SSTO Tips?

[ROFLCopter64bit]

I am horrific at designing SSTOs. That said, what is the key to making an SSTO? Is it the ISP, amount of fuel, or overall low weight? SSTOs look cool, it would be great if I could make one for the first time

[foamyesque]

I am horrific at designing SSTOs. That said, what is the key to making an SSTO? Is it the ISP, amount of fuel, or overall low weight? SSTOs look cool, it would be great if I could make one for the first time

Load up as much fuel as you can with engines that'll give you a TWR around 1.5 at launch and aim for 4.5km/s dV. An LV-45 can SSTO a one-man pod and a Mainsail can do the three-man.

[bsalis]

An SSTO rocket or Spaceplane?

[ROFLCopter64bit]

An SSTO rocket or Spaceplane?

An SSTO spaceplane.

[bsalis]

I guess more than anything, creating an SSTO Spaceplane is about balance, and then some. To start with you probably want to start small, then pick an engine count. Look at some existing designs as well.

Might be easier if you post some pics of your attempts and the problems you have.

[foamyesque]

An SSTO spaceplane.

Oh.

Well, that's a bit more complicated. In general, the idea is to blast your plane to >22km and >1.2km/s or so, then close your intakes and turn off the turbojets and engage rockets to bring you up to orbital speed and altitude. Lower overall weight, as with any rocket, makes it easier. You'll want at least a 2:1 ram intake:turbo jet ratio (no other intake or engine is worth using for a spaceplane, so don't bother). If you put your fuel tanks squarely on the CoM of the plane, they will not cause a CoM shift as they empty. Doing this properly necessitates not stacking any tanks one on top of the other. Put your wings either on or just in front of your CoM and add a stabilizers at the back. Wheels should be a nose-gear and two mirrored wheels that are just a hair behind the CoM.

Take a look at this, for example (I was able to get away without a proper tail stabilizer because of how small the plane was):

Edited April 22, 2013 by foamyesque

[RoboRay]

I am horrific at designing SSTOs. That said, what is the key to making an SSTO? Is it the ISP, amount of fuel, or overall low weight? SSTOs look cool, it would be great if I could make one for the first time

All of the above, and then some.

And for planes, the arrangement of Center of Lift and Center of Mass really become important, not just in relationship to each other, but also in relation the airframe components and flight controls.

But the smaller your design, the easier it becomes. My smallest two-seat SSTO plane weighs in at just 12 tons...

Edited April 22, 2013 by RoboRay

[V1rtua1An0ma1y]

*Disclaimer* This is not the end-all of how to SSTO and I'm a very patient person. This may not work for you, but here are a couple points that I think would've made my life easier in designing a true SSTO from the get-go:

0.) This shouldn't need to be said, but if you've ever built anything in KSP, use struts liberally. Everything wobbles, and your plane will veer off the runway unexpectedly if things are flopping around.

1.) (And this has been said already) The bigger it is, the more difficult it'll be to get in to orbit. I'd aim for something like 20tons and wouldn't exceed 40 if you're new to the SSTO scene.

2.) Intake Air. I know it's been said a few times about 2:1 intakes to engine, but I've found that something like 3 or 4:1 works well for me. That means more altitude and more speed before you start in on your precious rocket fuel.

3.) Lift. If your 40-ton space plane has just 1 set of the delta wings, you might find it tough to get off the runway. Give it a decent wingspan, and place your center of lift behind your center of mass. Just remember that the farther back that lift is, the harder it'll be to get the nose up and climb effectively. (I like putting canards far forward to at least coax the nose up off the runway). Also, have your rear landing gears slightly higher up (shorter) than the front. This causes the plane to sit on the ground pointing upwards a bit already.

4.) Efficient rocket engines. Your thrust to weight doesn't have to be much over 1. You're going to have burned your jet fuel, you'll be in less-dense air, and gravity will have diminished somewhat. A thrust to weight of 1.5 or 2 just isn't necessary. As long as you can manage a greater than 1 ratio, go for efficiency over power every time.

5.) Precision. This has to do with how you ascend.

- The faster you're going in the atmosphere, the more you'll be paying for it via drag. If your horizontal speed is 400m/s and you're only 5km off the ground, you're doing it wrong. You could have used a lot of that energy to climb instead.

- Know how much fuel you have. If you brought 2 Mk1 fuselages, then you have 300 liquid fuel to use on the jet engines. If you're in orbit and carrying 100 of that around, you did it wrong. You can't use it via rockets since you won't have enough oxidizer.

- Push your turbo-jets to the limit. Watch your resource tab and specifically your intake air. You'll probably want 2 jet engines, and they'll function until your intake air hits about .10 . The higher you can go before you turn on the rockets, the less fuel you'll need to get to orbit. If you're climbing slowly, your plane will start to pull to one side as one of the engines suffocates. If you can catch this as soon as it starts, this is exactly when to kill the jets and fire the rockets.

6.) Your SSTO is not a rocket. I can't stress this enough: It's a plane. If you're taking off vertically or not equipping jet engines, or staging a bunch of parts during your ascent, then you've built a weird rocket, not a plane.

A standard ascent for me - Take off and climb as quickly as you can with only jet power. Once you're above around 10km or more, start to level out and gather speed. The higher you go, the less the air is going to fight you. Your goal should be to get to 600m/s when you're at about 18km (this doesn't have to be exact). Watch your intake air and continue climbing slowly. Ideally you'll run out of liquid fuel for your jets at the same time you run out of intake air. At this point you want to light the rocket and shutdown the jets (use an action group so you can close all of your intakes as well -> less drag). You're still a plane. If you start to climb at an angle higher than 60, you're wasting energy to gravity losses; not to mention you need substantial horizontal speed to orbit anyway. Climb at around 45 degrees, get your Ap above 70km, and circularize accordingly.

*Using these rules I can put a plane into a circular orbit at 80km with almost 2000 delta-v remaining. That's more than enough to dock with any station or satellite, assuming similar orbital planes.

Anyway, I'm no expert, I've just found something that works for me. Please ask questions or provide better means of doing this if you've got them!

Edited May 15, 2013 by V1rtua1An0ma1y
Grammar/Spelling

[Van Disaster]

Step 1: Install FAR. Your planes can then look like planes, you can design them using 100 years of aviation knowledge as a reference, and you get to choose what control surfaces do.

I seem to do ok with one intake per jet; I let the plane accelerate at 21km or so until it's hit 1500m/s and then pull it up to 10-12 degrees & light rockets. NERVAs work fine, you'll want a 1:1 ratio with jets ( unless it's a tiny plane, then I'd use one NERVA & two jets ). Make sure the rocket thrust is aligned with the centre of mass, it's less important for the jets.

Wings. Wings are light, usually you can't have enough wings. You're attempting to fly level at around Mach 5, at 70,000 feet, on a planet 1/6th the diameter of Earth ( and I think 1/6th the atmosphere height? I forget where Earth atmosphere actually ends ); you're also hauling enough rocket fuel up to that height to see you the rest of the way. You need much bigger wings than you probably think you need.

[K^2]

There are basically two things you need to understand about SSTO.

1) Don't try to design it as a plane. Design it as a rocket. Fly it like a normal rocket. You simply use the first stage that's air breathing and you don't separate it out when you switch to second stage. This is the lightest SSTO I've ever built.

As you can see, it does not have any wings. The fins are just for extra stability. And while it has separators, it cannot actually drop the tanks, since the rocket fuel tanks are on the outside. It does separate to land. This is just experimental craft, and I didn't want to add weight with parachutes big enough to lower the whole thing.

All you need to make this work is about 1.5 - 2.0 TWR and about 3 ram intakes per jet engine to get sufficient altitude. You should also do gravity turn a bit earlier than with the normal rocket. It increases time you spend in atmo, but that just lets you get more work out of your jet engines. And don't forget to close them intakes when you switch to rocket power.

2) If you want to actually land it as a space plane, you'll need to make sure your SSTO is well-balanced regardless of how much fuel it used up. This is an illustration of how I usually do this.

Since CoM is aligned with centers of tanks, as the tanks drains, CoM stays put. That means that CoM is always just a touch ahead of Center of Lift, and that lets you aircraft stay well-balanced throughout the flight. This particular design, while still being a work in progress, has been successful at taking off from the runway, climbing near vertically at full jet thrust, performing gravity turn, switching to rocket power, establishing orbit, doing a turn around Kerbin, perform de-orbiting burn, and use jet power to land at KSC landing strip.

Note also the long "neck" of the ship. I'm using that to counter-balance the engines, so that the center of mass is where I want it. Naturally, you don't want to waste this space, but if you need such a bit ship, you probably have the payload to put there.

[Rune]

That fuel manifold. Can I suppose the ones in the back go to the engines, and they work? Because I am totally stealing the idea for a thing I'm working on.

Rune. I mean, I'm totally being inspired ;P

[K^2]

Yes. I've adjusted it slightly since taking that screenshot, though. I moved the hard point onto the actual jet tank in the middle and got rid of the line between hard point and jet tank. It wasn't pulling fuel quit right otherwise. But yes. You can take the lines directly to engines so that they all feed from the same point. Makes managing fuel much easier.

[Van Disaster]

As a different example, here's one I threw together in 15 mins as a demo for someone:

Notice how much dV it has left over... that was the smallest usable tank for that one.

That by itself is a bit useless, but all my small spaceplanes look like that.

That's a SAR version for rescuing stranded Kerbals. Occasionally the game likes to throw them in orbit, for some reason. I've no idea why that wing design is so successful - I started throwing parts together to make something that looked wierd and it just worked - but it's not at all unstable no matter what the fuel use is. I use TAC fuel balancer a lot, that is pretty essential - I'd quite like it to be programmable.

As the planes get bigger you're more likely to want to use delta wings instead of straighter ones.

[Rune]

Yes. I've adjusted it slightly since taking that screenshot, though. I moved the hard point onto the actual jet tank in the middle and got rid of the line between hard point and jet tank. It wasn't pulling fuel quit right otherwise. But yes. You can take the lines directly to engines so that they all feed from the same point. Makes managing fuel much easier.

K, thx! Now I get the concept, I might even use something else instead of a hardpoint.

[Firov]

I am horrific at designing SSTOs. That said, what is the key to making an SSTO? Is it the ISP, amount of fuel, or overall low weight? SSTOs look cool, it would be great if I could make one for the first time

My first tip for designing SSTO space planes, and probably the most important, would be to download the Ferram-Aerospace-Research Mod.

The stock "aerodynamic system", and I use that phrase loosely, is extremely rudimentary and inaccurate. For whatever reason, drag is based off of the mass of the part, and lift is purely a function of angle of attack. Also, drag calculations don't take into account part positioning or function. For example, if you place a nose cone on a fuel tank, not only does that not reduce drag for the fuel tank, it actually increases drag because the nose cone is generating it's own drag in addition to the fuel tank.

Not ideal, by any means. That's where the FAR mod comes in. It completely removes the stock system and replaces it with a more detailed, more realistic aerodynamic system. So things like nose cones, payload fairings, and cargo bays actually help. Drag is no longer a function of mass, and so on.

Now, some people do consider it more difficult, and in a way, since its deeper than the stock system, I can understand where that belief would come from. However, the stock system isn't intuitive at all, since it's not based in actual physical laws that we're familiar with. FAR is better in that regard since it's based on a realistic aerodynamic model. If it looks like a plane, it's likely going to fly like a plane. So you can draw inspiration from actual aircraft.

Before you install it, just make a backup of your parts folder, so you can quickly and easily reverse the install, if you choose.

Anyway, onto more general tips.

Generally, you're going to want the center of mass (CoM) in front of the center of lift (CoL). This is going to give you a more stable and easier to control aircraft. Also, be mindful of the shifting center of mass as you deplete your fuel during ascent. If your CoM shifts behind your CoL while you're in atmosphere you will not go to space today. Try to design your aircraft so that using fuel won't cause a huge shift in CoM. For my SSTO shuttle below, notice how the wings are near the very rear of the craft.

Also, as others have mentioned, you're probably going to want to start small. My first spaceplane was a little one man craft with just a couple of engines and weighed maybe 10 tons. As the size increases, so does the difficulty of designing it. Use small space planes just to teach yourself the basics of spaceplane design and flight. Once you understand that, then you can move up to the big space planes.

Fly with trim!! A lot of people don't know this, but KSP includes a functional trim system, which is a lot more accurate, and convenient than trying to manually fly the craft. You can trim in a direction by holding alt and hitting a directional key. Holding alt and hitting "X" will reset all trim to default, which you'll want to do when you get into space, since trim affects all directional systems, from control surfaces, to RCS, to the reaction wheel. Past initial takeoff and minor course corrections, I almost never manually control my shuttle while still in atmosphere.

Your ascent profile is going to depend on the specific design of your spaceplane, so that's going to take a little experimentation. Don't get discouraged if you fail a few (dozen) times. For my SSTO shuttle below, I climb out at 45 degrees until 10km, at which point I let gravity slowly pull the nose down to a 20 degree angle, which I maintain (with trim!) until just before the jets cut out due to lack of oxygen, usually around 23km at Mach 4. At this point I cutover to the rockets and maintain thrust until my apoapsis is where I want it. 70km if I'm trying to rendezvous with something, or 100km if I'm just releasing a payload. With this profile, I usually still have around 800 delta-V to play with, depending on the weight of my payload.

As you've probably already noticed, at high altitudes, jet engines will flame out due to lack of oxygen. The altitude they flame out is a function of air density, air speed, and intake area. More intakes can help you stay on jet propulsion for longer. A number of people like a 3:1 or even 4:1 ratio. Personally, I use a 1:1 or 2:1, but that's just for aesthetics.

Also, don't overdo it on jet fuel. Jet engines are extremely efficient and sip fuel. Try not to carry more than 100 to 150 units of extra fuel into space for your return to the KSP, otherwise its just wasted mass. For my SSTO shuttle, I use a modified docking clamp that has a huge liquid fuel and oxidizer capacity to transfer excess fuel out of the plane before launch. At launch, I only carry about 250 units of fuel for the jet engines, and that's for a four jet engine 35 ton spaceplane.

If you have multiple jet engines, flameouts can cause a flat spin, since jet engines don't all flame out at the same time. This can be extremely difficult to correct for once it happens, and so you're going to have a bad time if you don't shut down your jets before they start cutting out. Rather than manually trying to toggle the engines by right clicking on them, set up a series of action groups to shut down your jet engines, close your air intakes to reduce drag, and activate your rockets. Then do the inverse with a second action group. That way, with a single keypress you can instantly switch over your propulsion systems.

This SSTO space plane has a ~5 ton cargo capacity and is the pride and joy of my space program. I use it for cargo delivery to orbit as well as crew transfers. It was originally designed with just KW Rocketry parts and the FAR mod. Over time, I've added additional components, usually just to make it look better. At this point, it's using wings and intakes from Taverio-s-Pizza-and-Aerospace. The cargo bay and inline Mk3 docking are from Mk3 Fuselage Expansion Pack. Finally, I use RLA Stockalike for smaller, weaker, RCS ports. I'm still trying to perfect the RCS placement though. It's probably gone through 8 or 9 design revisions at this point.

Looking back at this post, I think I might have been a bit too verbose.