De-orbiting from 200km+?

[Blexie]

Title says most of it really. What's the best way to de-orbit from 200km+? For lower orbits I'm just burning straight down until I run out of fuel and hoping for the best, and while not exactly elegant, it works. This doesn't work from higher up (I.E, I miss and slingshot around the planet). Is it best to burn at an angle that would reduce speed as well as altitude? Or am I missing something glaringly obvious?

TLDR: How do you de-orbit from higher altitudes?

Thanks in advance!

[thorfinn]

The best way to leave an orbit, at any altitude, is slowing down

The most fuel-efficient orbit change maneuvers are performed at the lowest point of the orbit (perigee) and the highest (apogee). A change in speed at these points reflects at the opposite end without changing other parameters; acceleration at perigee equals to raising apogee, deceleration at apogee equals lowering perigee, and the other two combinations.

A burn which isn't oriented directly towards your current velocity vector (empty circle on the 8-ball) or opposite to it (retrograde burn, crossed-out circle) will act on many orbital parameters at once ad will take you nowhere near to where you pointed, since gravity will distort your trajectory in time. Real astronauts stop thinking with orbital mechanics and go back to 'aim where I want to go' only in the last hundreds of meters of a rendez-vous.

So, for deorbiting, the least expensive maneuver is a retrograde burn at apogee. A change in velocity of a few tens of meters per second equals to tens of kilometers of perigee height change. Burning nearer to perigee gets progressively more expensive, though upper stages in KSP usually aren't short of delta-V for such simple maneuvers (the usual LFE+1LFT+decoupler+CM has ten times the delta-V of a Soyuz...)

[Blexie]

I'm surprised that slowing down has a BIGGER effect than just going Jeb style on things. But yeah, that's that cleared up. Thanks for the fast response!

[Ferrit]

Something you might want to search on these forums for is Hohmann transfers. They're basically a fuel efficent method of increasing or decreasing orbit altitude. For accurate re-entry (like when trying to land at KSP) performing a transfer to a lower orbit should allow for greater accuracy on the final de-orbit. (That said, I've not managed to land near KSP without a lot of luck and/or flying in Jeb-style)

Best of luck!

[rdfox]

The way I deorbit from just about any orbit, low or high, is a five-step procedure.

1) Start up KSP Calculator.

2) Use the 'elliptical orbit' feature to calculate an orbit with an apokee of whatever my current orbit's apokee is, and a perikee of about 34km. (Yes, I'm still old-school about getting that deep, just to make sure.)

3) When I hit apokee, burn retrograde until I'm down to the speed given as apokee speed for that orbit.

4) Turn to a reasonable angle, jettison my retrofire stage, then turn BEF (blunt-end-first) and wait for entry.

5) Let atmospheric drag take me the rest of the way.

It lets me carry less fuel into orbit (so I could throw more payload up there with the same booster), and once Harv adds re-entry heating and G-load limitations, it'll also keep both heat and G loadings down to lower, safer levels than just thrusting straight down into the atmosphere, so it's good practice for the future. (Same reason I always put Moach's add-on heat shield on the command pod; that way, I'm used to doing so for when it's needed--and used to its weight cost, too!)

[Cannon_Fodder]

Just burning towards the planet doesn't always work. You might just end up in a more eccentric orbit and now stuck in orbit out of fuel.

Like others have said, slow down and let gravity work for you. From really high orbits I like to save a little fuel to burn just before I enter atmosphere to make sure the g-load on the capsule doesn't kill the pilots from too much speed.

[Jarnis]

Like others have said, slow down and let gravity work for you. From really high orbits I like to save a little fuel to burn just before I enter atmosphere to make sure the g-load on the capsule doesn\'t kill the pilots from too much speed.

Do not worry. Kerbonauts are mighty tough. In our detailed experiments they have routinely survived 'straight down' re-entries at nearly 7000m/s, resulting in peak G loads of over 170G. Highest recorded spike is about 215G (suspected to be due to a parachute deployment too early ) and highest that resulted in soft landing about 190G.

Jeb even smiled all the way down!

(That, or the game is missing a feature that turns the contents of the crew can cabin into a suitably green paste and distributes it all over the wall facing the blunt end if G loads get too high... not sure...)

8)

[Ivan Ivanovich]

It\'s likely that the game doesn\'t calculate that yet, 215G would turn any living organism into goo. Likewise, the reentry heat isn\'t in yet, which would make you burn up like a snowball if you reentered at this speed.

[Jarnis]

It\'s likely that the game doesn\'t calculate that yet, 215G would turn any living organism into goo. Likewise, the reentry heat isn\'t in yet, which would make you burn up like a snowball if you reentered at this speed.

Re-entry heat... mmm... crispy Kerbonauts...

If it is in any way realistic, re-entries from trajectories outside of simple circular orbits may get interesting... as in have to re-enter at this specific angle - too shallow and you\'ll just skim through the upper atmosphere, too steep and crew can gets all crispy (and flat). Should make initial Mun missions... *Fun

*Fun is trademarked by Dwarf Fortress

[shadowfax]

A way to really help understand how orbital maneuvering works is to fire up orbiter and go through the Delta Glider - ISS / Mir scenarios. It\'s a good way to crystallize the idea that pointing at something and hitting the gas isn\'t the way to get to that something. Actually, the maneuver you described in your first post is the one I use to cheaply achieve a fast escape velocity. You slingshot around the planet, using its gravity to help accelerate you instead of having to use fuel to do all the work.

[Herra Tohtori]

A way to really help understand how orbital maneuvering works is to fire up orbiter and go through the Delta Glider - ISS / Mir scenarios. It\'s a good way to crystallize the idea that pointing at something and hitting the gas isn\'t the way to get to that something. Actually, the maneuver you described in your first post is the one I use to cheaply achieve a fast escape velocity. You slingshot around the planet, using its gravity to help accelerate you instead of having to use fuel to do all the work.

Gravity wells are symmetric, you are not going to be able to utilize the gravity to increase your total momentum in this case.

The only way to utilize planets is to bleed a little bit of their angular momentum around the central star, and add that to your angular momentum around said star. That has been used to make probes get to outer planets faster, but the angular momentum around the planet itself stays constant.

What you\'re doing here is simply burning fuel to first change your direction closer toward the Kerbin (as opposed to ballistic trajectory of your original orbit with no thrust), and while it may seem that the planet\'s gravity makes you go faster (it does), it will also start to slow you down as soon as you pass the lowest point of your trajectory and start climbing.

You would get a better result simply doing a prograde burn until you achieve the desired velocity, because that way you use all your delta-v for accelerating your ship instead of turning it. Zenith and antizenith burns should only be used for minor orbital circularization maneuvers. Altitude changes should be done with Hohmann transfer orbits initiated with prograde or retrograde burns, and orbital plane changes should be done with +/- normal burns at ascending or descending nodes (defined by where your target orbit plane crosses with your original orbit).

Orbiter definitely offers an unfair advantage in understanding how orbital mechanics work, and while this game is... different in many ways, the basic flight principles are the same.

[shadowfax]

First, I should clarify that I\'m not pointing straight at the planet when I do this - I dip my nose a little below the horizon.

You\'re assuming an acceleration determined only by gravity. If you use the gravity to help accelerate you to a given velocity, and then use engines to maintain that velocity, you can (in KSP anyway, from my experimenting) get to your end velocity faster because the really hard work of accelerating was helped along by the planet. This won\'t make you faster than if you launched and went straight to an escape velocity, but it does work if you\'re simulating a launch to normal orbit, and then an escape burn (for instance, you launch to a normal orbit, then take on fuel from an orbiting supply, and then head out). With most of the rockets that I build (other than the ones that are intentionally stupid) I have boatloads of fuel once I reach orbit, and so I tend to measure 'cheaply' as a function of time rather than fuel.

[nivvydaskrl]

'In astronautics, the Oberth effect is where the use of a rocket engine when travelling at high speed generates much more useful energy than one at low speed. Oberth effect occurs because the propellant has more usable energy (due to its kinetic energy on top of its chemical potential energy) and it turns out that the vehicle is able to employ this kinetic energy to generate more mechanical power.' (http://en.wikipedia.org/wiki/Oberth_effect)

The rocket is more efficient at a higher speed.

[Herra Tohtori]

Ah, that.

Incidentally, that\'s also the reason why you can\'t really define 'power' as a meaningful physical specification for a rocket engine (or jet engine for that matter). I was thinking of the gravity sling phenomenon, which has popped up here a few times before.

[Guest LoSboccacc]

The rocket is more efficient at a higher speed.

a question remains: does it work also in this game that way?

[Zeroignite]

a question remains: does it work also in this game that way?

It\'s a secondary physical effect due to dumping propellant at the bottom of a gravity well, so I\'d be astonished if it didn\'t occur.

[Rickenbacker]

Well, the correct way to de-orbit from a high orbit, would be to transfer into a much lower orbit (just above the atmosphere) so you can re-enter at a survivable angle. Just burning retrograde to slow down works too, but will probably result in splattering the kerbonauts against the back wall, shortly before they burn to a crisp. Or will do, once those effects are in.

[thorfinn]

Well, the correct way to de-orbit from a high orbit, would be to transfer into a much lower orbit (just above the atmosphere) so you can re-enter at a survivable angle. Just burning retrograde to slow down works too, but will probably result in splattering the kerbonauts against the back wall, shortly before they burn to a crisp. Or will do, once those effects are in.

Why? If you can survive lunar return, you can survive return from any Earth orbit. The entry angle is adjustable in both cases, though the corridor will become narrower the higher you start from. In the real world, it\'s true that some aerodynamic lift is needed to keep a vehicle inside a lunar return corridor; in KSP, only Harvester knows for now... (though I suspect that he will make things so that it\'s the case here too)

[shadowfax]

Why? If you can survive lunar return, you can survive return from any Earth orbit. The entry angle is adjustable in both cases, though the corridor will become narrower the higher you start from. In the real world, it\'s true that some aerodynamic lift is needed to keep a vehicle inside a lunar return corridor; in KSP, only Harvester knows for now... (though I suspect that he will make things so that it\'s the case here too)

Because in order to de-orbit in a reasonable amount of time from a high orbit, you have to come in at a steep angle, which is exactly what happens with lunar return. You then do a burn to change your orbit so that you enter the atmosphere at a shallow angle, whether that burn is at the end of a lunar-earth transit or a high earth orbit. You COULD do a shallow de-orbit burn from a high orbit, but you\'d be up there for a very long time slowly spiraling down toward the atmosphere. That\'s fine if you have a boatload of oxygen, water, and food available, and you\'re willing to spend all that time doing nothing but falling very slowly, but otherwise, you need to descend a bit faster initially.

[rdfox]

Again, if you can survive a direct lunar re-entry, you can handle any orbital re-entry without exceeding thermal or G-load limits. Your entry angle will be flatter, and interface speed will be lower than you\'d get in a direct lunar re-entry.

[shadowfax]

Again, the more you slow down, the steeper your reentry angle is. If you slow down enough to deorbit from a high orbit in a reasonable amount of time, your reentry angle will be too steep. You need to make the angle shallower or you will not survive. Also again, when they came back from the moon, they set their trajectory so that they would reenter at a shallow angle, which is possible when you\'re intercepting the planet, but not possible with a single burn when you\'re already closely orbiting it unless you want to spiral down for an unreasonably long time. If they just bombed straight for the earth, which is what would happen if you slowed down too much in a high orbit, they\'d have come in too steep. Instead, they set their course so that they would intercept earth just to the side, rather than head on.

[rdfox]

Why would you need to take an unrealistically long time 'spiraling down?' You enter a transfer orbit to a perikee of about 30km, you get atmospherinc interface half an orbit later at a shallower angle than you\'d get on a direct entry from any other planetary body, with a lower speed, and you deorbit nicely with temp and g-loads in limits. The only way you\'ll end up with an excessively steep entry is if you try to shorten distance from deorbit burn to atmospheric interface to less than about half an orbit.

[Entroper]

There are no (unpowered) spiral orbits. If you\'re slower than escape, you\'re elliptical.

[Herra Tohtori]

There are no (unpowered) spiral orbits. If you\'re slower than escape, you\'re elliptical.

Except in reality where all spacecraft at low Earth orbit encounter a continuous small drag force from the thin atmosphere that doesn\'t have any clear border, just asymptotically approaches perfect vacuum.

All spacecraft in reality experience constant reduction in their peri- and apoapsis altitude.

[Guest LoSboccacc]

I thought we were talking about this game and not reality.

also, not every real spacecraft experience that drag, specially at apoapsis. heck, some of them doesn\'t even has an apoapsis ;P