So you want to build a space plane (.25 stock)...

[GoSlash27]

This is a quick how-to guide for constructing a safe, reliable, and efficient spaceplane for stock KSP.25. This design relies on proper piloting, so there is very little room for error due to an inefficient launch profile. This little guy has just enough supplies to do the job with 20% reserves and fuel to get you home safe.

Not to worry, though; I'll include an example of a proper launch profile for this type.

This tutorial will consist of 3 parts: Part list, Assembly, and operation.

NOTE: While many of these philosophies and techniques apply to FAR, this SSTO will only work in stock installations. FAR derates turbojet thrust to make this sort of thing impossible using turbojets.

Prologue: What spaceplanes are for

While SSTO spaceplanes can be used to perform a wide range of tasks, they are rarely best- suited to many of them. This design is optimized to fulfill the role that is best- suited for SSTO spaceplanes: Shuttling Kerbals and supplies safely and cheaply from KSC to LKO and back. While you *can* use SSTO spaceplanes to do other things, that's not what this primer will teach you to build. Nevertheless, the concepts and procedures taught here are universal for spaceplanes, so feel free to apply them to what you are building.

Part 1: Mommy, where do spaceplanes come from?

From experience, I have distilled the spaceplane down to a simple collection of parts put together in the proper fashion. It's very much like a recipe.

Required parts

-1 Mk 2 cockpit (drained of monopropellant)

-1 Mk 2 docking port (15 units of monopropellant)

-1 Mk 2 fuel section, short (180 units of fuel)

-1 Mk 2 F+O section, short (135 fuel, 132 oxidizer)

-1 Turbojet

-2 48-7S rockets

-4 XM-G50 radial intakes

-14 "strake" wing sections

-3 short mark 2 sections of payload of your choice. These can be passenger compartments, LF+O, monopropellant, or payload bay contents.

-assorted anciliary bits and pieces (landing gear, control surfaces, lights, solar panels, RCS thrusters)

You can mix and match your payload as required to make a hauler that best suits your needs. This tutorial uses 1 load of LF+O, 1 load of monoprop, and 1 load of xenon tanks in a cargo bay.

Part 2 will deal with how to use these parts to make a safe and easily flyable spaceplane.

Edited December 28, 2014 by GoSlash27

[GoSlash27]

Part 2: Insert tab A into slot B

So here we see the core sections of our future spaceplane duct taped together. We have our mission supplies (marked in red) and our payload (marked in green).

Problem is, this arrangement wouldn't fly very well because it has unacceptable static balance.

Static balance: The property of a spaceplane that puts it's center of gravity in alignment with it's center of lift and keeps it's center of gravity from moving very far as the fuel drains off.

An aircraft with poor static balance will be either nose heavy(bad), tail heavy (catastrophic), or will change this tendency as fuel drains (really not good).

We need to get this fixed by re-shuffling the sections to put the center of mass in the middle of our fuel tanks.

This arrangement seems to work pretty good.

We will drain our fuel tanks and re- check it to see how far our center of gravity moves as we burn fuel.

Very good. It didn't move much at all.

We'll leave the fuel tanks empty for a while to represent a "worst case" while we continue building. Last thing we want is a tail- heavy plane.

[GoSlash27]

Static balance part deux: Electric Boogaloo

Since we've come up with a happy arrangement, we'll fix the stuff in place that we won't be adjusting.

We've added our rockets and intakes, and flipped our docking port to the bottom (to make room and because docking ports on the bottom are very easy to align)

Now for the second half of static balance, center of lift.

Here's our 14 "strake" wing sections. Obviously, they're too far back, so we'll adjust them and add a few additional sections to make them look pretty/ aid us in making our balance.

That was easy. Now on to the next part: Dynamic balance.

Edited December 7, 2014 by GoSlash27

[GoSlash27]

2B: It practically flies itself...

We could pretty much slap some parts on this as- is and go flying, but we'd be fighting with the plane the whole way because the airplane isn't trying to help us. That's where "dynamic stability" comes into play.

Dynamic stability:

The tendency of an aircraft to seek straight and level flight after being disturbed.

The 2 big culprits here are roll and yaw. We want an airplane that has been knocked out-of-level in roll to level it's own wings so we don't have to do it ourselves. Likewise, an airplane that is side-slipping with the nose pointed off to the side of where it is going should naturally want to weathervane back into line.

Positive roll stability is achieved by gently sloping the wingtips uphill (called "dihedral") and positive yaw stability is achieved by hanging a big vertical board out back to act as a weathervane (called an "empennage").

A couple options for our plane that would work just fine:

A single vertical tail and gently sloped wingtips.

Twin vertical tails with gently sloped wingtips.

Either of these would work fine, but here's my favorite:

Wingtips set way back and sloped up 45 degrees to perform both functions with a little less mass and drag.

I'll take door #3, Monty.

[GoSlash27]

2C: I went to 'Ludicrous Speed' and it went plaid...

During the terminal phase of launch and reentry, KSP spaceplanes can often exhibit a tendency to want to fly backwards. This is a result of "drag misalignment". The builder accidentally put high- drag parts in the front and low- drag parts in the back. Much like an arrow fired backwards, it doesn't like to fly that way.

High-speed stability:

The tendency of a spaceplane to to respond to high- drag situations by pointing it's nose in the direction it's headed.

We already solved this in the 3rd post by putting the intakes as far back as we can mount them. Intakes are the highest- drag part of the plane, so keeping them aft is enough to give us good high- speed stability.

Another big culprit is having wings too far back and using canards to balance them.

Of course, we won't do that...

[GoSlash27]

2D: The finishing touches

Time to fill in the last little bits of detail work.

Refill the tanks. Fuel tank gets 180 and the LF+O tank gets 135 fuel and 132 oxidizer.

We will add control surfaces, but very little. We're making a spaceplane, not an air superiority fighter. Stability and precision are our friends.

So we add a couple Elevons in the back for basic controllability and (and this part is important) a couple canards in front to counter- balance the elevons. Why? High- speed stability. We don't want a longitudinal imbalance in drag by having control surfaces at just one end.

Rudders? We don' need no steenkin' rudders!

Almost done now. Add gear. Nose gear gets active steering and no brakes.

Adding anciliary systems (lights, solar panels, RCS quads), setting up action groups (1= close intakes, 2= activate orbital subsystems 3= disable rockets for flying home) and setting up staging.

Ready to go crashin'!

Edited December 7, 2014 by GoSlash27

[GoSlash27]

2E: This is only a test...

Take it out and fly it to find out where you went wrong.

Try it with full tanks, nearly empty tanks, full payload, and empty payload. Make sure it flies well, and takes off/ lands without any embarrassing 'spontaneous disassembly' problems.

In the case of this example, I had to reposition the main gear a little closer inboard and further back to keep from accidentally removing the turbojet.

2F: Movin' On Up

Take it into a few high- speed runs to make sure it's stable throughout the launch profile and at high speed. There probably won't be any issues, but best to make sure.

[GoSlash27]

3: How to fly an SSTO Spaceplane:

This is an example of a proper launch profile:

http://s52.photobucket.com/user/GoSlash27/slideshow/KSP/SP%20profile

The first 10 KM of altitude:

Climb as fast as you can while maintaining at least 100 m/sec velocity.

10-25 KM:

Slowly taper off your climb rate to 100 m/sec so you don't find yourself too high and slow

25-32KM:

Build speed and keep your angle of attack at 22 degrees or less.

Above 32 KM:

Keep the engines lit by throttling back and continue to climb at 100 m/sec. Once you've stopped gaining speed (well over 2300 m/sec), it's time to engage the rockets. Close the intakes (action group 1)

Once apoapsis is achieved, engage your orbital systems (action group 2) and treat it like any other rocket.

Landing:

Do a normal retroburn for KSC, then

Open the intakes (AG1) shut down orbital systems (AG2) and disable the rockets (AG3).

Rotate to prograde do attack the atmosphere head- on and verify that your action groups performed properly.

Once reentry heating subsides, throttle up to 1/3. Now you're just a regular airplane. Fly back to KSC and land.

Best,

-Slashy

[GoSlash27]

Multiengine spaceplanes

The process of building multiengine spaceplanes is the same as single engine spaceplanes. You just double the specs and go flying.

Except for one important difference...

"Houston, we have a problem..."

This is an example of asymmetric thrust due to oxygen starvation. The way the game engine works, one of the engines will starve and throttle back, leaving a surplus of oxygen for the other engine. Before you know it, you're doing the rhumba at mach 6.

It's too stupid to realize that the left intakes are meant to feed the left engine and vice-versa, so all the air you grab goes into one big pool and the engines draw from it in the order they were placed. Whichever engine was placed first gets all the air it wants, and the second engine gets the leftovers.

Thankfully, there's a fix for this...

[GoSlash27]

We've got the balance that we want, so first we remove all intakes and engines.

Now with symmetry turned off, we place only the intakes meant to feed the left engine.

Next, we place only the left engine.

Then we repeat the process for each remaining engine.

Place only the intakes for the right engine

Finally, place only the right engine.

This helps trick the game into doing what we intended. Now it will

1) Draw air from the left intakes

2) Feed the left engine

3) Draw air from the right intakes

and

4) Feed the right engine.

Let's give it a go.

It now gets much higher and faster without spinning out of control. We still have some yaw starvation effects, but they are tempered down enough to where they are manageable.

This is because excess air still gets carried over to the universal pool and one engine will always flame- out first.

There's no way around this limitation in the game engine, but there are ways to mitigate it's effects...

1) Don't go multi- engine if you don't have to. Feeding one engine is easier to manage.

2) Place your engines as close to centerline as you can. When one engine dies, it won't yaw so bad.

3) Use some reaction wheels for yaw authority. When one engine flames out, it won't spin you as quickly.

4) Throttle down earlier to keep the engines from flaming out. No flameout= no yaw

and finally...

5) Set it up so that the engines closest to the centerline flame out first. Maybe give them a little less intake area...

Stay thirsty, my friends!

-Slashy

Edited December 9, 2014 by GoSlash27

[cosmos33]

I have builted the first spaceplane with 1 engine ( i use FAR) and i cannot achieve your speed at 30 k. I have checked your lunch profile, but my space plane has a hardtime getting speed over 30 k.

Edited December 28, 2014 by cosmos33

[GoSlash27]

Cosmos,

Sorry, I should've specified:

This is only for stock. It will not work for FAR as FAR derates turbojet thrust.

I don't use FAR, so I'm not qualified to speculate about what works well using that mod.

Sorry!

-Slashy

[Pecan]

...This little guy...

Happy New Year and lots of hugs, etc. Slashy but why's it so BIG?

ETA: Sorry, I live near Brighton, England - this question is liable to misinterpretation. Especially as it's pantomime season (Oh no it isn't!)

[GoSlash27]

That's definitely not what *she* said

Seriously, it's delivering 6+ tonnes of cargo, so <13 tonnes total is pretty small.

Best,

-Slashy

[Pecan]

Seriously, it's delivering 6+ tonnes of cargo, so <13 tonnes total is pretty small.

Ooh no missus, titter ye not.

Worth a mention in the OP then, I'd think? A 6t launch-vehicle with nearly 50% payload ratio is a different thing to just 'a spaceplane' :-)

(Or maybe I'm just being dense, sorry)

[PTNLemay]

I just wanted to say thanks for this guide. At first glance I wasn't sure about it, but I followed many of your suggestions (not all of them, I didn't want to just make a carbon-copy of your plane after all) and sure enough... for the first time I've succesfully put a plane in orbit in a single stage.

The two bits that REALLY helped me were the notion of levelling out after 10 km (I would always just race upwards as quickly as possible) and throttling back when your air intake approaches zero. I didn't realize the turbojet could be kept running for so long. That and keeping the spacecraft relatively light, I usually try to make the plane bigger to make it go higher, but this thing is a featherweight.

Also, why is that ugly little radial intake so powerful? It seems like the more expensive/advanced ones were less effective compared to this little guy. I'm not complaining though, ugly or not, he's going to be my go-to intake from now on.

(here's my historic first success)

http://i.imgur.com/KceiqBF.png

http://i.imgur.com/pVJ8DNB.png

[numerobis]

Hmm... obviously I see ways to improve, though you've got a nice plane already.

The 1-kerbal cockpits would save you a bunch of mass. The pretty one has a nice cockpit view for landings; the other one has less drag and mass, and allows putting a ram intake on the nose.

That would give you plenty of leeway to replace the strakes with bigger wings, to reduce part count.

Can you replace the Mk2 short F+O with an Mk2 adapter? Same fuel, 50kg less dry mass, and you get a tapered tail. You'd have to fit the 48-7S elsewhere of course. And the center of mass would move forward as you burn your fuel.

I recommend a vertical stabilizer. No yaw control on it, just the stabilizer itself -- a small delta, for example. Really helps you go straight and level.

[GoSlash27]

I just wanted to say thanks for this guide. At first glance I wasn't sure about it, but I followed many of your suggestions (not all of them, I didn't want to just make a carbon-copy of your plane after all) and sure enough... for the first time I've succesfully put a plane in orbit in a single stage.

The two bits that REALLY helped me were the notion of levelling out after 10 km (I would always just race upwards as quickly as possible) and throttling back when your air intake approaches zero. I didn't realize the turbojet could be kept running for so long. That and keeping the spacecraft relatively light, I usually try to make the plane bigger to make it go higher, but this thing is a featherweight.

Also, why is that ugly little radial intake so powerful? It seems like the more expensive/advanced ones were less effective compared to this little guy. I'm not complaining though, ugly or not, he's going to be my go-to intake from now on.

(here's my historic first success)

http://i.imgur.com/KceiqBF.png

http://i.imgur.com/pVJ8DNB.png

PTNLemay,

Glad you found this helpful. It's always gratifying to hear about the success stories!

The XM-G50 intake is actually pretty unremarkable AFA it's stats. The reason it tends to fare well is because it doesn't require additional structure to mount. All the stackable intakes require a flat forward facing surface, which means more structure, which means more weight and drag.

I also put together an intake comparo if you want to check it out.

Best,

-Slashy

[numerobis]

The ram intake works well on a cube strut, which is physicsless. It's also not super aesthetically pleasing.

[GoSlash27]

The ram intake works well on a cube strut, which is physicsless. It's also not super aesthetically pleasing.

That's true, but a ram intake also lacks the surface area to do the job with just one intake, which means you'll either need 2 (which is overkill) or assist with the XM-G50.

More importantly, it conflicts with drag balancing, which demands intakes in the back.

The stack mount intakes are really good, but I don't recommend bothering with them unless you're dealing with a multiengine design. They tend to have unused real estate that's perfect for stackable intakes.

Best,

-Slashy