Jump to content

Skip reentry is very well possile wihout FAR!


Recommended Posts

I was returning my Apollo replica from a Mun flyby. I did a periapsis of 30 kilometers above Kerbin's night side. During entry from orbit you would land immediately, but with 3 km/s I "skipped" out of the atmosphere up to 72 kilometers; after which I had a very shallow descent. I landed on the opposite side of Kerbin from where my first entry was; yes, as a bonus I got to land on the day side.

I did I very well without FAR.

Link to post
Share on other sites

I don't thing that is as much skipping out of the atmosphere due to the aerodynamics of the capsule and more due to the fact that the atmosphere didn't slow you down enough to bring your periapse inside the atmosphere.

Link to post
Share on other sites
Skip reentry is a reentry technique involving one or more successive "skips" off the atmosphere to achieve greater entry range or to slow the spacecraft before final entry, which helps to dissipate the huge amount of heat that is usually generated on faster descents.

...

The basic concept is to 'clip' the atmosphere at such an angle that the craft is 'pushed' back out into space, conceptually similar to a pebble skipping across the surface of a lake. Each time, the craft's velocity is reduced so that it can eventually drop into the atmosphere at a low suborbital velocity.

There does not seem to be any restriction on that to be caused by aerodynamic forces so yes, passing through the atmosphere back to space before you dip to the atmosphere again can be called skip reentry even if it's driven just by inertia.

And by the way, it is possible to do it using aerodynamic forces with a spaceplane under stock aerodynamics, too.

Edited by Kasuha
Link to post
Share on other sites

Be careful :) what you did was simply an aerobraking pass :)

The Apollo Skip reentry is used, of course, to limit the reentry spped and heat, but it's also used to steer the reentry module to target a specific landing zone.

The basic concept is, the reentry module has an offcentered COM, which means the module is tilted relatively to the reentry vector. This tilt ends up 'deflecting' more aire towards one side than the other, relative to the module's path - and by reaction, the module is pushed the other way.

The reentry module combined this with an ability to still perform rolls during reentry - so if they thought they'll overshoot their landing zone, they roll the module so the air is deflected upwards instead of downwards - resulting in the module going down faster, allowing them to target a much more reduced target landing zone.

Link to post
Share on other sites

I do this all the time coming back from Mun. When you do your burn to leave Mun orbit and return get your Kerbin PE around 34km. The result for me is often a skip re-entry with a maximum of 3-3.5g. On the other hand a precision landing would not be easily done like this so only do it if you have a large army of kernels for recovery missions.

Edited by wolfedg
Link to post
Share on other sites

I've been doing this since the Apollo 44th anniversary and watching a docu about the re-entry process. Even thou I don't play with deadly re-entry (or FAR), its now part of my standard return process.

When returning from Mun I aim to set my Pe at Kerbin to 32km, this varies a bit depending on the mass of the craft, 32km works reliably for just the 3man command pod. The skip can sometimes take you half way round Kerbin from where you first touched the atmo, so it does make projecting your landing site kinda hard (I'm sure someone can calc it, not me!).

In the real, the command pod has an offset weight and its blunt end acts to generate some lift. The amount of lift generated can be changed by rotating the command pod so the offset weight adjusts the angle of attack.

Command_Module_Aerodynamics.png

It would be kinda nice to have some aspect of that, but I can see that could be problematic/annoying for everything else if the command pods had an offset weight.

Link to post
Share on other sites

Skip re-entry is flinging back up out of the lower atmosphere for about 1 minute so the computer can make a more accurate landing by rotating the capsule and changing the associated aerodynamics.

What was described in the OP was just aerobraking.

Link to post
Share on other sites

Well, it 'should' be possible to somehow replicate in stock game, with fixed hidden control surfaces in the pod (for lift) a weight to slightly offcenter the COM and a counterweight in the service module (to have a centered COM before separation)

Link to post
Share on other sites

I always thought that skip-reentry was kind of like skipping rocks off of a pond. The mark 1 pod seems to use the rounded surface to act as if the atmosphere was kind of like water, so that is how I envisioned it!

Link to post
Share on other sites
What I did again was to enter the atmosphere, decelerate, fly back up to 72 km, and land on the other side of the planet.

Aerobaking is when your apoapsis is still above the atmosphere after the maneuver. Skip reentry is when it isn't.

Um, if you flew back up to 72km your apoapsis was above the atmosphere.

Link to post
Share on other sites

Let me add one more reference that skip reentry does not necessarily have to involve aerodynamic lift to expel the ship out of the atmosphere.

... However lift is not required for "skipping" out of the atmosphere, since unlike the apparent surface of a pond, the atmosphere is curved. All that is really meant in the case of a skip entry is that the entry flight path angle was not steep enough to prevent the object from leaving the atmosphere again. The trajectory of the ballistic skip flight does not curve up like that of a skipping stone. It still curves down. However the radius of curvature is greater than that of the radius of curvature of the atmosphere, so it departs back into space. ...
Link to post
Share on other sites

Haha. I did my first-ever Apollo style ship recently and I skipped through the atmosphere a grand total of FIVE times... I was actually raging a little bit because I simply wouldn't come down!

Link to post
Share on other sites
I'm not exactly clear what the difference between s*ip reentry and aerobra*ing is. Can one of you saying that this isn't s*ip reentry illuminate me?

What they're saying is that true s*ip reentry requires some sort of aerodynamic force to actually change the flight path. Hitting perigee and rising again isn't an aerodynamic change, it's simply physics. The ground is "falling" away faster than you are, the same as a periapse on an airless body. First you descend, then you rise again, relative to the surface.

What we get in *SP isn't *technically* s*ipping, but it does have most of the same problems & advantages (lower heating & G force, longer battery time needed) so it's close enuf for gummint wer*

Link to post
Share on other sites
What they're saying is that true skip reentry requires some sort of aerodynamic force to actually change the flight path. Hitting perigee and rising again isn't an aerodynamic change, it's simply physics. The ground is "falling" away faster than you are, the same as a periapse on an airless body. First you descend, then you rise again, relative to the surface.

What we get in KSP isn't *technically* skipping, but it does have most of the same problems & advantages (lower heating & G force, longer battery time needed) so it's close enuf for gummint werk

Yeah, that does nothing to distinguish between aerobraking and skipping. Both use aerodynamic forces to change the flight path.

Off topic, anyone else minimizing their use of the letter between J and L because of the April Fools shenanigans?

Link to post
Share on other sites
What we get in *SP isn't *technically* s*ipping, but it does have most of the same problems & advantages (lower heating & G force, longer battery time needed) so it's close enuf for gummint wer*

None of definitions of skip reentry I have found require aerodynamic lift to play any role in the ship return to space. That means what we have in KSP is technically skipping as well.

Aerobraking is part of the reentry and skip reentry is using multiple aerobraking phases to slow the ship down more gradually regardless of mechanics leading to that effect.

If you have any reference to a definition of skip reentry which would require aerodynamic lift, please post it.

Link to post
Share on other sites
None of definitions of skip reentry I have found require aerodynamic lift to play any role in the ship return to space. That means what we have in KSP is technically skipping as well.

Aerobraking is part of the reentry and skip reentry is using multiple aerobraking phases to slow the ship down more gradually regardless of mechanics leading to that effect.

If you have any reference to a definition of skip reentry which would require aerodynamic lift, please post it.

How about wikipedia?

The basic concept is to 'clip' the atmosphere at such an angle that the craft is 'pushed' back out into space, conceptually similar to a pebble skipping across the surface of a lake. Each time, the craft's velocity is reduced so that it can eventually drop into the atmosphere at a low suborbital velocity.

In theory, any craft could perform skip re-entry as it does not require much lift, but in practice it requires precise guidance.

Link to post
Share on other sites
How about wikipedia?

Zero lift also does count as not much lift. And all of the 'pushing' (note it's in apostrophes anyway) can be driven by centrifugal force, a.k.a. inertia.

I have read the wikipedia entry thoroughly and I don't see anything suggesting aerodynamic lift is required. Quite the opposite, it says you don't need it.

In theory, any craft could perform skip re-entry as it does not require much lift, but in practice it requires precise guidance.

That's also the only place where the whole entry contains the 'lift' word. The core of the explanation is above that and no lift is mentioned.

Now I can also provide this material about re-entry from the FAA which says:

But maximum g’s aren’t the only concern of re-entry designers. Too little deceleration can also cause serious problems. Similar to a rock skipping off a pond, a vehicle that doesn’t slow down enough may literally bounce off the atmosphere and back into the cold reaches of space.

And only much later they get to topic of aerodynamic lift, which is used very specifically:

In Sections 4.1.7.1 through 4.1.7.3, we assumed the force of lift on our reentering vehicle was zero, so we could more simply investigate the tradeoffs between re-entry characteristics. Adding lift to the problem takes it beyond the scope of our simple model but gives us more flexibility. For example, we can use the lifting force to “stretch†the size of the corridor and allow a greater margin of error in re-entry velocity or angle.

Also:

For Shuttles, meteors, and ICBMs entering the atmosphere at near

orbital velocities, it turns out that

• The re-entry vehicle is a point mass

• Drag is the dominant forceâ€â€all other forces, including lift and

gravity, are insignificant. (We’ll see why this is a good assumption

later.)

Edited by Kasuha
Link to post
Share on other sites
Zero lift also does count as not much lift. And all of the 'pushing' (note it's in apostrophes anyway) can be driven by centrifugal force, a.k.a. inertia.

I have read the wikipedia entry thoroughly and I don't see anything suggesting aerodynamic lift is required. Quite the opposite, it says you don't need it.

That's also the only place where the whole entry contains the 'lift' word. The core of the explanation is above that and no lift is mentioned.

Now I can also provide this material about re-entry from the FAA which says:

And only much later they get to topic of aerodynamic lift, which is used very specifically:

Also:

Yes they also compared it with aerocapture who is more accurate than aerobraking and the maneuver is similar to an aerobrake except that it leaves you on an pretty flat suborbital trajectory, note that an aerocapture to low orbit tend to leave your Pe below ground and you need to raise it

For landing it reduces both g-forces and thermal shock.

I see the lifting body of the pod as more as an terminal guidance function than anything else. You set the reentry profile before dropping the service module.

Link to post
Share on other sites

"Does not require much lift" is equal to "requires some lift, but not much". All capsules were able to provide some lift. Skip reentry that "requires some lift, but not much" was a feature of Apollo capsule, but was never utilized in manned flight.

Reference here.

Even before the first Mercury flight, however, it was recognized that a purely

ballistic vehicle was not the ideal approach. In addition to g-loads and heating

during reentry, the poor predictability of the final impact point forced the use

of a large contingent of ships spread over a wide area of ocean for recovery.

Ultimately, Mercury was the only U.S. piloted spacecraft to use a purely bal

-

listic reentry without some ability to steer the vehicle.

29

Providing a small amount of liftâ€â€an aerodynamic force perpendicular to

the flightpathâ€â€reduced the severity of reentry and improved the accuracy

of recovery. For the capsules, lift could be created by slightly offsetting the

center of gravity from the vertical axis. The trajectory could then be changed

by rolling the vehicle about its vertical axis so that the offset lift vector could

be pointed in any direction. The roll maneuver could be accomplished via a

reaction wheel (gyroscope) or by using small reaction-control thrusters, which

is the approach that ultimately was selected. The lift could also be directed

upward to maintain a small flightpath angle for as long as possible. Once the

vehicle passed the high-speed heating pulse, it could be banked to turn toward

the desired recovery point, producing a relatively small cross-range capability.

30

A lift-to-drag ratio (L/D) of only 0.2 could cut the maximum deceleration in

half and stretch the range by 280 miles

Link to post
Share on other sites
This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...