Why orbit before landing?

[dlazerka]

While looking at ÃŽâ€v maps, I don't quite understand why to land on the Mun I ought to first orbit it. And the same for leaving Kerbin -- is it really better to first orbit it, and then Hohmann transfer to Mun?

That would make sense if I were going to leave an orbiting station, land on a detachable module. But if I'm going to just land, isn't it better to shoot straight to the Mun, and then decelerate to land?

And a related question: how is better (in terms of ÃŽâ€v) to inject into Mun orbit -- from behind it (counter-clockwise, same direction as the Mun itself around Kerbin), or from the front (clockwise)?

Of course, all that is valid for other bodies, Kerbin and Mun are just examples.

[lajoswinkler]

In the terms of ÃŽâ€v, it doesn't matter. It's basic conservation of energy. Whatever your path is, net energy exchange between a system and environment is the same. You still have to brake, i.e. expell gas.

Orbiting is done beforehand for the convenience of choosing a landing spot, leaving an orbital module, etc.

[Wanderfound]

Lajoswinkler has the right answer, but as another consideration: heat, for those of us using Deadly Reentry.

Although you can do a successful direct-from-Duna-to-runway extreme aerobraking maneouvre, it's a lot easier to keep your temperatures down if you stabilise in a low orbit first.

As for insertion style...I think coming in from behind is more efficient (uses the Mun's velocity to reduce the required retroburn), but double check that; I routinely get things like that backwards.

Edited August 27, 2014 by Wanderfound

[Nuke]

ive done direct landings before. those odd occasions where my maneuvers were so precise my orbit intersected the object i was going to land on anyway. i figured the deltav cost to enter orbit from that point was greater than what it would have cost to land, so i landed instead to save fuel.

i routinely aerocapture, but its still somewhat easier to come in with a little bit of wiggle room so that i remain in orbit after the maneuver. if i dont like where my periapsis is in relation to the surface, i can always do a small prograde burn at apoapsis to push the trajectory out of the atmosphere. i also have the option to continue aerobraking maneuvers to bring my orbit in closer if so desired.

Edited August 28, 2014 by Nuke

[K^2]

In the terms of ÃŽâ€v, it doesn't matter. It's basic conservation of energy. Whatever your path is, net energy exchange between a system and environment is the same. You still have to brake, i.e. expell gas.

Orbiting is done beforehand for the convenience of choosing a landing spot, leaving an orbital module, etc.

Energy may be conserved, but delta-v is path dependent. Or we wouldn't have such a thing as Hohmann Transfer. In other words, keep Oberth Effect and conservation of angular momentum in mind.

On the other hand, specifically for landing on a body with no atmosphere, it matters very little. Sufficiently low orbit is nearly identical to direct landing, while providing some additional safety margin.

Establishing a high orbit and landing from there does cost a bit of extra delta-V compared to direct landing.

[lajoswinkler]

Yeah, I meant for bodies without atmosphere. Obviously, in KSP you can just throw something right at Eve and avoid the fuss, but that's not how things work in real life if you want your astronauts to survive the reentry.

I forgot about the angular momentum. Yes, that counts. Anyway, the differences are not dramatical for slowly rotating bodies.

[K^2]

It's not about rotation of the body. It's about angular momentum of craft in central potential. If your aim point is low, with periapsis of hyperbolic trajectory almost kissing the surface of the target body, the angular momentum you need to kill to perform landing is also low. With insertion to high orbit prior to landing, you start out with higher angular momentum. That's more angular momentum to kill.

With energy, the situation is reversed, but also plays against you with high orbit. The total amount of energy you need to kill is constant either way, but you use delta-V to reduce your energy more efficiently at higher speeds. (dE/dt ~ v I_sp dm/dt) So again, a low periapsis helps you.

Ideally, if we picture target body as a perfect sphere and we have ability to accelerate instantly, your best landing is hyperbolic trajectory that just touches the surface, at which point you perform an instant suicide burn to land. In that scenario, you can also do a partial burn, transfer to circular orbit that perfectly shadows the surface, and then finish the burn to land anywhere. Delta-v in both scenarios is identical.

In reality, accelerations aren't infinite, and you need some minimal height over surface to circularize. That results in losses due to circularization as opposed to direct descent. But they are very minor, so long as parking orbit is very low.

[PakledHostage]

In the terms of ÃŽâ€v, it doesn't matter. It's basic conservation of energy. Whatever your path is, net energy exchange between a system and environment is the same.

This post inspired me to go looking for my old "Minimum Delta-V to the Mun" challenge thread from a couple of years ago but I think it might have been lost in the Great Forum Derp...

While that challenge is only peripherally related to this thread, it is still relevant to Lajos' post above: The point of my "Minimum Delta-V to the Mun" challenge was to get from a 100 km starting orbit about Kerbin to a landing on the Mun using the minimum Delta-V. Guys tried all sorts of things from bi-elliptic transfers to simple Hohmann transfers and everything in between. The two methods that resulted in the lowest delta-V expenditure in the competition were Stochasty's, who built a craft that could survive a roll-on landing at over 100 m/s and my own. Stochasty still beat me, but I got close to his score by using a double gravitational assist off the Mun, then intercepting the Mun for a landing on the third encounter. I used the first munar slingshot to raise my periapsis about Kerbin and the second Munar slingshot to lower my apoapsis about Kerbin. When I entered the Mun's SOI the third time, I was actually on an elliptical trajectory rather than a hyperbolic trajectory. Using Stochasty's landing technique at the end of the multi-gravitational assist trajectory would have shaved even more delta-V off the transfer.

[magnemoe]

Energy may be conserved, but delta-v is path dependent. Or we wouldn't have such a thing as Hohmann Transfer. In other words, keep Oberth Effect and conservation of angular momentum in mind.

On the other hand, specifically for landing on a body with no atmosphere, it matters very little. Sufficiently low orbit is nearly identical to direct landing, while providing some additional safety margin.

Establishing a high orbit and landing from there does cost a bit of extra delta-V compared to direct landing.

If you have low TWR going into orbit might save fuel as you will do more of the burn close to the target getting more out of the Oberth Effect.

Yes you could just continue the braking burn into deorbit and landing but that would limit the landing areas.

Now if your target has atmosphere you might go strait for landing, Apollo did then returning from moon. This require you to wait with the return burn until earth rotation give you the wanted landing spot. Did many direct landings in KSP too before 0.24 and you could land anywhere with the same result.

[K^2]

Stochasty still beat me, but I got close to his score by using a double gravitational assist off the Mun, then intercepting the Mun for a landing on the third encounter. I used the first munar slingshot to raise my periapsis about Kerbin and the second Munar slingshot to lower my apoapsis about Kerbin. When I entered the Mun's SOI the third time, I was actually on an elliptical trajectory rather than a hyperbolic trajectory.

It shouldn't be possible to get into elliptical orbit from assists alone without perturbations, n-body physics, or another body for assists. Though, you can get very close, so maybe the difference was made up with burns. Or even numerical errors. (I seem to recall from previous discussion that numerical errors occasionally result in capture due to numerical errors in KSP.)

But this is an interesting approach. It's basically doing what Rosetta mission did in reverse. I'll have to keep it in mind for future missions.

[magnemoe]

It shouldn't be possible to get into elliptical orbit from assists alone without perturbations, n-body physics, or another body for assists. Though, you can get very close, so maybe the difference was made up with burns. Or even numerical errors. (I seem to recall from previous discussion that numerical errors occasionally result in capture due to numerical errors in KSP.)

But this is an interesting approach. It's basically doing what Rosetta mission did in reverse. I'll have to keep it in mind for future missions.

You can get an almost elliptical orbit, it look like an ellipse but it has an small opening on the top.

I think elliptical trajectory is one who change your trajectory more than 180 degree inside the SOI, an hyperbolic trajectory does not.

[r4pt0r]

It shouldn't be possible to get into elliptical orbit from assists alone without perturbations, n-body physics, or another body for assists. Though, you can get very close, so maybe the difference was made up with burns. Or even numerical errors. (I seem to recall from previous discussion that numerical errors occasionally result in capture due to numerical errors in KSP.)

twice last week I entered stable elliptical orbits just slingshoting around the joolian system. once with vall and once with bop. IDK if its an error, but I only did burns when ejecting from the previous moon, and ended up with orbits at the target moons for free.

[cantab]

Magnemoe has it right. In KSP - and for that matter in the real world - you can get in an orbit that's elliptical but has apoapsis outside the body's SOI. You'll actually be going slower than escape velocity on said orbit, but escape by virtue of another body's gravity coming to dominate your trajectory.

[Mr Shifty]

It shouldn't be possible to get into elliptical orbit from assists alone without perturbations, n-body physics, or another body for assists. Though, you can get very close, so maybe the difference was made up with burns. Or even numerical errors. (I seem to recall from previous discussion that numerical errors occasionally result in capture due to numerical errors in KSP.)

But this is an interesting approach. It's basically doing what Rosetta mission did in reverse. I'll have to keep it in mind for future missions.

With patched conics, you can leave/enter a planet's SOI with an elliptical trajectory no problem. (This is particularly true for moons.) You just need your Mun-relative velocity when you cross into the Mun's SOI to be less than escape velocity at the SOI edge, which for the Mun is 232 m/s. (For Laythe, it's over 1000 m/s.)

[Superfluous J]

I'm a bit late to this party, but...

If you wrap your head around the idea that launching as close to horizontal as possible on an airless body is the ideal way to launch (because you're cutting your gravity losses to a minimum and putting more of your thrust into speeding up than you are to not falling down) then just think of your question as the opposite of that.

It's more efficient to orbit first because when you're burning at your periapsis¹ you're spending ALL of your fuel reducing your SPEED. If you just come straight in for a suicide burn (as fun as they are) a portion of every liter of fuel is spent fighting gravity.

Obviously, you have to fight a LITTLE gravity when you finally land, but the less of that you do the better. For the same reason you shouldn't fight it during takeoff.

¹ Make this as low as you dare. I like 6-10km when coming in to Mun. 6km is scary though!.

[No one]

When escaping something it's best to orbit first because you can get into orbit with a gravity turn and spend less time fighting gravity.

When landing on something, I don't think it is best to orbit first, but I don't know. I know that historically we landed probes on the moon before we sent probes to orbit it, so I think that it's probably easier not to orbit.

[Mr Shifty]

When landing on something, I don't think it is best to orbit first, but I don't know. I know that historically we landed probes on the moon before we sent probes to orbit it, so I think that it's probably easier not to orbit.

Seems like the reason this occurred had more to do with growing pains regarding rocket development. The very first lunar mission launched was an attempted orbiter, Pioneer 0 in 1958, which failed during the launch. There were no (soft) lander missions attempted until 1963-4. It just happened that the first successful soft landing (Luna 9) happened 2 months before the first successful orbiter (Luna 10.) And interestingly the first successful US lander (Surveyor 1) happened 2 months before the first successful US orbiter (Lunar Orbiter 1). All of these successful missions in 1966, which seems to have been a banner year for lunar exploration.

[cantab]

I'm a bit late to this party, but...

If you wrap your head around the idea that launching as close to horizontal as possible on an airless body is the ideal way to launch (because you're cutting your gravity losses to a minimum and putting more of your thrust into speeding up than you are to not falling down) then just think of your question as the opposite of that.

It's more efficient to orbit first because when you're burning at your periapsis¹ you're spending ALL of your fuel reducing your SPEED. If you just come straight in for a suicide burn (as fun as they are) a portion of every liter of fuel is spent fighting gravity.

But surely it's then even more efficient to come in on a trajectory with periapsis level with the surface, and suicide burn at the last minute to zero your velocity and land there.

The differences are small though. The main reason to orbit before landing is the same as to orbit Kerbin before departing: getting the time to set up your manoeuvre properly. On the landing end of things this includes being able to select your landing zone and make the appropriate deorbit burn for that.

[Superfluous J]

But surely it's then even more efficient to come in on a trajectory with periapsis level with the surface, and suicide burn at the last minute to zero your velocity and land there.

Most assuredly. You save even more fuel (or at least I do) when you (or at least I) crash into the surface at 500m/s and your entire craft explodes.

The differences are small though. The main reason to orbit before landing is the same as to orbit Kerbin before departing: getting the time to set up your manoeuvre properly. On the landing end of things this includes being able to select your landing zone and make the appropriate deorbit burn for that.

Yes. But it's important to make sure people know that by doing this you're getting a benefit (picking your landing zone. Taking a breather. Hitting quicksave in a save area) with NO DETRIMENT. It's not like we do this, get all these benefits, but sacrifice fuel. By doing this we're actually SAVING fuel AND getting the benefits.

[K^2]

With patched conics, you can leave/enter a planet's SOI with an elliptical trajectory no problem. (This is particularly true for moons.) You just need your Mun-relative velocity when you cross into the Mun's SOI to be less than escape velocity at the SOI edge, which for the Mun is 232 m/s. (For Laythe, it's over 1000 m/s.)

Then you'll reach edge of SOI going at the same speed and exit. If you had non-zero speed coming in, you'll have non-zero speed coming out. There is now way to get around that.

So you can't get captures in patched conics without either an assist from another body (entering and leaving its SOI) or burning fuel.

And yes, in KSP, if you managed to get a free capture, that's due to numerical errors.

[Mr Shifty]

Then you'll reach edge of SOI going at the same speed and exit. If you had non-zero speed coming in, you'll have non-zero speed coming out. There is now way to get around that.

So you can't get captures in patched conics without either an assist from another body (entering and leaving its SOI) or burning fuel.

Right, I agree with you, and I'm pretty sure PakledHostage wasn't implying capture; just an elliptical instead of hyperbolic trajectory (positive sma) upon SOI entry. I was just clarifying that leaving/entering the SOI of a body in KSP doesn't necessarily imply a hyperbolic trajectory. "Capture" in KSP just means you have an elliptical orbit that lies wholly inside the SOI of a body.

[Superfluous J]

Then you'll reach edge of SOI going at the same speed and exit. If you had non-zero speed coming in, you'll have non-zero speed coming out. There is now way to get around that.

Mathematically speaking I agree with you.

Doesn't change the fact that I've gotten captured multiple times in the game. I've always chalked it up to rounding errors in my favor.

[K^2]

Doesn't change the fact that I've gotten captured multiple times in the game. I've always chalked it up to rounding errors in my favor.

Essentially. Has to do with the way physics is computed for joints and other forces. Won't happen on rails.

[PakledHostage]

Right, I agree with you, and I'm pretty sure PakledHostage wasn't implying capture; just an elliptical instead of hyperbolic trajectory (positive sma) upon SOI entry.

Thanks for the support! You are absolutely right that I wasn't implying that my craft had been captured. Only that my multiple gravitational assists had resulted in sufficiently slow speed relative to the Mun on the third encounter that my craft was in an elliptical trajectory relative to the Mun rather than a hyperbolic one. Clearly that is advantageous when attempting to reach the Mun's surface from LKO using minimal delta-V.

Edited August 29, 2014 by PakledHostage

[magnemoe]

Essentially. Has to do with the way physics is computed for joints and other forces. Won't happen on rails.

Can it also be the SOI change during warp bug? if it miscalculate your speed or position you could get capture.