficient than the most efficient?

Gravity turn is *most efficient* for getting to LKO

If your ultimate goal isn't LKO, then of course, something can be more efficient than it...

[arkie87]

How do you get the all-vertical numbers?

EES Code:

{!Constants}

{Distances}

R=600e3

A=100e3

H=12000e3

{Gravitational Parameters}

mu=3.5316e12 [m3/s2]

{!Vertical Ascent}

{Initial Climb}

-mu/R + 1/2*V[0]^2 = -mu/(R+H)

{Circularization}

V[1]^2 = mu/(R+H)

{Delta-V's}

dV[0]=V[0]

dV[1]=V[1]

{!Horizontal Ascent}

{Ascent to LKO}

-mu/R + 1/2*V[2]^2 = -mu/(R+A) + 1/2*V[3]^2

R*V[2] = (R+A)*V[3]

{Ascent to Mun}

-mu/(R+A) + 1/2*V[4]^2 = -mu/(R+H) + 1/2*V[5]^2

(R+A)*V[4] = (R+H)*V[5]

{Circularization}

V[6]^2=mu/(R+H)

{Delta-V's}

dV[2]=V[2]

dV[3]=V[4]-V[3]

dV[4]=V[6]-V[5]

{!Horizontal Ascent from Surface}

{Ascent to Mun}

-mu/R + 1/2*V[7]^2 = -mu/(R+H) + 1/2*V[8]^2

R*V[4] = (R+H)*V[8]

{Circularization}

V[9]^2=mu/(R+H)

{Delta-V's}

dV[5]=V[7]

dV[6]=V[9]-V[8]

{!Summing Up Different Approaches}

dV_vertical = dV[0]+dV[1]

dV_horz = dV[2]+dV[3]+dV[4]

dV_horz_surf = dV[5]+dV[6]

[numerobis]

By the way, I think you missed the 175m/s sidereal rotation you get for free on launch. That's a gift to the standard launch approach, and does almost nothing for the vertical launch.

--

Do you have words to explain what you're trying to calculate with that EES code? It's best to talk about concepts so we can each do the math in a different way and get similar numbers.

Edited December 11, 2014 by numerobis

[arkie87]

I get roughly the same numbers as you for a spacecraft that does an impulse at 0m altitude on Kerbin to reach orbital velocity, and simultaneously another impulse to do a Hohmann to 11.4e6 m altitude (Mun's orbit), followed by the second burn up there. It's not accurate for a lunar orbit injection, since that throws away Oberth effects on arrival along with possibilities for gravity assists, but it's close enough.

I hear what you are saying, but those effects should be potentially equal depending on the encounter. Regardless, the best comparison we can make is to get both ships into a circular orbit with same altitude as the Mun and compare.