Decelerating an airplane... in time, without airbrakes. How do?

[Laie]

So, I'm trying to land a spaceplane. I know that moar pitch will ultimately lead to more deceleration and a sooner landing, except when it lifts me back into the upper atmosphere for what is, in effect, a suborbital hop. I know that I can avoid such hops by rolling, so that my pitch won't lift me up.

I do all of the above, but cannot really tell out how much is called for at what time. In the stock game I can work around the problem by building planes that can pitch up 90° just like that, coming to a Looney Tunes-like screeching halt in mid-flight. In RSS+FAR, that's not quite as easy to pull off, and besides, I'd rather not resort to such shenanigans.

I've got a similar problem at the final approach, either falling short with the runway in sight or coming in too fast for comfort.

So, I'm looking for good advice on how to do better approaches. Any rules of thumb how as to what data displays to watch, or how I can gauge or estimate the amount of energy I've still left?

[bewing]

For the first 99% of the reentry, the overall gist is that it's about trying it multiple times, seeing what approximately works, and then remembering (or writing down) landmarks from the terrain below, related altitudes, and related speeds.

When you are still in the high upper atmosphere (since you are saying RSS, I'm not going to try to quote numbers) there is very little actual drag on your plane when you are oriented prograde. When you want to slow down, in stock, yes you can use reaction wheels to force your plane to a 90 degree AoA. Outside of stock, you use RCS thrusters to accomplish exactly the same thing. If that slows you down too much, then you need to know some aerodynamic details about your plane -- what's the AoA that gives you the most lift overall, and what's the AoA that gives you the best lift/drag coefficient?

In general, it's a good idea to come in with an AoA somewhere between max. lift/drag and max. lift. This keeps you from falling out of the sky way too short. Then you hit your first landmark, and you say, "Too fast/slow? Too high/low?" And you start making adjustments. If you are too fast and low, you start maximizing lift. If you're way too fast, you go to 90 degree AoA and cross your fingers. If you are too high/slow, you drop your nose to prograde and trade some altitude for some speed. if you are too low and slow, then you reentered way too soon and you need to either call this a mere computer simulation and hit Alt-F9 to try again, or light your engines a bit.

Then you start looking for the next landmark.

For landing, I like coming in really high and fast in the high atmosphere. Where I have plenty of aerodynamic control, but I can easily supercruise for a many kilometer overshoot. Again, you need to pick your spot. And stall your plane -- with RCS or reaction wheels, or just aero. Point the nose mostly straight down, dive to 1km altitude, pull up, and stall again. Now you are close to the runway at the minimum possible flyable speed.

If you really need help stopping on the runway, then (of course) you can always pack a parachute or two.

 

[Xd the great]

Or, just land like a bfr. Tbh, landing on water isnt that bad...

[Daveroski]

When I have a new craft to test, the objective is getting it into orbit with as much fuel remaining as possible.
This in and of itself can be a stumbling block. your craft should end up with as much fuel as needed to do the job it was designed for.
If it doesn't need to go to the moon/Mun then it doesn't need the fuel to get there.
The problem with extra fuel is it gives extra mass (fuel AND fuel tanks) and the extra mass needs extra fuel to get it to orbit and extra drag to slow it down on re-entry.

Each craft has different mass and drag. A bunch of air-brakes as far back on the craft as possible will help to slow you down. your Angle of Attack can make a big difference to your glide profile.

However if like me, you prefer to re-enter the same way each time regardless of the craft design then something has to change for each craft.

Each craft therefore requires it's own re-entry point. The sweet spot where you first touch atmosphere.

Usually after around three test-flights with a given craft, I can work out the best spot to re-enter. The first test over-shooting. The second too short. If I know where I re-entered on both of these re-entries then the third time is usually (but not always) the charm. Once I have the re-entry nailed. I don't change anything. EVER.

I always circularise at 75k before starting my re-entry.

I always re-enter at between 5-10 degree pitch without changing the Angle of Attack on the approach.

Once I built a craft that needed almost a whole circumnavigation to land at KSP gracefully.

Most of my craft now tend to start their decent towards the furthest edge of the deserts to the west.

Some craft get much more in the way of testing. If it is a craft that I intend to use on a regular basis for transporting kerbals to and from orbit then I get to know the vehicle. I know just where to re-enter. The only place I want to slow down is right before touchdown with air-brakes.

I have three lady pilots in rovers. One 10k west (270 deg from the runway) and one 5km West (270 degrees from the runway) Their job is to wave at pilots as they go past. to make sure that they are lined up for the runway. The third is close to the waters edge to the east of the runway.

If all three are lined up on approach then I know I am lined up for the runway.

Using this method, I will reiterate, I only ever have to change the point at which I re-enter. Nothing else.

I am aware that few people have my patience and tenacity but I hope this helps.

[Xd the great]

Or just make the plane land elsewhere, mine and refuel, and fly home.

[Laie]

10 hours ago, bewing said:

For the first 99% of the reentry, the overall gist is that it's about trying it multiple times, seeing what approximately works, and then remembering (or writing down) landmarks from the terrain below, related altitudes, and related speeds.

Yeah, that's probably the best there is. Only that stock descents easily take 20min, and in RSS it's typically half an orbit. Including deceleration and everything, it can be an hour -- not the kind of experiment you repeat ten times for every new vessel design.

[Gordon Fecyk]

On ‎7‎/‎27‎/‎2018 at 7:39 PM, Laie said:

In the stock game I can work around the problem by building planes that can pitch up 90° just like that, coming to a Looney Tunes-like screeching halt in mid-flight. In RSS+FAR, that's not quite as easy to pull off, and besides, I'd rather not resort to such shenanigans.

If you build Wheesley turbofan engines into your design, you can reverse thrust with them, but that only works well in the lower atmosphere after you've dropped below Mach 2.

I've built airbrakes in FAR by using overlapping elevons and changing the spoiler deploy angle to a negative value on one set. This gets you a similar effect to the real world STS tailfin's airbrake.

Neither of these help you with your original problem though, which is when to de-orbit so you are reasonably close to the Space Centre when you're done decelerating. I have three re-entry examples for FAR with three very different styles of craft that all use the same re-entry approach: set periapsis to about 25 km directly above the KSC and glide gently down. These all start skipping off the air and climb a little bit, but are still losing speed as they do and will start dropping down again.

The really big one (the Fat Star) even alternates between positive and negative angles of attack to maintain a roughly constant rate of descent, but it might not be strictly necessary.

In all of these craft, you can stretch or shrink your path by adjusting your angle of attack. Enabling and using flaps can help.

[Laie]

4 hours ago, Gordon Fecyk said:

Neither of these help you with your original problem though, which is when to de-orbit so you are reasonably close to the Space Centre when you're done decelerating.

Nah, that's not the problem. From the same entry point I've plunged into the Gulf of Mexico, or overflown Florida at Mach 6. It stands to reason that I should be able to reach any point in between, and probably even further if necessary. Stock is more forgiving, the circle is more like 20km around KSC so I can usually stretch it by gliding, or do a 180 and come back from the sea. Still, the fundamental problem in getting the approach right is just the same.

While I know quite well what I can do to affect my trajectory, I have no sense or measure of how much of it is called for, or when. The effects of any action don't become obvious until much later, and it seems a bit unintuitive -- which probably only means that I need to hone my intuition.

Even though it's the best suggestion so far, I don't like the the idea of maintaining a table of airspeeds and altitudes. I'd still welcome any advice on how I can do it more seats-of-my-pants style.

 

[Daveroski]

16 minutes ago, Laie said:

From the same entry point I've plunged into the Gulf of Mexico, or overflown Florida at Mach 6. It stands to reason that I should be able to reach any point in between, and probably even further if necessary.

 

Did you read my post?

Standardise your flight profile - get repeatable results.

 

[Kryxal]

The way to "develop" your intuition is by giving it more to go on ... just repeat it, making note of airspeed, altitude, and position.  You pretty much need to have a feel for how a good profile is and how and when to fix it, or make a table to tell you.

[Gordon Fecyk]

7 hours ago, Laie said:

From the same entry point I've plunged into the Gulf of Mexico, or overflown Florida at Mach 6. It stands to reason that I should be able to reach any point in between, and probably even further if necessary

I would have expected you could just glide further or shorter as you needed to. My stock space planes usually have some air breathing range after re-entry, to compensate for missing my target, but I understand the real world STS couldn't do that.

I know it isn't RO, but stock Eve is the closest I've come to seeing how a periapsis difference of only 3 km makes a difference between landing on target and landing a good 120 degrees of latitude off target. I imagine RO Earth further exaggerates that, and a few hundred metres PE difference could mean missing by hundreds of kilometres. But without a flight computer recalculating your descent path at all times... wait, does the Trajectories mod work in RO?

[Mahnarch]

Have you tried flying short final from cockpit view?

Sometimes, all you need is a fresh perspective.

[blakemw]

Use the Trajectories mod (it works with FAR). You need to tune it a little bit, tell it you'll be entering prograde and estimate your angle of attack at different regimes (or use 10%). Naturally the prediction made by trajectories is not perfect, it's probably not perfect even if you maintain the angle of attack you told it you would, but the prediction is much more useful than not having it, generally speaking if the prediction is undershooting the target you can pitch up a little, and if it's overshooting you can pitch down.

[bewing]

The basic facts are that you always have to aim to overshoot -- if you run out of velocity too early you will always fall out of the sky short of your runway and that's all there is to it. So you maintain some speed and altitude right to the end. Now the problem is stopping. What are you willing to do to stop? You complain about the lack of realism in coming to a Looney Tunes stop. So you set yourself to overfly your target, and then you aren't willing to do what it takes to stop, and then you complain that you can't land? You're setting this up as an impossible problem right from the beginning.

 

 

[Lego_Prodigy]

TBH, I would recommend airbrakes, or if not that, some kind of SRB retro thruster to slice off some excess velocity. You really should use airbrakes on airplanes, as most have them IRL. When I forget them, I simply do it the Kiting way

[blakemw]

1 hour ago, bewing said:

The basic facts are that you always have to aim to overshoot -- if you run out of velocity too early you will always fall out of the sky short of your runway and that's all there is to it. So you maintain some speed and altitude right to the end. Now the problem is stopping. What are you willing to do to stop? You complain about the lack of realism in coming to a Looney Tunes stop. So you set yourself to overfly your target, and then you aren't willing to do what it takes to stop, and then you complain that you can't land? You're setting this up as an impossible problem right from the beginning.

 

I'm sorry but this advice is almost completely invalid when using FAR. The FAR atmosphere is a lot more "slippery" and lift is a lot more effective, if you try a "looney toons stop" then for one you'll retain more velocity because steering tends to be very effective in changing direction without losing velocity, for two you probably won't be able to because of the air restraining the plane's movement or you'll succeed but rip the wings off. Because lift is a lot more effective (i.e. lift/drag ratio is higher) by adjusting the angle of attack you can glide very long distances, much further than in stock. It's hard to bleed off velocity and overshooting is a very realistic possibility. Because of the slipperiness of the atmosphere heating can also be a serious constraint, you can get down to lower altitudes where it's hotter without slowing down enough.

Even in stock, but moreso in FAR, with the right plane it's possible to glide further than the ballistic trajectory would indicate, with a high enough lift/drag ratio the vertical velocity can be converted into horizontal velocity effectively enough to more than compensate for drag - to the point where you can fly at least a 3rd of the way further around kerbin (than ballistic impact point) through the power of gliding. If you're re-entering a capsule then aim to overshoot, if a plane then aim to undershoot and glide to make the distance. (and if using trajectories it's essential to set a positive AOA so it knows you'll be gaining lift, the 0 AOA prediction is nearly meaningless for a plane)

Edited July 31, 2018 by blakemw

[Laie]

On 7/31/2018 at 4:42 PM, bewing said:

What are you willing to do to stop? You complain about the lack of realism in coming to a Looney Tunes stop. So you set yourself to overfly your target, and then you aren't willing to do what it takes to stop, and then you complain that you can't land?

a) what @blakemw said.

b) looney tunes is not practical in FAR. I guess that it's possible to design a plane that can pull it off, but nowhere as easy as in stock. However, I'd suppose that there is some leeway between looney tunes and doing nothing; I'd like to explore that.

Sticking with the problem of approaching the runway too fast: If I even manage to get into that situation, it only takes a few rounds of saveloading to get the plane down safely. However, that does not translate into experience: next time I approach the runway at a somewhat different speed, I will take just as many iterations to get it right.

Airbrakes make it easier, of course. But the lesson I'm trying to learn is not "just bring enough airbrakes and don't fret".

I've learned to do RO launches where I can neither throttle nor restart; it's actually quite easy once you know how it works. I hope that FAR landings can similarly be coped with once I know what things to look out for. But as of now, I'm lost in the woods.

[bewing]

Even in FAR, you gotta be able to stall your plane. Especially if it has RCS on it to help the rotation.

> next time I approach the runway at a somewhat different speed, I will take just as many iterations to get it right

That's why you stall. Your stall speed is invariant. So it'll be the same speed every time.

 

Edited August 1, 2018 by bewing

[blakemw]

9 hours ago, Laie said:

Airbrakes make it easier, of course. But the lesson I'm trying to learn is not "just bring enough airbrakes and don't fret".

 

I think the key is understanding lift.

Lift is very potent at transforming vertical velocity ("falling") into horizontal velocity ("distance"). Even with the stock aerodynamics, a good spaceplane (one which needs wings to fly, not one with enough power to do it without wings) can glide about 1/4 of the way around Kerbin further than the ballistic prediction would indicate (and that's the simple suborbital prediction, not even the ballistic-with-drag prediction of a poorly configured Trajectories). The FAR aerodynamics are more "slippery" than stock and higher lift/drag ratios are possible, meaning gliding even further. When landing spaceplanes you need to aim to undershoot by A LOT because the plane is going to glide around the world - that is assuming you are pitching up a bit to slow descent and ease heating. Trajectories mod is so useful for modeling this but it's vital to tell it you'll be gaining lift (and nice thing is, you can adjust the angle of attack you want it to use for its prediction and see the projected trajectory change in response).

If you've never seen it I super highly recommend this video about how the Apollo capsule used lift to narrow down, though mainly just for inspiration, the main take-home is that it's damn hard, you descend too fast and burn up/get destroyed, or you overshoot by half a world (and it's computer stuff, not "seat of the pants" stuff):