Landing on Mun straight from Kerbin or from LKO - what's more efficient?

[Ksa222]

Hi! I'm new here, so sorry if this is the wrong category or sth :/

I was wondering what is more efficient, going into LKO and then performing a transfer burn to the Mun or heading for it straight from the launchpad? Every time I go to the Mun I don't do a LKO, just straight for the Mun. It seems to consume less delta V, but I'm probably mistakem

So which option requires less fuel/delta V?

[FlyingPete]

Theoretically, a direct ascent should save some delta-V. This is because effectively your Pe is below the surface of Kerbin. If your Ap remains the same for either trajectory, that means a lower-energy orbit compared to launching from LKO. The trouble is that unless KSC is in exactly the right location, you end up almost going to LKO anyway. It's usually simpler to make a parking orbit first, then you can fine-tune your TMI burn without having to be overly-precise about your launch time. Also gives you time to reconfigure/check the spacecraft's working properly on missions where this matters.

I've done direct ascents before, but only where by chance my circularisation burn ended up being iat the same longitude as my TMI burn, so I just carried on through.

[Alshain]

The Oberth Effect says that any maneuver will be less expensive closer to the parent body's center of gravity.  This means that the effect of thrust is always greater the closer you are to Periapsis.  At launch you are closest to your periapsis while sitting on the launchpad.

So as FlyingPete says, theoretically it's cheaper to make the straight shot.  However, in practice the savings are minimal and the procedure is exponentially more difficult to accomplish over a parking orbit. It does make a good challenge, and can be done.

 

EDIT: BTW, this is more of a gameplay question than tech support.

Edited January 26, 2016 by Alshain

[cantab]

Good: Launch as normal at the right time, making a gravity turn, but keep burning to raise your apoapsis out to get a Mun encounter.

Bad: Launch straight up to get a Mun encounter.

[Specialist290]

Just dropping a note to let y'all know I'm moving this over to the Gameplay Questions subforum

As others have said, in theory direct ascent is indeed more efficient if you get it just right -- but in practice, the savings are minimal compared to the amount of effort that setup takes, at least if you fly everything manually.

[Empiro]

11 hours ago, Ksa222 said:

Hi! I'm new here, so sorry if this is the wrong category or sth :/

I was wondering what is more efficient, going into LKO and then performing a transfer burn to the Mun or heading for it straight from the launchpad? Every time I go to the Mun I don't do a LKO, just straight for the Mun. It seems to consume less delta V, but I'm probably mistakem

So which option requires less fuel/delta V?

One thing to keep in mind is that using less fuel and using less delta-V aren't always the same thing (and using less fuel isn't necessarily cheaper if you need to spend more money on engines). Depending on your design and how you fly it, it may very well be the case that your craft spends less delta-V by doing a direct ascent. However, by changing both your design and your piloting, you can probably improve on the overall cost.

I remember that someone here has done a pretty detailed analysis with mechjeb and multiple designs (I can look for the original post if you're interested). The most efficient (in terms of cost) were crafts that had low TWR (~1.3 on launch, meaning fewer expensive and heavy engines) but lots of fuel. You fly it by doing a gravity turn, but completing the transfer to the Mun while you're still inside the atmosphere. You're essentially going into "orbit" at an altitude of 50km, taking as much advantage of the Oberth Effect as possible while balancing that against gravity drag and atmospheric drag losses. However, at the end of the day, the saving is pretty minimal compared to just going into a stable orbit of 80km, which is why most people just do that.

Edited January 27, 2016 by Empiro

[Snark]

Just to add on to the comment that others have made, i.e. that "theoretically a direct burn could be more efficient, but in practice the savings would be minimal":

A direct burn would actually be worse, by a lot, if you do it wrong.

You said

12 hours ago, Ksa222 said:

just straight for the Mun

...what exactly do you mean by that?  Do you mean literally straight for the Mun, i.e. you launch up and keep going up?  That would be bad wrong wasteful.  Reason:  you'll be paying a heavy penalty due to gravity loss.  Every second your engines are directly fighting against gravity, you're throwing several m/s of dV down the toilet.

Ideally, you want as much of your burn as possible to be horizontal, or at least not steeply above it, in order to minimize gravity losses.

If Kerbin didn't have all that pesky atmosphere getting in the way, then you'd launch from Kerbin the same way you launch from the Mun or other vacuum worlds:  hit max throttle and keep it there, then crank it over to only slightly above horizontal immediately upon liftoff so that you're thrusting mostly sideways.

Since there's all that annoying air on Kerbin, you can't do that-- you have to start out by going up.  But you want it to get over to a more horizontal path as early as practical-- thus the gravity turn.

An efficient direct-to-Mun launch would look essentially identical to a launch to LKO... it's just that where you'd ordinarily stop burning as soon as your Ap gets up to 80 km or whatever, you'd just keep burning until it goes all the way to the Mun.

[PLAD]

There was a challenge a while back to get from Kerbin's surface to Minmus' surface for the minimum dV expended. At first I tried the standard method of launching into the lowest possible Kerbin orbit and setting up a burn to Minmus from there that used a Mun flyby. Then I tried a straight-up launch that flew straight to Mun for the flyby. Conclusions: For a minimum dV flight to Minmus straight up is better, but for just to a Mun landing it is worse. Note that the ship is much larger than the lightest possible ship to Minmus or Mun, the challenge just cared about dV expended. The key is a very high TWR. If you accelerate hard enough the gravity losses are reduced to less than you lose from a standard ascent, and the shorter time in the atmosphere reduces the drag losses a bit too. If you go too high with TWR though (like a Jules-Verne-style explosive launch)  the drag destroys your ship. Here are the numbers:

Orbit first- 3010m/s surface to 72x72km orbit, then 845m/s LKO to Mun encounter = 3855m/s.

Straight up- 3780m/s surface to Mun encounter.

Savings to get from Kerbin surface to a Mun encounter: 75m/s.   From my experiments the break-even point was with a TWR of about 5. Less than that and the gravity losses dominate. Note that a simple LKO to Mun trajectory requires 856m/s but you can reduce it to the 845m/s I give by using a complicated multi-flyby of Mun that isn't possible with a straight-up path.

But now the part 2- if you use a straight-up path to Mun you will arrive at Mun with a higher relative speed to it. This completely eliminates the advantage of the straight-up launch. For a standard LKO to Mun path you will need about a 271m/s capture burn to get into low Munar orbit, but with the straight-up path you will require about a 371m/s to brake into the same orbit (and it's the same difference if you go straight to a landing). The reason for this is that you have less prograde velocity when you arrive at Mun's orbit after a straight-up launch, so Mun overtakes your ship at a higher relative speed. You do have a little bit of prograde velocity since the KSC is moving sideways thanks to Kerbin's rotation, but it's a lot less. Your speed relative to Mun at the moment you enter Mun's SOI is about 340 m/s for a standard launch and about 538m/s for a straight-up. (Note that Mun's orbital speed is 542.5m/s.)

I do wonder if there is a hybrid path that goes almost straight-to-Mun but with a little 'sideways' during ascent that would do better, but the aiming would be a nightmare.

[tater]

20 hours ago, Alshain said:

The Oberth Effect says that any maneuver will be less expensive closer to the parent body's center of gravity.  This means that the effect of thrust is always greater the closer you are to Periapsis.  At launch you are closest to your periapsis while sitting on the launchpad.

So as FlyingPete says, theoretically it's cheaper to make the straight shot.  However, in practice the savings are minimal and the procedure is exponentially more difficult to accomplish over a parking orbit. It does make a good challenge, and can be done.

 

EDIT: BTW, this is more of a gameplay question than tech support.

No, the Oberth effect says that such maneuvers are more effective when the craft is moving faster. In an orbit (even if it intersects the atmosphere somewhat) that point is at periapsis. It is NOT moving faster at the launchpad.

KE goes as v², so when you square 0+dv (at pad) it's vastly smaller than (orbital_velocity_at_periapsis + dv)². 

Edited January 27, 2016 by tater