Best entry angle for shuttles?

[CaptainPanda]

Splashdowns are easy. Landings are hard.

I usually wind up coming in too steep too late, or come in too shallow and go through a long reentry where I don't lose altitude fast enough. Either way, I usually wind up overshooting the runway at KSC and ending up in the drink (or the island runway, if I'm really lucky). Can anyone advise me on the best reentry path for 'aircraft' style shuttles from an orbit of about 100 km to land on the runway at KSC?

[bewing]

It depends on how much drag your shuttle has when it's flying prograde, how much heat tolerance it has, and just how much control authority it has in thin air. And whether you need to line up on the runway from a long distance, or whether you have enough maneuverability to keep making major adjustments up until the end.

So it'a per-design thing.

You can usually come pretty close by having some landmarks. 2100 m/s by the east coast of the Great Desert. 1900 m/s at the west coast of the KSC continent at 35km altitude. 1500 m/s at the mountains west of KSC at 22km altitude. Something like that. If you are too high or too fast, then the landmarks let you know that you need to make adjustments. Usually by telling you that you need to do some extreme maneuvers to scrub some speed and/or altitude.

 

[Spricigo]

Kind of issue that is solved in 3 steps:

1.practice

2.practice

3.practice

In any case the best path is craft dependent. How much heat it can resist and which AoA  it is able to hold make a huge difference.

My spaceplanes tend to rely more on thrust than lift. Such flying bricks reentry with Pe below the ground a bit ahead of KSC coming with little AoA, miss the "too short" mountains by some hundreds meters and land without much style at the runaway.

Mind you, I'm more of a rocket guy. Someone else probably will offer better advice.*

Edit: *e.g. @bewing post. Which I didn't notice because how long took to post mine.

 

 

Edited November 4, 2017 by Spricigo

[invision]

 

[CaptainPanda]

2 hours ago, bewing said:

You can usually come pretty close by having some landmarks. 2100 m/s by the east coast of the Great Desert. 1900 m/s at the west coast of the KSC continent at 35km altitude. 1500 m/s at the mountains west of KSC at 22km altitude. Something like that. If you are too high or too fast, then the landmarks let you know that you need to make adjustments. Usually by telling you that you need to do some extreme maneuvers to scrub some speed and/or altitude.

 

So, using those landmarks as guidelines, is there a good general rule of thumb as to where to begin a deorbit burn, and where to aim the flight path's intersection with the ground?

@invision, does this guide account for lift from spaceplane wings? Are the guidelines pretty much the same?

Edited November 4, 2017 by CaptainPanda

[bewing]

3 hours ago, CaptainPanda said:

So, using those landmarks as guidelines, is there a good general rule of thumb as to where to begin a deorbit burn, and where to aim the flight path's intersection with the ground?

Some spaceplanes need to spend a lot of time above 58km to bleed off the first couple hundred m/s of speed. For those, you need to start in a very low orbit and aim your deorbit burn to put the Pe 200km past KSC (above the ocean) at about 60km altitude.

Other spacepanes have high heat tolerance and can dive in quicker. I put the Pe right over KSC for those, at about 50km altitude.

And then you always should come in with an AoA of 90 degrees, for as long as you can hold it.

You should never set the orbit of a spaceplane to intersect the ground. That wastes fuel. Spaceplanes have the ability to stop by themselves -- only rockets should intersect the ground.

 

Edited November 4, 2017 by bewing

[Geschosskopf]

8 hours ago, bewing said:

And then you always should come in with an AoA of 90 degrees, for as long as you can hold it.

This^ is key.  Coming in belly-first maximizes drag so you slow down quickly.  The quicker you slow down, the less time you spend burning, so the less heat becomes a problem.  It often means that many parts of the ship get only slightly warm instead of 1 or a few parts getting very hot.

 

8 hours ago, bewing said:

You should never set the orbit of a spaceplane to intersect the ground. That wastes fuel. Spaceplanes have the ability to stop by themselves -- only rockets should intersect the ground.

I do the landmarks a bit differently.  I find that on any stock planet with an atmosphere, if I begin my descent from low orbit about 90^ of longitude ahead of where I want to end up on the ground, I come out OK.  That profile seems to give good accuracy and keeps things from getting too hot.  I also recommend using Trajectories.

There is a difference, however, between a shuttle (assumed to be essentially a glider) and a spaceplane (assumed to have a fair amount of powered flight available once in the atmosphere).  With a glider, the main thing is, you don't want to undershoot because you can't fix that.  If you overshoot, no bid deal, just turn around or do a split-S and come back.  So with a glider, try to aim your de-orbit burn to come down a bit on the far side of your target, to make sure you don't undershoot  

[Spricigo]

8 hours ago, bewing said:

You should never set the orbit of a spaceplane to intersect the ground. That wastes fuel. 

Oh well, convenience has its price. I'd rather use that bit of fuel and bring that craft down quick than worry about a few extra funds.

 

[CaptainPanda]

1 hour ago, Geschosskopf said:

This^ is key.  Coming in belly-first maximizes drag so you slow down quickly.  The quicker you slow down, the less time you spend burning, so the less heat becomes a problem.  It often means that many parts of the ship get only slightly warm instead of 1 or a few parts getting very hot.

90^o AoA seems a bit steep, though. I know the space shuttle had a 40^o AoA through reentry, which is what I'm aiming for so that I can avoid spinning out of control. Is a higher AoA really better?

[Geschosskopf]

1 minute ago, CaptainPanda said:

90^o AoA seems a bit steep, though. I know the space shuttle had a 40^o AoA through reentry, which is what I'm aiming for so that I can avoid spinning out of control. Is a higher AoA really better?

Yes, for the reasons stated.  You can only spin out of control if the air is already thick enough that it cares which way your plane is pointing.  The idea is to get all (for smaller, lighter planes) or at least most (for bigger, heavier planes) of your flaming deceleration done before you reach air that thick.  That way, when you do have to start worrying about your orientation, it won't create excessive heat build-up on the nose.

[Spricigo]

55 minutes ago, CaptainPanda said:

90^o AoA seems a bit steep, though. I know the space shuttle had a 40^o AoA through reentry, which is what I'm aiming for so that I can avoid spinning out of control. Is a higher AoA really better?

I'm convinced that the major advantage (if not the only) is style points.

As long you stay below the critical temperature, it makes no difference if heat peaks at 50% or 99%.

Off couse if you come with a higher velocity (e.g. from Mun) it's a different story, but from LKO most spaceplanes can take the heat without such high AoA.

[AeroGav]

If I'm flying an unfamiliar space plane home,  I stay in Map (planetarium) view mode so i can see the predicted trajectory blue line on re-entry, but keep the navball visible so i can see what attitude the airplane is in.      I typically come in with the nose 10 degrees above prograde to start with,   and have the blue line intersect the western edge of the space centre continent or the east edge of the desert continent beyond.   If you see the blue line's impact point with the ground moving further away, with a trend to overshoot,  then you need to worsen your lift / drag ratio so you're more like a ballistic capsule - raise the nose, up to stall if you have to ,  you can also drop the landing gear (makes a small difference).    If you look like you might be coming up short,   lower the nose so it's only 5 degrees above progade,  this is close to your "best glide" angle and will stretch the glide considerably.   As in most of the way round the planet potentially.

3 hours ago, CaptainPanda said:

90^o AoA seems a bit steep, though. I know the space shuttle had a 40^o AoA through reentry, which is what I'm aiming for so that I can avoid spinning out of control. Is a higher AoA really better?

I don't think there's much to be gained coming in steeper than 20 degrees AoA TBH.   What you  gain from higher drag, you loose in terms of less lift, faster descent, and the heat pulse getting concentrated into a shorter space of time.     Just don't come in on prograde lock on a craft without an inline cockpit, or you deserve everything you get.   Craft design plays a large part.    The further back in the stack the crew bits are, the better.  Large wing area means more lift (heat pulse spread out for longer) with the option to create more drag if you pitch up to stall AoA or beyond.

I am not a fan of extreme AoA on re-entry.   Not only is there the risk of spinning out - yes, you can usually regain control lower down, but it's not easy to predict how long that will take and what effect that will have on your trajectory - you are also giving up one of the benefits of being in a spaceplane - cross range capability.    I can retro burn to 30km PE  anywhere on the orbit, and stretch my glide as much or as little as needed to reach the space centre, by coming in at a more moderate AoA.    On something with an inline cockpit (heat tolerant) and generous wing area,  you can aerobrake over a few orbits from an unpredictable interplanetary trajectory,  and stretch/squash the glide into KSC just by manipulating AoA.   Magic !

[ForScience6686]

On 11/4/2017 at 9:48 AM, Spricigo said:

Oh well, convenience has its price. I'd rather use that bit of fuel and bring that craft down quick than worry about a few extra funds.

 

This is what I do.  Intersect at about the island runway, then adjust aoa to keep my trajectory just past ksc.  Once in sight it's quite easy to get on the runway.  I've fine this from a 500km orbit, but it is dependant on your drag.  One tip, if you are coming in to high, roll over and pull up, you still get the drag with the added benefit of increasing your vertical speed.  I also tend to come in too fast and have a means of slowing down once on the final approach.  Drag chutes are also pretty common on my craft.  They help maintain control on landing and require less braking.

[Silverwood]

Hi Folks,

I was about to ask a similar question to this, so am pleased with the answers. I was wondering if any of you had landed the stock Dynawing shuttle? I have managed to get a good reentry twice and a landing once. It seems that once you are around the 30K mark and an AoA of about 50 deg, it starts to spin, yes you can recover, but there must be a better way. As no one mentions the stock shuttle, I wondered how you would do the re-entry and final land. I am quite happy with the last few minutes, drop off the speed and flare around 50 m per second holding off till touch down at around 10 or so. It is the re-entry bit that is a problem for me.

Any help appreciated.

Keith

[bewing]

17 hours ago, Silverwood said:

 It is the re-entry bit that is a problem for me.

Any help appreciated.

Keith

Well, I'm sure there are a million ways to do it. But when I bring up that scenario, the first things I say to myself are:

OK, I'm driving a super-draggy truck that can stop on a dime in midair, because it's so draggy. And this reentry is from a really low Ap, and the Pe is really far down in the atmosphere, and I'm a really long way from KSC.
So clearly, I want to get a hell of a lot closer to KSC before I start doing any braking. So I turn off the RCS to save the monoprop, and go prograde. Then I wait until I'm down to maybe 50km and halfway across the Great Desert. Then I go to stability mode, and let the nose rise by itself to give me some lift. Since I'm not trying to stop yet, I manage the AoA to around 10 degrees max, to provide a lot of lift and not much drag yet.

By the time I'm halfway across the Western Sea, I'm at about 42km altitude, and my descent is about 30 m/s. Now it's time to hit the brakes, so I turn on RCS, and lift the nose to 90 degrees, and go to Radial Out.
By the time I'm at the Too Short Mountains, I'm at about 25km, and 1380 m/s, and the Dynawing wants to do one backflip (because the CoM is a little too far back). (Note that you can avoid the CoM issue before reentry by pumping your monoprop forward out of the cargo bay.) But I let the plane do one backflip, and set it to prograde while it's flipping. Then back to Stability when it's close to prograde.

And that's the reentry part. The last bit is just flying it down to the runway. Of course, it's still going 300 m/s too fast for a nice landing, so you have to do a bit more braking.

 

Edited November 8, 2017 by bewing

[Silverwood]

Ok that sounds good, I will try those tips. I had a quick go again last night and managed a fairly good re-entry, but way too far off course, so the landing would have been on top of a mountain!!! Shame really as the final descent was good.

Oh by the way anyone know of a fairly reliable larger version of the shuttle?

[AeroGav]

The AoA numbers people are quoting here still sound far too steep to me.   The real NASA Space Shuttle re-entered at 40 degrees , at which angle it is not stalled  because the stall angle increases in supersonic flight and because highly swept delta planforms with strakes like the Shuttle's wing have a high stall angle to begin with.

Because it is not stalled it is not in danger of spinning out and it is also producing lift (which it won't be if stalled) so it takes longer to drop into thicker air meaning 

• thermal pulse is spread out over a longer period but less intense
• g forces are less compared to capsule style re-entries

KSP aerodynamics does not take account supersonic airflow or wing shape so the stall angle remains 30 degrees,  coming in even at the angle of the real shuttle means you are stalled, not making any lift and in an aerodynamically unstable situation.        If you want to imitate real life come in at 20-25 degrees AoA on your mk3 shuttle,  though in reality a mk3 cockpit should not overheat coming back at 10 degree AoA if you're only returning from low orbit .

A design with an inline cockpit  will do even better.        Having the option of lowering the nose closer to best glide, without fear of blowing up,  means you can correct if you find yourself undershooting, makes it easier to reach the space centre.