A repeated question about spaceplane reentry with MK1 cockpit

[SalehRam]

Hello everyone,

 

I have this bad time with a spaceplane I am trying to get it to re-enter Kerbin from an orbit ranging between 75k-75k to 90k-100k, things are fine up to the point I reach an altitude of 40k...

The plane is using MK.1 cockpit, and MK.1 passenger cabin, with the rest of it using MK.2 fuel tanks...

I am unable to lose enough speed with the reentry process, I set my PE to various heights while testing including 30k, 35k, and 40k, and when I hit the atmosphere at 69k, I do it with an AoA of 90 degrees...

My only "semi-working" attempt was so far I keep the AoA to 90 as long as I can with my RCS, then once I hit below 50k, I just do massive pitch up/down as quick as I can to force more drag and speed bleeding... But this is very edgy process, as I get the cockpit dangerously overheated while I am doing that, and if I ease a bit on the pitching maneuvers (they are just crazy fast ups and downs), it just explode.

I am wondering if there is no point of wasting time on MK.1 cockpit, as I feel it is very weak, and instead to use MK.2 cockpit...

 

Appreciate your feedback and input!

Regards

[m4ti140]

Protip: if you lower your AoA you get more lift in this case (even if stall is not modeled you only produce drag at 90 deg) and thus descend more slowly, so you shouldn't heat up this much.

[Gojira1000]

I drop Mk1s in all the time at about a 45-60 pitch from 75-120km. They do get toasty, but should get through. At around 45 degrees you can skip along through the upper atmo a bit longer. I'm usually setting my pe somewhere between 25 and 35. Your Mileage May Vary.

[SalehRam]

I have already tried having my AoA between all these values, from 90 down to 30 degrees, but still overheating... My atmosphere entry speed usually is about 2300m/s +/-... do I need to get it slower than this?

[mrclucks]

Try setting your periapsis to 50km.  Not sure about 1.1 but in 1.0.5 I would set my periapsis at roughly 45-50 km and would keep my AoA around 25-50 and then negative if it started having altitude again

[LameLefty]

29 minutes ago, mrclucks said:

Try setting your periapsis to 50km.  Not sure about 1.1 but in 1.0.5 I would set my periapsis at roughly 45-50 km ...

I posted a topic asking more or less exactly like this in the 1.1 Pre-release forum several weeks ago. THIS (^^^) is the answer I got that has served me very, very well. Set the Pe right at 50-51 km and point your nose straight up at the zenith - yes, a 90º AOA for as long as you can hold it. You will slow down fairly quickly in the upper atmosphere, well before you get to the thick, dangerous stuff below 40km and heatsplode. Since I learned this trick, I haven't had a single failed spaceplane re-entry, entering from 75km - 125km orbits. 

In fact, this technique even works to return a spaceplane from Minmus (!!!)  There's a scenario in 1.1 that has you stranded in Minmus orbit with an Aeris 4B spaceplane. I played it just yesterday. You have to dock with a nearby tanker, refuel and get home. I played it last night. Make a direct return to a Pe of 50km, stay nose up all the way through. I required one or two aerobraking passes, but it works. If you fully fuel from the tanker, you'll also have enough fuel to cruise back to KSC on turbojets if you miss your entry and come in too short or too long. 

[purpletarget]

You may want to relook at this one in 1.1.1

As per the Patch Notes:
 

Quote

* Tweak skin<->internal conduction, slightly increase internal max temps of Mk1 cockpits.

If it's still an issue, then feedback will always be good to see in the bug tracker. 

 

 

[NathanKell]

I looked at this a fair bit. I think what's going on is people have too high a periapsis, not too low. The Mk2 and Mk3 parts are designed to be able to reradiate well; they have a nice high max skin temp, and a higher internal max temp. By contrast, the Mk1 parts need to not sit there and bake. So try for a 20km periapsis instead. Fly at the maximum AoA you can--85 degrees or so until you can't hold it that high, then keep the nose as high as you can. With that I reentered the Aeris 4A with no trouble at all.

Remember that heat is proportional not to density but the square root of density, whereas speed loss is proportional to density. That means that you should get lower faster, up to the limit of the flux your parts can withstand.

[Alshain]

I agree with @NathanKell, I drop my Pe down to 10km.  The lower your Pe, the faster you hit the hard air, the hotter it gets but also the faster you slow down (speed = heat in the atmosphere), this is better than the alternative which is sitting less hot air for a longer duration because the air isn't slowing you down fast enough.  Heat is cumulative, so extended periods of low heat ends up being worse than a quick extremely hot burst through.   Really if you enter the atmosphere at 69,999km with a positive Pe, you are probably going to be ok, so don't worry about being too steep, as long as you don't take it negative (until you start getting slowed by the atmo of course).

Edited April 29, 2016 by Alshain

[Fortunateson1969]

On 4/23/2016 at 0:02 AM, SalehRam said:

I have already tried having my AoA between all these values, from 90 down to 30 degrees, but still overheating... My atmosphere entry speed usually is about 2300m/s +/-... do I need to get it slower than this?

Okay, so I just landed one of my shuttles just to make sure my info was as up to date as possible.

On 4/28/2016 at 9:04 AM, LameLefty said:

Set the Pe right at 50-51 km and point your nose straight up at the zenith - yes, a 90º AOA for as long as you can hold it. You will slow down fairly quickly in the upper atmosphere, well before you get to the thick, dangerous stuff below 40km and heatsplode.

I can't affirm the rest of the post but the major point worked for me.  Here's my setup:

I have a passenger shuttle with the Mk1 Cockpit, two Mk1 crew cabins, an advanced inline stablilizer, two (per wing) big elevons, a bunch of RCS and the puff RCS thrusters.  I have my Mk-55 Thuds on there, but at this point they're just for show.  I do not have any liquid fuel or oxidizer on the craft at this point.  I have arrays of Place-Anywhere 7 Linear RCS ports pointing up, down, left and right on the nose and two of those plus two RV-105 RCS Thruster Blocks on the tail.  I've attempted to put as much of the weight as aft as possible.  I have my spacedock at 71km.  These little baby Mk1 shuttles can make it easy to 250km easy but coming back from 250km is the hard part.  Haven't nailed that one yet.

1.) I leave my spacedock topped up with RCS.  For me that means 377.5 units of RCS.

2.) When I am west of the impact crater on Kerbin, I burn retrograde until my Pe = 50km.  (I'm still dialing in my burn location, I usually end up east of the runway.  I would like to undershoot than overshoot, but my shuttle is maneuverable enough while gliding that I can usually muscle it back over to land and not get my crew wet.  Or dead.)

3.) (Here's where people may have qualms) I engage mechjeb *boos and groans* Yeah yeah.  Anyway.  I engage mechjeb.  My settings are SmartA.S.S.->Surf->SVEL+->ROL = 0, PIT = 90, YAW = 0.  I turn and leave RCS on, making sure that SmartRCS->Use RCS for Rotation = ON.  (TLDR; Pitch up.  Way up.  90 degrees up.  Hold it fast and hard)

4.) I descend.  For a long time.  Long.  Eventually, the drag on the craft will overcome the reaction wheels and the RCS thrusters and you'll nose down.  You'll be well into the fire and flames at this point.  

5.) When my vertical speed reaches around -10m/s (the dial on your altimeter), I disengage Smart A.S.S. entirely as well as the RCS jets.  I fly it in by hand using the SAS on the craft to maintain stability.  Usually as I drop down through 1000m/s Surface Speed I start to regain control and can bank or do what I need to (because historically I've overshot my entry vector by 10km or so).  However, look at your Mk1 Cockpit.  For me, at this point the skin temperature is decreasing.  The internal temp usually is still on the rise, but won't hit catastrophic levels before it too starts to cool.

6.) Bring her home.  I usually keep the pitch at about -10 degrees for the remainder of the flight.  I'm gliding at this point, and that puts me at about 1000m @ 100m/s.  Nice, slow, gentle landing.  Thanks for flying the friendly skies.

Notes:  Make sure you have a bunch of control of your craft during your descent.  I brought a shuttle back from orbit and forgot to turn the elevons back on (had their limiters at 0) and I burned up and exploded.  Not enough RCS juice?  Burn up and explode because you haven't decelerated enough.

 

If you want, I'd be happy to run another shuttle landing and post the profile, weight, and craft pictures.  The "pilot involved" steps are pretty obvious though.  I don't pay much attention to speeds and such as I'm re-entering.  Just hold that 90 deg pitch as long as possible, and even when it isn't possible.  Any pitch will continue to decelerate you.  This is what worked for me.  NathanKell and Alshain seem to have different approaches...and while I won't discount their experience, it didn't work for me...I've literally tried every altitude from 65km to 0km with rcs on and off (and a bunch of other modifiers) and this is the only way that worked for me.

 

TLDR: 60km periapsis x 71km apoapsis, pitch @ 90deg, assisted with stabilizers, elevons, and RCS.  Pitch will drop naturally, but try to keep it as high as possible until vert speed = -5m/s.  

[mk1980]

interesting. i usually do a much more aggressive deorbit, which works niccely for me. if i can afford the slightly more expensive burn, i do the retro burn about halfway above the desert on the continent west of KSC and burn the PE below 0. basically i burn until the projected trajectory intersects the ground about halfway in the gulf/ocean east of KSC. 

seems to work nicely. mk1 parts get hot but usually survive. mk2/mk3 are often down to safe speeds without even showing heat bars. whenever i try to bleed off speed in the higher atmosphere (PE at 50km or something), it just builds up lots of heat and doesn't really slow down. so my reasoning was - if the braking effect only really kicks in at ~35km, i want to get down to 35km ASAP so the parts are exposed to the heat for only a short time.

with the retro burn above the desert continent, i'm usually down to ~35km somewhere near the west coast of the KSC continent and then speed drops very quickly to "safe" velocities where even Mk1 parts start cooling down again.

seems to work, so i thought that's actually the way it was supposed to work. 

[biomecaman]

I circumvent the 2000 degree threshold by adding radiator panels to the underside of the cockpit thereby raising the temp threshold to 2500 deg.

not sure if that helps you out. its a bit of a cop out

[bewing]

1 hour ago, mk1980 said:

seems to work, so i thought that's actually the way it was supposed to work.

It usually does, as long as your RV is not too heavy. Steep works most of the time, very shallow (maximizing drag) seems to work all of the time. It's the middle ground that will kill you every time.

[Fortunateson1969]

2 hours ago, mk1980 said:

seems to work nicely. mk1 parts get hot but usually survive. mk2/mk3 are often down to safe speeds without even showing heat bars. whenever i try to bleed off speed in the higher atmosphere (PE at 50km or something), it just builds up lots of heat and doesn't really slow down. so my reasoning was - if the braking effect only really kicks in at ~35km, i want to get down to 35km ASAP so the parts are exposed to the heat for only a short time.

Hm.  My problem was always skin temp, not built up internal temp.  

I'd put my Freyja Light Shuttle through the rounds again to test that but...eh.  That prospect doesn't really grab me haha.  Until the next round of shuttle development fires up.  I need a way to get liquid fuel into orbit cheaply...and the mk1 liquid fuel tanks are about the only way I can do it right now :\

[SalehRam]

Shallow re-entry, seems to be the best solution for me, I set PE between 45k to 55k, pitch to 60 degrees, and so far things were fine for me...

I have been working on an MK3 crew transport SSTO, which so far was my best SSTO I ever built and the most successful one in control and overall performance...

For cargo deliveries, I have not yet done anything, but I will experience with them in the next few days...

 

Appreciate your input and the great discussion...

[AeroGav]

 

I've done  a few re-entries on my Citation mk1 spaceplane, from LKO.   It does seem to have got harder in version 1.1.2.    The problem is if you come in at low AoA, the wings (Fat 455 airliner wings) overheat, but if you pitch up the mk1 inline cockpits and passenger cabins get exposed to the airstream and are no longer protected by the nose cone.   In some ways this is game balance, you save mass with mk1, but don't expect to aerocapture Kerbin from Eeloo.

I'd probably choose 50km or higher because it's always skin temp that gets me. But for the purposes of the test i chose 35km periapse to make it challenging.  Interesting, what i found is that there may be a better approach than just holding 25 degrees nose up all the way down in Mechjeb.  It might be worth thinking of it like "landing",  ie, come in at best glide angle (6 or 7degrees aoa),  then at 50km the airplane will start to level off by itself due to lift.   At this point i start increasing pitch to try delay the fall into the max heating area (45km) as long as i can, so in ways this is like flaring for a landing.  The key is when to flare - not too late, or you hit the atmosphere with excess energy - nor too early as you'll stall and plunge faster. 

 

[ForScience6686]

I wouldn't waste time on the mk1 for space planes.  It's better suited for airplanes, and the added lift of the mk2 helps, including on re entry.  If you have a hard time keeping nose up, try centering your wings as much as possible.  Many space planes I see have the wings at the rear, and since we all know how levers work, it will naturally pitch the nose down as the forces begin to overwhelm your control authority.  Even more so if you require rcs to maintain pitch as you have a finite range, and I'd rather use that mass for other needs.

Now I haven't dabbled in 1.1 yet, but I used a very steep re entry in my space planes, pe well below surface.  This gets you to the thicker air for better drag, then pitch down to increase glide.  This method is dependant on a high drag ratio.  A heavier craft with a small surface area would probably not survive it.  Granted I tend to use very little wing, but design allows me to hold high aoa during reentry, without rcs.