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Reinhart Mk.1

Slowing down in an SSTO

Question

So much like my whole Eve adventure im having some trouble on rentry to Kerbin, just to spare everyone with the giant list of details for this mission ill break it down: I just got back from the Juul system but im currently going 4000mps upon rentry and while i COULD just let the ship be destroyed and parachute to the ground... well that hardly seems classy. I KNOW there's ways to slow down your orbital speed before actually encountering Kerbin but all my attempts just seem to end up in gravity assists that seem to ramp my speed up (accidental gravity assist?). Help?

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4000m/s reentry is fine. Doable. All the parts that are exposed to plasma need to have at least 2000K max temp., like Mk2 spaceplane parts for example. Aim for 48-50 km periapsis and max angle of attack (belly first). With luck, you’ll lose enough velocity to capture in Kerbin orbit. Then make another pass with 30km Pe and land. I did this exact thing just yesterday, actually. Aerobraked at Kerbin in a Mk2 spaceplane coming from Jool, nothing exploded.

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6 minutes ago, sh1pman said:

Aim for 48-50 km periapsis and max angle of attack (belly first). With luck, you’ll lose enough velocity to capture in Kerbin orbit.

tried this but i seem to be going so fast it's either i break apart if i try to go lower that 50k and be risky, but if i stay at 50k i just shoot right past it

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Posted (edited)
4 minutes ago, Reinhart Mk.1 said:

tried this but i seem to be going so fast it's either i break apart if i try to go lower that 50k and be risky, but if i stay at 50k i just shoot right past it

Yea, you need to go just a bit lower than 50km, I think I went 48 or so. Almost exploded, some bars were completely red. Try tumbling and rolling when things get too hot to disperse heat

Edited by sh1pman

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Have you tried entering a highly elliptical orbit and skimming the upper atmosphere repeatedly to bring your apogee down? eventually you will be able to dive deeper into the atmosphere as your speed becomes lower. I did this with a probe that had 1200 degree temp parts and my start speed was close to yours.

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55 minutes ago, Reinhart Mk.1 said:

I KNOW there's ways to slow down your orbital speed before actually encountering Kerbin but all my attempts just seem to end up in gravity assists that seem to ramp my speed up (accidental gravity assist?). Help?

Do you have a copy of your save? Is it Stock or are there a collection of modded or DLC parts?

Coming back from Jool it's simple enough to use Kerbin for a gravity assist at least once. This will lengthen your trip by two Kerbin years if done right, but it should reduce a good chunk of your speed. Assuming you're encountering Kerbin at your craft's Sun Periapsis, passing in front of Kerbin should lower your Sun Apoapsis some. Just adjust your Kerbin PE so the resulting orbital period is about two Kerbin years. The next pass should then be a little easier re-entry wise.

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2 minutes ago, Dale Christopher said:

Have you tried entering a highly elliptical orbit and skimming the upper atmosphere repeatedly to bring your apogee down? eventually you will be able to dive deeper into the atmosphere as your speed becomes lower. I did this with a probe that had 1200 degree temp parts and my start speed was close to yours.

YES, but i dont think im doing it right or even understand 100% what that means... like when i get into orbit of the sun my peri is low enough for Kerbin but my apo is still very high

you are all so smart and im just barely winging my way through this game and i feel bad that all of you reply with great information but im just too dumb to really absorb it put to good use ;.;

7 minutes ago, Gordon Fecyk said:

Do you have a copy of your save? Is it Stock or are there a collection of modded or DLC parts?

and nah no mods or DLC just stock parts

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Posted (edited)

I just mean, can you skim the atmosphere enough on your encounter to enter any orbit of Kerbin without blowing up? Even if you only touch 68km that might burn off enough speed to let you enter into a highly elliptical orbit of Kerbin, even if your apogee is over 70 million km you can just spend a heap of orbits skimming 68km until you don’t generate as much heat and then carefully over successive orbits bring your skims lower and lower  67, 66, 65, 60 etc... 

At least that’s what I think you’re trying to do. I might be reading wrong.

Edited by Dale Christopher

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1 minute ago, Dale Christopher said:

mat least that’s what I think you’re trying to do. I might be reading wrong.

no that's correct! the only problem is i can't get into orbit, i just either fly by or my ship would be HEAVILY damaged on rentry at lower alts (also dont have enough fuel to circularize either)

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1 minute ago, Reinhart Mk.1 said:

no that's correct! the only problem is i can't get into orbit, i just either fly by or my ship would be HEAVILY damaged on rentry at lower alts (also dont have enough fuel to circularize either)

How deep do you usually go into the atmosphere. It starts at 70km 

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@Gordon Fecyk i could upload a small video to show you

1 minute ago, Dale Christopher said:

How deep do you usually go into the atmosphere

lowest was around 37, that was bad immediately, then i went for 47 and that was...better? then i was around 57 and i could avoid the damage but i couldn't get low enough to do an aerobrake

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1 minute ago, Reinhart Mk.1 said:

@Gordon Fecyk i could upload a small video to show you

lowest was around 37, that was bad immediately, then i went for 47 and that was...better? then i was around 57 and i could avoid the damage but i couldn't get low enough to do an aerobrake

That’s strange. I would have thought going past Kerbin at 4k m/s and dipping down to 60km altitude would have been enough to let you enter an orbit. 

Im just guesstimating in my head though, maybe someone else will be able to help >_<

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Just now, Dale Christopher said:

Im just guesstimating in my head though, maybe someone else will be able to help >_<

it MIGHT be like 4.5 mps though i have to check again, i have two different saves and im trying to replan the trip haha

but no man! any bit helps and you were definitely helpful, i just always get myself in odd shenanigans in this game lol

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My typical solution to this sort of problem was to do a gravity assist or two off of Kerbin. If you're coming back from Jool, you should be able to pass by the nightime side of Kerbin around 175km to drop a bunch of speed for a future encounter a few years down the line.

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I just realized i could get through this but i could also get @Matt Lowne to rescue me which would be cool to appear on one of those episodes but im not gonna tap out :(

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Do you have any fuel left?  The really important thing is capturing on the first trip through atmo.  Once you're in Kerbin orbit you can slowly bleed off speed in pass after pass.  So it's generally worth it to use almost all your fuel on a capture burn, and just leave a bit left for maneuvering.  

Are you angling your plane to be as draggy as possible on your atmosphere passes?

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19 minutes ago, Reinhart Mk.1 said:

I just realized i could get through this but i could also get @Matt Lowne to rescue me which would be cool to appear on one of those episodes but im not gonna tap out :(

Definitely give the assist a shot before you tap out. If you've never had an opportunity to try one before no time like the present, am I right? :D . The way to do it: When your craft is about a couple weeks away from Kerbin's SOI, you can maneuver the Periapsis to be ~150km-200km away from Kerbin (I believe on the night side). You'll notice when you do this, the craft will still fly through Kerbin's SOI, but when it emerges your Apoapsis with respect to the sun will be lower than Jool's orbit. The lower you can get that Apoapsis the better! If you put a maneuver node out at the Ap you can click the button that lets you see Kerbin close encounters on future orbits. Simply try different Periapsis altitudes until you find one that gives you a nice close encounter with Kerbin again somewhere a couple years down the line. When you finally return, you'll have much less speed than before, so if the reentry is a close thing now it should become easy. When done properly, this should only takes 20-50m/s of deltaV.

Good luck!

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Posted (edited)

I tried testing a bit and yer it might be hard if your craft dosent have enough drag.

Spoiler

Stardate: 9.7.2019

Test shuttle name: Ascent from Chaos

Mission: Deceleration from interplanetary trajectory 

XD

Ascent-from-Chaos-01.png

Ascent-from-Chaos-02.png

I was able to capture but I could only get up to 3400 m/s with the deltaV I had. Have you tried burning all your DV  retrograde just before perigee while deep in the atmosphere? maybe that would give you the extra juice to capture?

Edited by Dale Christopher

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Posted (edited)

Here's how I slow down from orbital speeds:

Doing it a'la SpaceShipTwo is very effective.

FF01flight_Photo-by-Clay-Center-Observat

Edited by Wjolcz

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3 hours ago, Wjolcz said:

Doing it a'la SpaceShipTwo is very effective.

Rotation servos from BG make it easy. However, what I noticed is that feathered wing configuration doesn't really help you to slow down more than fixed wings. It just makes reentry orientation very stable. If you have issues with overheating at high speeds (>4km/s), it won't help.

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Posted (edited)
42 minutes ago, sh1pman said:

Rotation servos from BG make it easy. However, what I noticed is that feathered wing configuration doesn't really help you to slow down more than fixed wings. It just makes reentry orientation very stable. If you have issues with overheating at high speeds (>4km/s), it won't help.

Correct me if I'm wrong but from my experience slowing down early on should make the ship heat up less, shouldn't it? That's why the best way to aerobrake is to slow down by perofrming multple passes in the upper atmosphere and then, once the ship isn't going interplanetary speeds anymore, enter the atmosphere for landing.

I'm also assuming we are talking about spaceplanes here since they are known for orienting thmselves nose-first (making them aerodynamically stable and very fast) then having proper viking burial.

I do realize that it depends a lot on the ship and how it enters the atmosphere, but a bigger surface will slow you down more. Sure, the crew will experience a lot of gees and black out but if the probe core is active everything should be fine. You should either maximize your surface or change the entry/aerobrake profile.

BTW, @Reinhart Mk.1 may we see your ship, please?

Edited by Wjolcz

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1 minute ago, Wjolcz said:

I'm also assuming we are talking about spaceplanes here since they are known for orienting thmselves nose-first and not slowing down quickly enough then having proper viking burial.

Yes, I'm talking about spaceplanes, having in mind a typical Mk2 spaceplane. Their natural reentry orientatnion usually depends on where thier CoM is relative to CoL. But if we're talking about super-fast interplanetary reentry (4km/s), it doesn't really matter because much of your aerobraking will happen in the upper atmosphere (47-50 km Peri, any lower and you'll burn up), and at these altitudes the atmosphere is thin enough that you can keep belly-first orientation with just RCS and reaction wheels. 

10 minutes ago, Wjolcz said:

That's why the best way to aerobrake is to slow down by perofrming multple passes in the upper atmosphere and then, once the ship isn't going interplanetary speeds anymore, enter the atmosphere for landing.

If you can't slow down from an interplanetary speed to ~3km/s in one pass, you're going to be thrown back into an interplanetary space. That is usually an undesirable result. Once you're captured in Kerbin orbit, you can take as many passes as you like.

14 minutes ago, Wjolcz said:

I do realize that it depends a lot on the ship and how it enters the atmosphere, but a bigger surface will slow you down more.

Yes. That's why the best orientation is when the whole spaceplane is pointing straight up. At ~50km good enough RCS is all you need to keep it.

Just yesterday I tried reentry with my spaceplane coming from Jool. With Pe any lower than 47 km it burns up regardless of feathered or normal wing configuration. But with normal configuration, it does slow down a bit better (more wing area exposed to reentry plasma), enough to get captured in Kerbin orbit.

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16 hours ago, Dale Christopher said:

Have you tried entering a highly elliptical orbit and skimming the upper atmosphere repeatedly to bring your apogee down?

The problem is that to be in a closed orbit around Kerbin, you can't be going much over 3000 m/s at Pe-- otherwise, you're over escape velocity.  If he's arriving at 4000+ m/s, that means he has to shed at least 1000 m/s on his first aerobraking pass, otherwise he'll just go flying past Kerbin and back out into solar orbit.

And shedding 1000 m/s of speed from aerobraking requires diving fairly deep into the atmosphere; if you're over 50 km, it'll heat you up a lot but not slow you down by any appreciable amount.

16 hours ago, Dale Christopher said:

Even if you only touch 68km that might burn off enough speed to let you enter into a highly elliptical orbit of Kerbin

It doesn't work that way.  If you only graze the atmosphere and your Pe is 68 km, it won't slow you down at all.  You'd be lucky to lose 1 m/s of speed.  You need to lose 1000+, and there's simply no way to do that without getting down into the "meat" of the atmosphere.  The problem is that reentry gets hot at a much higher altitude than it gets draggy.

 

One option would be to use rocket engines to slow the approach, of course, but that assumes having the dV to spare.

 

16 hours ago, sh1pman said:

Aim for 48-50 km periapsis and max angle of attack (belly first). With luck, you’ll lose enough velocity to capture in Kerbin orbit.

16 hours ago, Reinhart Mk.1 said:

tried this but i seem to be going so fast it's either i break apart if i try to go lower that 50k and be risky, but if i stay at 50k i just shoot right past it

One thing that matters a lot is your ship's ballistic coefficient.  Without going into fancy details:  ship design matters.  In particular, is it big-and-draggy, or dense?

If you've got a relatively lightweight vehicle that's got acres and acres of wing surface, it can do very well with reentry, because when you enter belly-first (i.e. holding all that wing surface perpendicular to the airflow), it generates a lot of drag relative to its mass, so it slows down rapidly and doesn't get fried.

On the other hand, if you've got a heavy / dense vehicle with little wing surface, i.e. built like a javelin... then it's gonna have problems, because it's hard to generate enough drag to slow down.  It'll punch through the atmosphere without losing much speed, and the only way to slow down enough would be to dive really deep, which then fries it.

Moral of the story:  A ship that is lighter weight, and has more wing surface, will do better at spaceplane-style reentry than one that's heavier / denser / less wing area.

@Reinhart Mk.1, could you post a picture of your ship?  Might help in diagnosing the problem.  Maybe you just need more wing.

 

15 hours ago, Dale Christopher said:

That’s strange. I would have thought going past Kerbin at 4k m/s and dipping down to 60km altitude would have been enough to let you enter an orbit.

Not even vaguely.  60 km is plenty hot enough to fry you if you're going fast enough, but it'll never slow you down to any appreciable degree in just one pass.

 

14 hours ago, Aegolius13 said:

The really important thing is capturing on the first trip through atmo.  Once you're in Kerbin orbit you can slowly bleed off speed in pass after pass.

If he's coming in at 4000+ m/s, that means he has to bleed off like 1000 m/s of speed on his first pass, which is also when he's going the fastest (and therefore hottest).

Surviving that first pass is the key.  If he has a ship that can aerobrake 1000+ m/s in one pass and survive, then he's good to go.  If he doesn't, then it's hopeless.  If he does survive that first pass... there's no need to "slowly bleed off" on multiple passes, because he'll only need one more pass at most.  If he could bleed off 1000+ m/s on the first pass, then he'll be able to do that even more easily on the second pass when he's slower, and that puts him down to suborbital speeds.

14 hours ago, Aegolius13 said:

So it's generally worth it to use almost all your fuel on a capture burn, and just leave a bit left for maneuvering.

Definitely agree with this.  :)  There's no point in landing with any fuel.  Burning the fuel slows you down (thus making your reentry easier), and also makes your craft lighter (and therefore slows down faster from drag), which also makes reentry easier.

 

3 hours ago, sh1pman said:

Rotation servos from BG make it easy. However, what I noticed is that feathered wing configuration doesn't really help you to slow down more than fixed wings. It just makes reentry orientation very stable. If you have issues with overheating at high speeds (>4km/s), it won't help.

^ This.  Honestly, all that matters is that you have your wings facing perpendicularly to the airstream.  If you simply point your nose :radial: and hold it there, then that gives you exactly what you need, without requiring any servos at all.  You just need to design your craft's balance so that it can hold an attitude like that, or close to it.

 

3 hours ago, Wjolcz said:

Correct me if I'm wrong but from my experience slowing down early on should make the ship heat up less, shouldn't it? That's why the best way to aerobrake is to slow down by perofrming multple passes in the upper atmosphere and then, once the ship isn't going interplanetary speeds anymore, enter the atmosphere for landing.

You're right for some cases, but (I believe) wrong in the current instance.  It depends on situation.  In particular, there's a world of difference between starting out already in orbit, versus approaching on an interplanetary / escape / hyperbolic trajectory.

Let's take the easy case first.  Let's say you're already in an orbit around Kerbin, with your Ap waaaay out near the edge of Kerbin's SOI, and Pe is brushing atmosphere.  This means your speed will be slightly over 3000 m/s at Pe.

Well, in that case, you're already in a closed orbit, which means you have the luxury of doing as many aerobraking passes as you need.  If you want, you could set your Pe to something really high, e.g. even over 60 km.  Each pass, you'll slow down only a teeny tiny bit, whereas you'll heat up a lot.  But the heat-up-a-lot doesn't matter, since it's not heating you enough to actually explode, and you can cool off again as soon as you leave atmosphere.

In this way, it will take many many passes to bring your Ap down and shed speed, but hey, why not?  You've got all the time in the world.

However... if the approaching ship is on a hyperbolic trajectory-- i.e. it's going way higher than Kerbin's escape velocity-- then the situation is completely different.  Because then, you have to shed a whole lot of velocity on the very first pass, so "just barely graze atmosphere" simply won't work.  You have to plow deep into the atmosphere to knock off tons of speed in a single pass.  The dip-a-toe-in-the-water strategy simply won't work.

 

3 hours ago, Wjolcz said:

That's why the best way to aerobrake is to slow down by perofrming multple passes in the upper atmosphere and then, once the ship isn't going interplanetary speeds anymore, enter the atmosphere for landing.

So this is true only if you're already in orbit and have the luxury of making many little passes.  Otherwise, the exact opposite strategy (plow right in, to a deep level of the atmosphere) is often better.  Depends on circumstances (i.e. trajectory) and ship design.

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This question about how to play the game has been moved to Gameplay Questions. 

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You can use the Mun for a gravity slingshot -- you can often bleed off several hundred m/s at the Mun. That's often enough to make the difference between a survivable aerobraking and not. It depends on how much dV you have left. You always need a little to make minor adjustments to your Ap and Pe on aerobraking passes.

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