Planes and their rear ends (plus a couple side questions)

[G'th]

So, after accomplishing the great rigors of creating a functional plane in KSP and then getting used to FAR's flight model (Because now its totally worth it), I've been working on an SSTO. And I've gotten a design that should work (after tweaking it a bit after the last test flight it should be able to establish orbit) but I want to refine its tail end, because the way I have my plane constructed I end up with my engines getting destroyed during landing. (and during take off I have to either be very careful pitching up or waiting until the end of the runway gives me clearance) Even coming in as slow as I can without losing control of the plane ends up with the engines colliding into the ground. The rest of the plane is fine, but the entire engine assembly just goes kablooey.

I did manage to land it once, fully intact, but that required me pointing my plane at the largest, flattest area I could fine and then giving myself a side view so I could refine my AOA as I got closer and closer to the ground. And while I could just maintain that angle (which I tried on the next two flights), there its literally zero wiggle room. And while I trust in my skills as a pilot to nail that angle every time, I don't like that a plane is locked to a single angle for landing, with any other angle resulting in some kind of rapid dis-assembly.

So, here's my plane:

For those that know of B9 and how its wings work, don't worry about the larger set of stabilizer wings i put my control surfaces on. I turned off that ones control surfaces so I could use it as a painless base wing. I didn't like trying to piece together the other bits into something that looked good provided my ample space for the control surfaces I needed).

Also, the reason for the second of wings closer to the front is due to the nature of what I want my SSTO to do (carry both a sizable crew and cargo to orbit rather than one or the other). Without something in the front I can't get enough lift to get my plane off the ground without letting run off the runway, and when landing I almost always can't get my nose up without them). The large amount of RCS i'm carrying is intended to be used as a de-orbiting measure, both in emergency and in general. As I refine the plane more and figure out its max payload, I'll probably end up removing a lot of it, or at least minimizing it to just the B9 tank.

What could I do to prevent this? I've thought about lowering my rear landing gear, but I'm unsure on whether the distance between the gears will keep my plane from wobbling as it speeds off down the runway. (That and rule of cool states that the gears look better at the height they're at now). I'm not opposed to rearranging where the different compartments are either, so any suggestions toward that end are welcome.

Now, some side questions.

First is about FAR. I know all the ins and outs of what it does to Kerbin's atmosphere. But what about flying elsewhere? While I've never actually flown anything on any other planet (nor have I ever really been to any other planet after playing this game for a year ), I'm wondering what to expect and what I should keep in mind once I eventually design something to fly on another planet. Is it easier like it is on Kerbin, or is it harder in places? Or both?

I'm also wondering how you guys determine your max payload for a spaceplane. I do have a small satellite I'm aiming to take into orbit with this thing, but that isn't very heavy at all, and if it can't take that into orbit then I'm back to the drawing board anyway.

[Sirrobert]

I don't know a thing about Far or spaceplanes (let's just say planes and me don't get along).

But I do know that, if you want to use jets for spaceplanes, only Kerbin and Laythe have oxygen, so you can only use jets there

[technicalfool]

One fix for the issue of clonking your engines on the runway, is to move the rear wheels further back. This however creates the problem of being unable to nose-up or rotate to take off. This can be fixed in a number of ways, including:

1) Put the front and back wheels at different heights, so your plane is nose-up while at rest. It will naturally want to take off as you gain speed along the runway.

2) Use canards at the front rather than (or as well as) elevators at the rear. This will more effectively drag your nose end up if you have a lack of leverage caused by the rear wheels being far back.

3) Carry on and ignore it. The runway has a dip at the end, and that can be enough to allow you to lift the nose up before it splashes all over the grass off the end of the runway.

Edit: 4) I have had some luck using sepatrons and decouplers attached to the nose of particularly stubborn aircraft. This is not a recommended approach, but it's definitely a kerbal approach.

Edited May 24, 2014 by technicalfool
For more sense, use edit!

[Pecan]

FAR changes atmospheric flight so, apart from Kerbin and Laythe as Sirrobert mentioned, that's Eve and Duna. Eve has soup for air and crushing gravity which, together with no oxygen, means spaceplanes have no practical value there. Duna, on the other hand, has hardly any air at all making 'flight' useless for the opposite reason.

Put all that together to answer your payload question: spaceplanes are ok for crew-transfers.

[Taki117]

Alright, here's what I see (Without both a rear and 3/4 view)

1.) Ditch the two wing arrangement. It's not helping you at all. Make a single delta wing in the rear and if needed add some small canards on the front. I don't know where those engines are from, but if you don't have it already you should check out Dtobi's Space Shuttle Main Engine Mod They are much smaller than you would find normally, but they have comparable thrust and decent efficiency for SSTOs. (I use 3 of the small OMS on my SSTO to achieve orbit) I would also move the Rear Wheels back more, (And the wing in general) to help with the tail strikes, alternatively you can add a Small Gear Bay under the rear of your aircraft so that you can pitch up and not have to worry about hitting your tail because there is a landing gear to stop it. Assuming your plane is symmetrical you don't need that much RCS. I can understand using it to de-orbit in an emergency, but that what the OMS engines are for, if you are running out of fuel before that you need more efficient engines, and sometimes it is better to have purpose build SSTOs for things. (I currently have 3, one is a crew shuttle to my space station, one is for general payloads, and the third is for repairing my Telescope)

[phoenix_ca]

Tail wheels.

If you want to test weights, grab the NRAP.

[Hodo]

Let me help you out a bit, as I person who only builds SSTOs. You have to remember that your take off angle should not be much more than 10-15deg normally anyway. If you have to pitch up more than that you have a problem. The thing is with refined piloting skills you should be able to get off the ground with only 5-10 degrees of pitch up. No need for silly tail wheels or any of those things that are not on 90% of real aircraft. You may have to move really long engines to a more centralized location on the aircraft, like along side the tail section instead of it being the tail.

There are some minor problems with your current craft that I can see from that picture.

The air brakes are mounted high on the nose of the aircraft, and you have none towards the bottom. If you hit your air brakes at high speeds it will pitch your aircraft in a direction you don't want to go. Try to match your airbrakes top and bottom.

The mono-tanks on the top of the wings also are a problem, they add weight higher on the centerline of the aircraft, this raises the CoM, putting your CoM and CoT out of alignment ever so slightly. While it may not be a problem when the craft is full, it will become a problem as your craft burns fuel and the mono tanks stay full.

[Pecan]

Let me help you out a bit, as I person who only builds SSTOs..

Do you build spaceplanes as well?

[Hodo]

Do you build spaceplanes as well?

Yes, a few....

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http://hodo.imgur.com/

http://forum.kerbalspaceprogram.com/threads/56945-Hodo-Creations-SSTO-craft-designs-and-files-for-those-who-want-them

My thread about most of my SSTO space planes and more.

Edited May 26, 2014 by Hodo

[federally]

Flying on other bodies can be a lot of fun. I actually have turned strapping regular planes onto rockets to fly on other planets as a bit of a hobby.

With FAR Eve is super easy to fly on. The atmosphere is more like swimming then flying lol. Your problem is no oxygen, so you need a non typical way to generate thrust. Or just do what I do and drop gliders onto it and enjoy them till the fun ends lol.

Duna is a pain. It has an insignificant atmosphere and no oxygen. You need to design very special craft if you want to fly here. Something lightweight with tons of extra lift and an electric prop from a mod like KAX works.

Laythe will be your new home away from home. It has oxygen and a good atmosphere, though it's thinner then Kerbin's. Generally anything that flies well on Kerbin will fly here, though it will go slower and have a lower service ceiling.

Here is my first Corsair launched to Laythe

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I also have this Kethane miner/refiner/refueler on Laythe servicing my fleet. Though none of these pictures are taken therethere. It was most negatively effected by the thinner atmosphere but the huge wings still work well enough!

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[O-Doc]

Even coming in as slow as I can without losing control of the plane ends up with the engines colliding into the ground. The rest of the plane is fine' date=' but the entire engine assembly just goes kablooey.[/quote']

Your landing speed is too slow. Find your target speed where your AoA is between 5-10 degrees and you are losing 2-5m/s altitude. If that's uncomfortably fast then consider putting flaps(+ spoilers) on your plane to increase lift at slower speeds inside your required landing AoA.

I'm also wondering how you guys determine your max payload for a spaceplane

Ultimately, it's directly related to how much air intakes you have. I don't know what the FAR mechanics are.

[Hodo]

Determining the payload size for my space planes is a bit of math, and guess work.

Using FAR it is mostly determined by your wing surface area (lift you can generate) and the thrust to weight ratio of the aircraft.

I find if my TWR is lower than .75:1 using the jet engines, it will not reach orbit. After I design the craft I then put the heaviest possible load into its cargo bay and test to see if it can get to 25km and 1100m/s under jet power. I also test to see how it handles with that cargo, I will dump fuel to get it down to 50% or less total fuel on board and test how the CoM changed.

[LaytheAerospace]

You may be able to guess from my name, but I spend a fair bit of time designing spaceplanes and flying them to/on Laythe. I keep it as close to stock as possible, though I use FAR, MechJeb and some other random addons for external cameras, robotics and other stuff you simply can't do in stock. Here's some of what I learned.

PAY ATTENTION TO DRAG

Don't mount anything on your wings you can't live without. Those monoprop tanks are draggy as hell, and they throw off your CG (I'll touch on that later). Keep FAR's flight data open, and pay attention to your L/D as you fly, especially as you're pitching up. The higher your L/D, the more efficient your plane is. The more efficient the plane is, the less time and fuel you burn getting to rocket switchover.

Avoid nose canards. They're an easy way to get the nose up on the runway, but they do undesirable things to your aerodynamics in FAR. Run the simulations with and without the nose canards, and you'll see what I mean. The better solution is to raise the nose wheel by a small amount to give you some default AoA on the runway. It takes some fiddling, but you can balance the different heights with how far you move the tails aft, so your trail strike problem is mitigated. You can also try giving the wheels some additional clearance using cubic struts (lowest drag and mass, highest part count), regular girders (ludicrously heavy and draggy, lowest part count) or hardpoints (reasonably low drag, low mass, medium part count). I generally go with the cubic struts, 4-5 above each landing gear for my ships which have tail strike issues, though I'll admit it can look a bit goofy.

Next, keep your CG/CT as close to aligned as possible. Flying in atmosphere, you don't need your CG/CT to be in that great of alignment, because the aerodynamic effects keep you pointed more or less in the right direction. Once you get out of the atmosphere, though, small misalignments are going to make burns hard to impossible. Use hyperedit to do test vacuum burns to see if your plane is stable, so you don't have to spend half an hour flying it to space each time you make a tiny change. If your plane flies straight in vacuum without SAS on, you've made a very nice plane.

Keep your CL barely clipping through your CG, and keep movement to a minimum as tanks drain. If you have a CG/CL inversion, you're gonna have a bad time. If it drifts back, you're going to have a fairly bad time as well, since you'll lose all your maneuverability at hypersonic speeds and won't be able to nose up to get to space.

Enable a couple spoilers, and turn them on when landing. Helps drop speed and keep the nose up. Throttle up to 5% to prevent stalling. MechJeb can be used to maintain a good AoA to make contact with the runway with, but I usually don't bother.

[G'th]

Mkay so after several exhausting days of redesigns (and a couple trips to the Mun to get some more parts to play with. Believe it or not I haven't even gotten to Minmus yet) I've come out with a much better design, that hits LKO with 900+ dV to spare. And alas, it flies pretty well too.

After examining what all I wanted to do with this playthrough, I realized that I'd be better off designating simple rockets for substantial payload delivery, and instead leave my SSTO (which I've dubbed the Velocity) for resupply and crew rotation. (I may eventually utilize it on a Jool mission, but considering I haven't even gone interplanetary in my entire time playing this game, thats a long way off). So with that in mind, I was able to shorten the cargo bay, which I can now interchange with one or two crew tanks (or perhaps command seats if I feel silly) with minimal effect on performance. It also meant that I could pile on some more fuel, which made the rocket stage a lot less harrowing. I ended up throwing out the entire engine block entirely and restarting from scratch, which eventually necessitated some science gathering after realizing I wasn't satisfied with the engines I had. Did the same with the wings, which became easier with the shortened payload expectations. And no tail strikes below 10* pitch, which gives me a comfortable margin, so that issue is gone now.

Actually flying the thing is pretty good for what it is. I'm still running it through high altitude tests to see how well it does (which is more just an in-case option, as at these altitudes I'm either on my rocket stage or descending from orbit), but otherwise as long as I avoid my tendency to just want to land it straight down (i'm impatient ) I can land quite smoothly, especially now that I can maintain my pitch at low speeds. Found out after a rather disastrous test run (Talking large scale stall from 30km up straight down to surface impact) that my CM does drift rear of my CL just as I run out of fuel. My solution thus far is to just move everything I have left forward prior to reentry, which the VAB tells me should work out fine just fine. Is there a way to have this done automatically, besides TAC Fuel Balancer? That'd be nice, as my idea regarding fuel lines didn't do anything.

But as always, I still want to optimize. So here's a bunch of pictures for you guys to criticize. I am having an issue with my rocket engines (From Space Shuttle Engines), where the gimbaling and trim tends to wig my plane out of wack, particularly in space. I can compensate for it (as I set the trim speed for both down to 0. Even on a regular rocket they gimbal way too fast, so I did this first thing), but I'm still trying to find settings for them that lets me turn my ship about without having the gimbals get stuck somewhere, thus making the ship go all wacko. I have considered just turning gimbal off (I know have to keep Trim on to maintain the angle I have them set for) but then again, gimbals are always nice for that little extra maneuverability.

I'm also trying to weave around the intricacies of reentry. Basic reentry isn't an issue, and actually I've been having problems NOT descending in such a way that doesn't produce reentry heating (lowest I've gone is 8k). While I'm sure that's actually an awesome thing, it is kind of disappointing. It looks neat. But even so, I can't decide if I want to just accept that or find an angle to go for that gives me that classic harrowing reentry.

I'm also curious about the B9 control surfaces, and specifically the settings I should go for on them. Right now I have them set to default, but I'm wondering if different settings might provide some benefit.

Anyway, pictures!

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[Hodo]

I'm also curious about the B9 control surfaces' date=' and specifically the settings I should go for on them. Right now I have them set to default, but I'm wondering if different settings might provide some benefit.

Anyway, pictures!

[/quote']

Ok the control surfaces are simple. Right now with your current settings you have to many control surfaces doing to much. This will lead to some stability issues.

I suggest setting your outter most control surfaces as ailerons and thus only control roll. Then set your inner most ones as elevators, and thus set it for pitch. The ones between the two, I would set for both roll and pitch. Your vertical stabilizer I would set for yaw and only yaw.

This will give your control surfaces a realistic response and your aircraft will work a bit better because of it. Other than that I think your RCS thrusters are set sideways, they go length wise not vertically, but they should work either way.

[LaytheAerospace]

The ones between the two, I would set for both roll and pitch.

I'm generally not a fan of sharing pitch and roll control surfaces. If you're pitching up hard, and need to roll, you lose some pitch authority and nose down. This can either be irritating or scrub your launch, depending.

[Hodo]

I'm generally not a fan of sharing pitch and roll control surfaces. If you're pitching up hard, and need to roll, you lose some pitch authority and nose down. This can either be irritating or scrub your launch, depending.

I am not a fan of it either, but in the aircraft in questions options I really cant give much else for those control surfaces in the middle to do. They could just be used for pitch authority but then you may have to much pitch, they are to small to really be worth anything as flaps or spoilers. And if they are set as ailerons for roll and just roll they will give the craft a stupid fast roll rate which really isnt needed for a SSTO that isn't a combat vessel.

[G'th]

Okay wow that makes this thing SOOO much more stable. Its like flying a plane in a regular flight sim now. Granted it doesn't turn on a dime, but its smooth, and thats what counts (and without caps lock none the less). I did opt to not use roll on the middle CS so I think I'll stick with that.

[Hodo]

Okay wow that makes this thing SOOO much more stable. Its like flying a plane in a regular flight sim now. Granted it doesn't turn on a dime' date=' but its smooth, and thats what counts (and without caps lock none the less). I did opt to not use roll on the middle CS so I think I'll stick with that.[/quote']

Good to hear.

One more suggested mod if you are going to get serious about SSTO space planes, Procedural Wings. Best part saving mod out there for aircraft.