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Most efficient way to insert into orbit


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I've been looking around for an answer to this question but didn't know the best terms to even search for it.  The question is, what is the most efficient way to insert a ship into a particular orbit?  Let's use a ship going from Kerbin to Minimus as an example with the goal of landing.  Is it better to arrange the flyby at a very large periapsis, circularize, then elliptical and circularize again in low orbit before making a landing; or should you intercept the body at the intended low orbit periapsis and circularize from there?

So, intercept Minimus with a 1,000,000 m periapsis, or a 20,000 m one?

Is there a difference?  Is the difference big enough to care about?  Does this hold for other bodies such as Duna and Jool?  Are there other things to worry about such as length of burn?

Thanks in advance and apologies if this has been gone over a million times.

 

Edited by tranenturm
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Aim for a very low periapsis, as low as you can get it without smacking into terrain.

This conclusion comes from two things: the Oberth effect, and conservation of orbital energy. The Oberth effect says "the faster you're going, the more energy you gain or lose by accelerating", and conservation of orbital energy says "unless you make a maneuver, the sum of your kinetic energy and gravitational potential energy is constant".

The energy of a nice circular orbit around Minmus is much less than the energy of the capture, where by definition you have at least enough energy to escape Minmus orbit. So, you want to shed that energy.

Where's the best place to do that? At periapsis, where Minmus's gravity has accelerated you to the greatest extent possible, so your velocity at the capture burn is maximal, so you can shed more kinetic energy per m/sec of delta-V.

Where's the best place to have your periapsis? Scarily low, where Minmus's gravity will have accelerated you as much as it can without experiencing a lithobreaking event.

 

The length of the burn should simultaneously be very short (Oberth effect) and very long (engine mass is dead mass for the Tsiolkovsky rocket equation). It's a complicated optimization problem; I took a stab at the landing part of it a while ago and I think my conclusion was that the optimum was to have enough engine for about 2-2.5 m/sec^2 of acceleration (4-5 TWR for Minmus, ~0.2-0.25 TWR on Kerbin).

 

Landing should begin at as low of an altitude as you dare, managing vertical velocity to avoid a RUD event while shedding as much horizontal velocity as quickly as possible. Again, the Oberth effect comes into play here, and is a balance against adding too much dead-weight in the form of rocket engines. The ideal rocket, after all, is a giant, infinitesimally lightweight balloon full of propellant being fed through an infinitesimally lightweight (but still high specific-impulse, can't forget that) rocket engine*.

*And if you want to go real-world about it, that engine has an infinitely long de Laval nozzle... that again is infinitesimally lightweight.

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I'm not a rocket surgeon, but I would assume that the lower periapsis is more efficient due to the Oberth effect in most, if not all cases.  You should get more effect from thrusting closer to the planet where the gravitational force is felt more strongly than in a higher orbit.  In the case of a really small body like Minimus it's probably not a big difference unless you're running extremely lean.

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50 minutes ago, Gene Kerantz said:

I'm not a rocket surgeon, but I would assume that the lower periapsis is more efficient due to the Oberth effect in most, if not all cases.  You should get more effect from thrusting closer to the planet where the gravitational force is felt more strongly than in a higher orbit.  In the case of a really small body like Minimus it's probably not a big difference unless you're running extremely lean.

On that last bit: I suppose I should clarify that sometimes practical concerns can outweigh the theoretical.

For example, your descent trajectory to Gilly should be less concerned with optimizing gravity losses vs. engine mass, and more concerned with not embarassing yourself by crashing into Gilly.

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1 hour ago, tranenturm said:

The question is, what is the most efficient way to insert a ship into a particular orbit?  Let's use a ship going from Kerbin to Minimus as an example with the goal of landing.

If you plan to land on it, the fuel-optimal solution is not to circularize at all, and go for direct landing. The periapsis of the approach trajectory should just touch the surface just prior to the point of landing. You should kill off your ground speed while maintaining a constant altitude above the surface as soon as you reach the lowest point, and yes, that may even include burning downwards to prevent yourself from going up again. It's been demonstrated to be the most fuel-efficient way of landing on an airless body.

If you are forced to circularize before selecting a landing site, as people have indicated, lowest possible orbit is the best. In general, you have to reduce energy AND angular momentum to enter an orbit, which may require multiple maneuvers, but during entry from beyond SOI, you are pretty much guaranteed to have such an excess of energy that you need to squeeze Oberth effect for all it has. That means circularizing as low as possible.

If you are already in a fairly high circular orbit around the planet/moon, it can actually make sense to increase your apoapsis and then break at the apex into a direct landing. This has to do with aforementioned need to kill off angular momentum as well. Essentially, rather than killing off portions of energy and momentum at your initial circular orbit, you kill off nearly all of the angular momentum at a high periapsis point where it's really, really cheap, and then nearly all of your energy during the burn to landing at the lowest point. Optimizing both for angular momentum and Oberth effect.

This covers all of the standard cases that do not involve inclination change. If you are looking to change inclination, things get more complicated.

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Just now, K^2 said:

If you plan to land on it, the fuel-optimal solution is not to circularize at all, and go for direct landing.

The difference between circularizing into a low parking orbit and direct-to-landing is very minimal; the only real difference is that you can be slightly more aggressive with the altitude of your capture burn. Given that putting yourself into a parking orbit first makes it much easier to hit a given target, and gives you the chance to re-evaluate things*, I find no convincing reason to go direct to landing.

*For example: "do I have enough dV left for the landing, or should I scrub this one and just return home?"

 

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5 hours ago, K^2 said:

If you plan to land on it, the fuel-optimal solution is not to circularize at all, and go for direct landing. 

Actually you are just landing rigth after cirfularization. You inevitably need to reduce the orbital energy to that of a low circular orbit before reducing to the energy of a landed craft. 

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The periapsis of the approach trajectory should just touch the surface just prior to the point of landing. You should kill off your ground speed while maintaining a constant altitude above the surface as soon as you reach the lowest point, and yes, that may even include burning downwards to prevent yourself from going up again. 

Periapsis is by definition the lowest point of the orbit, if you have a periapsis touching the surface and wait for the lowest point to start to kill velocity you crash.  At this point you need to burn up,  if instead you kept the periapsis above the surface at least some of the energy would be reduced with a pure retrograde burn before committing to land. 

In the same vane burning downwards is inefficient because of misalignmentof velocity and change in velocity.  Let alone the fact that,  since you were actually in collision course, burning downwards is burning in the opposite direction you just burnt to avoid crashing. 

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It's been demonstrated to be the most fuel-efficient way of landing on an airless body.

A constant descent trajectory is indeed efficient. However if there is a demonstration that it is the most fuel-efficient way of landing I'm yet to see it.

Based just on my limited experience,  constant descent seems to be as efficient as reverse gravity turn,  depending on craft performance and pilot competence. For practical (game)  purposes maybe not that relevant given all the other factors that affect efficiency. 

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If  you are already in a fairly high circular orbit around the planet/moon, it can actually make sense to increase your apoapsis and then break at the apex into a direct landing. This has to do with aforementioned need to kill off angular momentum as well. Essentially, rather than killing off portions of energy and momentum at your initial circular orbit, you kill off nearly all of the angular momentum at a high periapsis point where it's really, really cheap, and then nearly all of your energy during the burn to landing at the lowest point. Optimizing both for angular momentum and Oberth effect.

It don't make sense at all. Burning to raise your apoapsis will increase both your orbital energy and also your angular momentum. Both will be conservated around the orbit but at the apoapsis the Oberth effect will be lower so not a efficient way to reduce your energy. And all that to end up in a trajectory with higher energy than you stated in, with all the issues raised above. 

 

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You have to be a tad wary of worrying about the Oberth effect as it can be pretty small. It might be useful if you are running really tight in terms of available dV but you are better building in a bit of a safety margin so that you don't have to worry about saving the odd 100 or so dV

Where you can also save a goodly amount of dV is getting the orbit altitude close to what you want as far away from the body as you can. So, when flying interplanetary, have a look at the displayed Pe for the upcoming intercept and use a manoeuvre node to adjust that down to your landing orbit altitude. If you leave it until close to the body then it can take a lot more dV.  

The other potentially huge dV saver is to use a gravity assist passing just in front of a body's moon to apply braking. This can save huge amounts of dV in places like the Jool system. 

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8 hours ago, tranenturm said:

Is there a difference?  Is the difference big enough to care about?... 

... Are there other things to worry about such as length of burn?

Yes there is a difference as @Starman4308 pointed out is the Oberth effect. If is big enough to care about depends

mostly on the mass of the celestial body in question and how much of a safety margin you have.  

Another things to worry about are:

1) you guessed it,  length of burn.  Since a manuever along a curved trajectory is less efficient.  So you want low because Oberth and high for a straight trajectory. But that is more a issue of heavy vessels with weak thrusters.

2)For missions consisting of a lander and an orbital vehicle (a. k. a.  Apolo style)  you want to stay somewhat higher in consideration to the cost of the returning trip. 

3) if a natural satellite can provide a gravity assisted capture it may be preferable instead of a propulsive capture near the planet. OTOH a satellite may be in the way of the ideal trajectory. 

4)aerocaptures gives lots to consider,  going too low may result in heating problems or an unintended landing (usually what in layman's terms is called crashing). Going too high may be ineffective. 

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3 hours ago, Spricigo said:

It don't make sense at all. Burning to raise your apoapsis will increase both your orbital energy and also your angular momentum. Both will be conservated around the orbit but at the apoapsis the Oberth effect will be lower so not a efficient way to reduce your energy. And all that to end up in a trajectory with higher energy than you stated in, with all the issues raised above.

Why don't we do math on this, shall we? We wish to land on planet of radius R from circular orbit of radius r. We have an engine with nearly unlimited thrust, so we can do our velocity changes instantly, but it otherwise obeys rocket formula. Therefore, to minimize fuel use, we wish to minimize total delta-V across all points where velocity changes.

I propose the following generic landing procedure. First, burn to increase velocity and add apoapsis at some altitude x. Then burn at that established apoapsis to bring periapsis to match surface R. Finally, when at the surface, perform final burn. You will notice that case x = r requires zero adjustment and so is equivalent to direct descent.

With me so far? Math time.

First burn takes us from (periapsis, apoapsis) pair (r, r) to (r, x).

Δv1 = Sqrt(2μ/(r + x))Sqrt(x/r) - Sqrt(μ/r)

Second burn takes us from (r, x) to (R, x)

Δv2 = Sqrt(2μ/(r + x))Sqrt(r/x) - Sqrt(2μ/(R + x))Sqrt(R/x)

Finally, we need to land at periapsis, so we completely kill our orbital speed.

Δv3 = Sqrt(2μ/(R + x))Sqrt(x/R)

Total Δv = Δv1 + Δv2 + Δv3

Since the only things that really matter are ratios r/R and x/R, and Sqrt(μ) factors out of absolutely everything, I'm going to give you a few plots of Δv(x) for R = 1, μ = 1.

r = 5, x = 5 to 500. Just what you expect.

oxMToKQ.png

r = 10, x = 10 to 1000. Starts to get curious. What's with the bump?

Yym0ACQ.png

r = 15, x = 15 to 1500. This is where you should start going, "Wha....."

QiiqhSX.png

So somewhere between initial altitudes of your orbit between 10R and 15R magic happens. It's no longer the most optimal to go for direct landing. It makes sense to go to higher orbit first! This works for the same reason that bi-elliptic transfer is sometimes noticeably better than Hohmann transfer. Energy isn't always the king. If your orbit is high enough, the thing you're trying to get rid of is angular momentum. And while sure, initial burns adds a bit of both energy and momentum, you actually kill the bulk of your angular momentum so much cheaper with high apoapsis that you end up with net savings.

 

Your intuition on direct landing is likewise wrong, but I'm all mathed out for tonight. If you want, I can continue tomorrow. But the really exciting fact is that if you have a limited thrust-to-weight ratio, suicide burn is not actually the best way to land.

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11 hours ago, Starman4308 said:

The difference between circularizing into a low parking orbit and direct-to-landing is very minimal; the only real difference is that you can be slightly more aggressive with the altitude of your capture burn. Given that putting yourself into a parking orbit first makes it much easier to hit a given target, and gives you the chance to re-evaluate things*, I find no convincing reason to go direct to landing.

*For example: "do I have enough dV left for the landing, or should I scrub this one and just return home?"

 

Except for on Gilly where landing from an orbit is so painfully slow that I usually prefer to aim for the center of the planet and brake at the last possible moment. It might cost a few 10s of m/s but saves sanity

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Disclaimer: by no means is my intention to be disrespectful or rude. But I'm aware of my tendency to present arguments in a somewhat blunt way. Be my guest to bash my arguments as hard as you can, if you prove me wrong that means I learned something.

 

7 hours ago, K^2 said:

Why don't we do math on this, shall we? We wish to land on planet of radius R from circular orbit of radius r. We have an engine with nearly unlimited thrust, so we can do our velocity changes instantly, but it otherwise obeys rocket formula.

(1)No, we don't have an engine with nearly unlimited thrust. All the issues I raised are related to that fact. Ignoring this fact is effectively avoiding to address my arguments.

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You will notice that case x = r requires zero adjustment and so is equivalent to direct descent.

(2)If we had unlimited thrust setting the trajectory before hand to that situation would be the ideal. We already agreed that one should aim for a periapsis as low as possible before proceeding to landing, being the fact that  you need time to perform the maneuver the only reason to not have the periapsis at the ground level

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With me so far? Math time.

...

Given (1) and (2) I'm confident to say no. Actually you are somewhat behind, delving in mathematics that is as irrelevant as correct. And I didn't verified, just assumed it is correct. Have you considered that we start in a hyperbolic trajectory, thus with a negative semi-major axis(/apoapsis)? Have you considered the actual ratio between the Radius and SoI of the celestial bodies? 

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Energy isn't always the king. If your orbit is high enough, the thing you're trying to get rid of is angular momentum.

You started with angular moment A, performed a maneuver that raised that angular moment to C,and at apoapsis burned again to reduce to B. If B > A, which is the case if you already intercepted/captured in a orbit as low as possible. Else: Why didn't you captured in a lower orbit to begin with?

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Your intuition on direct landing is likewise wrong

Assuming by "direct landing" you mean "constant descent trajectory", what I said is that I don't have a conclusion. Kinda hard to be wrong with "I don't know". I also stated that my experience is limited and I'm yet to see a comparison of constant descent trajectory VS reversed gravity turn, at least one that is not biased in favour of one or another because of other factors (specially craft performance and piloting competence).

 

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But the really exciting fact is that if you have a limited thrust-to-weight ratio...

In other words: depending on craft performance, which I already assume is an important factor. Do you have something to support your claim regardless of craft performance? Demonstrating it for limited cases is useful, but not conclusive.

 

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...suicide burn is not actually the best way to land.

Suicide burn is just doing the required maneuver to land as close to the ground as possible to get most of Oberth effect. Actually a constant descent trajectory can, and ideally* will, be a suicide burn. 

*if the objective is to minimize fuel consumption.

 

Edited by Spricigo
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5 hours ago, Spricigo said:

(1)No, we don't have an engine with nearly unlimited thrust. All the issues I raised are related to that fact. Ignoring this fact is effectively avoiding to address my arguments.

Largely irrelevant to the discussion. While it becomes more complicated when you factor in finite accelerations, the overall conclusions are largely the same: mainly, that in some edge cases, a bi-elliptic transfer is more efficient than a Hohmann transfer (particularly if you need to do a very large inclination change; I think at about 80-ish degrees, it's always more efficient to do it with a bi-elliptic).

5 hours ago, Spricigo said:

(2)If we had unlimited thrust setting the trajectory before hand to that situation would be the ideal. We already agreed that one should aim for a periapsis as low as possible before proceeding to landing, being the fact that  you need time to perform the maneuver the only reason to not have the periapsis at the ground level

Granted, I feel this point is kind of lacking in K^2's analysis: the cheapest thing in most scenarios is not to perform a bi-elliptic, it's to not have to do that bi-elliptic in the first place; for example, to get to a low retrograde orbit over Kerbin, the most efficient method is not a bi-elliptic... it's to launch retrograde in the first place.

5 hours ago, Spricigo said:

Assuming by "direct landing" you mean "constant descent trajectory", what I said is that I don't have a conclusion. Kinda hard to be wrong with "I don't know". I also stated that my experience is limited and I'm yet to see a comparison of constant descent trajectory VS reversed gravity turn, at least one that is not biased in favour of one or another because of other factors (specially craft performance and piloting competence).

Suicide burn is just doing the required maneuver to land as close to the ground as possible to get most of Oberth effect. Actually a constant descent trajectory can, and ideally* will, be a suicide burn. 

I'm pretty sure that's not actually what "suicide burn", "gravity turn", and "constant descent trajectory" mean to most people.

To me, a suicide burn is: you burn retrograde in orbit to set periapsis beneath the surface, then hold your vessel to the surface retrograde vector, and kick on the engines at the absolute last second, perfectly coming to a stop as you touch the ground. Key to this are that you're pointed exactly dead-retrograde the entire way, and light up the engines at the last possible second.

A gravity turn is mostly just something for ascent from atmospheric bodies, where you start by going straight up, induce a small tilt (usually eastwards), and then go get a coffee, letting aerodynamic stability keep you on track. It's not a good idea for airless worlds; you waste far too much dV going straight up, whereas the ideal for airless worlds is to go as horizontal as you possibly can without lithobreaking events the instant you take off.

A constant descent trajectory is something entirely different; you get to a circular orbit just above the surface, and then, instead of locking purely surface-retrograde, you start pitching up to maintain a constant-ish descent velocity, which is ideally 0.

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6 hours ago, Spricigo said:

If we had unlimited thrust setting the trajectory before hand to that situation would be the ideal.

The case discussed is for landing from a pre-determined parking orbit, as per my original post. Finite thrust does not change the fact that bi-elliptic transfer landing saves you fuel. Although, it does play into details of the final burn. I don't think there is a point in me replying to the other parts of it with this confusion in place. If you still disagree with derivation given a predetermined parking orbit, please, let me know.

For approach from parabolic or hyperbolic trajectory, I can actually derive an optimal descent for finite thrust engine. It's slightly more complex than constant altitude landing in that it starts out more similar to a suicide burn. That saves you the fuel you waste burning towards the ground in true constant-altitude landing. The difference, however, tends to be minute and isn't worth added complexity and requirements on timing. So in practice, constant altitude landing is ideal. Hopefully, someone will ping me in this thread to remind me to go through the whole derivation.

The only truly complicated case is landing from a "low" (1R - 10R) parking orbit. I have not found a way to properly generalize this case. The bi-elliptic transfer landing (>~12R) will follow exactly the same procedure as landing from parabolic orbit, where you first raise appoapsis to the edge of SOI (or however far you're willing to go in terms of available time) and from there, you're essentially coming down close enough to parabolic trajectory to use the same algorithm.

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4 hours ago, Starman4308 said:

Largely irrelevant to the discussion. While it becomes more complicated when you factor in finite accelerations, the overall conclusions are largely the same: mainly, that in some edge cases, a bi-elliptic transfer is more efficient than a Hohmann transfer (particularly if you need to do a very large inclination change; I think at about 80-ish degrees, it's always more efficient to do it with a bi-elliptic).

With infinite thrust and perfect piloting, there is no difference between constant descent trajectory,  reverse gravity turn or what you called a suicide burn: we just kill all the velocity instantly at the impact point,  where or trajectory is tangent to the surface, with a horizontal burn.  Assuming a finite thrust is what make the distinction relevant, instead of a comparison of gravity turn without turn VS a constant descent without descent.  

Likewise the fact a bi-elliptic transfer is more efficient than Hohmann transfer in some cases will be a Moot point if that cases don't happens because a previous intercept optimization. 

 

About what the terms means "to most people": we need to know what are we talking about,  so let's make it clear what we mean when using those terms.  

We are already in the same page in regard to constant descent.

And in divergence about suicide burn.  But I think we can avoid it because we agree that (a) doing a manuever to reduce energy is better at lower altitudes because Oberth  and (b)  the maneuver you described as suicide burn is inefficient. 

The definition I know (and use)  of gravity turn is: a ascent or descent* trajectory where the thrust is not used to steer the rocket but rather the rocket is steered by the gravity. **

*the descent version being the reverse gravity turn where the distinction is important. 

**if an atmosphere is present aerodynamic forces will also act on the craft affecting the turn. However what define a gravity turn is how gravity and thrust are acting on the vessel. 

3 hours ago, K^2 said:

The case discussed is for landing from a pre-determined parking orbit, as per my original post. Finite thrust does not change the fact that bi-elliptic transfer landing saves you fuel. 

Which pre-determined parking orbit?  In the first reply to this thread is suggested to aim for a periapsis as low as you can get it without smacking into terrain

As  far as I'm concerned we are assuming a parking orbit of radius R. Your demonstration that magic happens if the initial orbit is 12R only show that magic don't happens in the situation we are talking about. 

Again: your math seems correct. But is not applicable to the practical case at hand. Talking about 1,2R is already a stretch,  leave alone 12R.

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For approach from parabolic or hyperbolic trajectory, I can actually derive an optimal descent for finite thrust engine. It's slightly more complex than constant altitude landing in that it starts out more similar to a suicide burn. 

If by "start out more similar suicide burn" you mean "kill some  horizontal velocity first"   that is  a cirfularization burn and before the actual landing maneuver. If you start right away or several orbits ahead will not affect the efficiency. 

Also if that is the ideal height to start the landing (with finite thrust and safety margin)  you should had aimed to that heigh at the intercept.

 

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I agree with K^2. If (as the OP said) your goal is landing, then while you are outside the SOI of the destination CB, you set your line so it goes right through the heart of the CB. Then a massive suicide burn. Decent savings from Oberth. Decent savings from having zero gravity losses. It adds up.

 

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2 hours ago, Spricigo said:

Which pre-determined parking orbit?  In the first reply to this thread is suggested to aim for a periapsis as low as you can get it without smacking into terrain

As  far as I'm concerned we are assuming a parking orbit of radius R. Your demonstration that magic happens if the initial orbit is 12R only show that magic don't happens in the situation we are talking about. 

Again: your math seems correct. But is not applicable to the practical case at hand. Talking about 1,2R is already a stretch,  leave alone 12R.

Here is the quote from my original post that this spun out of.

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If you are already in a fairly high circular orbit around the planet/moon, it can actually make sense to increase your apoapsis and then break at the apex into a direct landing.

There can be any number of reasons to have a high parking orbit before you proceed to a landing. You might've dropped of a comm sat, or that's where you decided to park a station. Who cares? High parking orbit was the entire context. If you simply missed that, and you're comfortable with the fact that you have to burn to increase apopapsis prior to landing in that case, then we can just move on.

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If by "start out more similar suicide burn" you mean "kill some  horizontal velocity first"   that is  a cirfularization burn and before the actual landing maneuver. If you start right away or several orbits ahead will not affect the efficiency. 

Also if that is the ideal height to start the landing (with finite thrust and safety margin)  you should had aimed to that heigh at the intercept.

Correct, except height at periapsis during intercept needs to be somewhat above your "circularization" orbit. You will lose some altitude during the braking if you burn purely retrograde for maximum efficiency. The exact amount of altitude you lose will depend on TWR. This is what makes this approach difficult.

If instead you aim for periapsis to be at the exact altitude where you want to circularize, you will spend more fuel, as you'll have to have a radial component in the burn. However, the difference is fairly small. You lose about 5% at TWR = 1.5 and about 15% at TWR = 3. This is just from your circularization budget, so it's an even smaller fraction from total for landing.

In practice, because it's easy to misjudge the height you need to aim for with the most-optimal approach, you'll end up losing more fuel on corrections. This results in landings with constant altitude circularization being more efficient when piloted by hand.

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A few days ago someone posted a message seeking advice on reaching Mun in the Demo version of KSP.  This tempted me to load a design I used in it, to reach Minmus with a separate lander attached for the descent onto the surface of Kerbin's second moon, a mission not unlike what the Soviets planned form their proposed lunar landing in real life in fact.

Anyway, I have a really bad habit, I tend to tip over to horizontal way too early while flying into a parking orbit around Kerbin.  So I deliberately tried to resist this, and fly in a much more acceptable manner.  The result was that when I finally completed my orbital insertion, and then my injection burn for Minmus, I had only a very small amount of fuel left in the stage which would also be used to get into orbit around Minmus and do the bulk of the descent before the little RCS powered lander finally ditched it and switched over to its own on-board systems.

Curious I reverted to the launch pad to try again, this time flying my more traditional way-too-shallow flight profile to orbit, but from then as similar as possible to the first attempt.  The results were totally different; I could see immediately after establishing my parking orbit that I had loads more fuel in the tank, and indeed when I finally ditched the last stage during the descent onto Minmus, it still had a not insignificant amount in reserve.

Now I do appreciate this was all in KSP Demo 1.0 and things have changed a great deal since this was first released, and I haven't had time to try a similar experiment in 1.2.2 so it might produce totally different results, but if these results do hold up in the full game, then is there any advantage in not shallowing out your trajectory early, and thus increasing your velocity during the initial ascent from Kerbin?

Edited by The Flying Kerbal
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There is absolutely no reason not to try and get as much fuel reserve by the time you establish parking orbit around Kerbin. That's pretty much the only measure of a good ascent. But trajectory that achieves that will depend very much on your TWR and, as of the aerodynamics update, the shape and composition of the rocket.

Every rocket will have an optimal ascent profile, but there is no one-size-fits-all profile for all rockets. Just general guidelines that will, more often than not, get you to orbit. In general, pitching over too early will increase aerodynamic losses, and pitching over too late will cost you in losses to gravity. But where the too early and too late cutoffs are can vary a lot from one design to another.

Prior to the aerodynamics update, situation was a little different. There was an almost universally optimal ascent profile, so long as your rocket can manage at least a TWR of 3 when required and can perform all the necessary turns.

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Practically speaking, if you don't pass apoapsis and don't overheat, you didn't pitch over too early.  Going nearly straight sideways by 30 km is a good thing, you're piling on the energy in the best place for it.

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Note that these "optimal trajectories" on airless worlds, especially Minmus tend to be steeper than you can steer a retrorocket.

Anybody build a minmus lander in the spaceplane hanger (I think Scott Manley did a long time ago, presumably versions have changed too much to count)?  I'm guessing that landing on the flats as a runway is ideal.  I'm pretty sure you can't steer a tail-sitting lander sideways fast enough on Minmus for an ideal launch (and thus can't do an ideal landing either, even if you were Jeb himself).

Of course, this only works on Minmus (and the flats).

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20 hours ago, K^2 said:

Here is the quote from my original post that this spun out of.

There can be any number of reasons to have a high parking orbit before you proceed to a landing. You might've dropped of a comm sat, or that's where you decided to park a station. Who cares? High parking orbit was the entire context. If you simply missed that, and you're comfortable with the fact that you have to burn to increase apopapsis prior to landing in that case, then we can just move on.

OK, I missed it,  my fault. Assuming that because "reasons"  the capture orbit is high enough do a bi-elliptic transfer. 

Some considerations: 

(1) still relevant,  for practical purposes, to ask: why capture in such high parking orbit? There are "reasons",  but are those reasons good enough?  

(2a)SoI size (relative to celestial body radius)  is a important limiting factor.  Take the Mun as an example,  to a bi-elliptic transfer be more efficient your parking orbit need to be close to the SoI edge. 

(2b)In a similar vane,  leaving the SoI and  recapturing later in a lower orbit will often be more fuel-efficient than a bi-elliptic transfer within the SoI

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8 hours ago, wumpus said:

Note that these "optimal trajectories" on airless worlds, especially Minmus tend to be steeper than you can steer a retrorocket.

Really a matter of craft performance,  for lightweight landers the problem tend to be steering too quickly. At least that is my experience. 

Using the flats as a runaway (and landing gears instead of legs( gives an advantage: you touch down with a considerable portion of the orbital speed.  (odly enough, even a tailsiter can benefit from it) 

 

 

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