Difference between revisions of "Tutorial:Basic SSTO Design"

Revision as of 16:02, 28 June 2015

This tutorial explains how to build a single-stage-to-orbit (SSTO) spaceplane that is able to launch into space and return without jettisoning any parts.

Powerplant

Any craft that hopes to get anywhere needs engines. For SSTO craft there are two popular options.

The first is jet engines. Relatively high efficiency and thrust as well as low cost make these well-rounded and sensible engines. In the case of SSTO craft we will need jet engines that can perform as high as possible to justify the inability of these jet engines to perform in a vacuum. Currently the TurboJet Engine is by far the most sensible choice. While the Basic Jet Engine is lower cost and consumes less fuel and money, the turbojet is capable of decent performance at high altitudes where the Basic Jet Engine cannot keep up.

Jet Engines

Jet Engines are powered by liquid fuel and intake air. Liquid fuel can be supplied from regular rocket tanks, but jet fuselages would be a far more sensible choice due to their higher economy as well as their exclusive supply of liquid fuel. Intake air is provided by air intakes. Depending on how many you place, your jet engines' effective service ceiling can be anywhere from 15km to 30km. The Shock Cone Intake is currently the air intake with the highest amount of potential air intake.

Liquid Fuel

The second is liquid fuel engines. Liquid fuel engines are what will carry your craft into orbit. The Toroidal Aerospike Rocket is one of the better choices due to a low profile and high specific impulse (it performs better than other engines). For more economy-minded builders the LV-T45 is not a bad choice. However it's huge size can make it tricky to take off from the runway without destroying the engine.

Stability

One of the most important factors to how well an SSTO will perform is its aerial stability. A stable craft is going to be more desirable than an unstable craft. There are several forces in Kerbal Space Program which have an effect on the flight characteristics of SSTO craft.

Center of mass

One is Center of mass. This is the point from which there is an equal amount of mass in each direction and acts as a fulcrum for any rotational forces. The center of mass of any craft will shift as a result of burning fuel, but this is especially critical for SSTOs as spin-outs and otherwise catastrophic stability failures will hinder its ability to reach space.

Center of lift

The second is Center of lift. This is arguably the most important characteristic an SSTO designer should consider when building. To prevent craft spin-outs and otherwise catastrophic stability failures, it is important that the center of lift always be to the rear of the center of mass.

Center of thrust

A sometimes overlooked factor is Center of thrust. This is where force will be acting upon your craft to push it forward. Though this is may be a non-issue with most SSTOs, it would be wise to check if your center of thrust is more or less inline with your center of mass and lift.

The bottom line: Ensure your center of mass is always ahead of your center of lift, and your center of thrust is inline with your center of mass.

Additional Systems

These are common features of SSTO crafts (however the Command Pod is a must, like always).

Command Pod

The possibilities are limitless. You can create either manned or unmanned SSTOs. Just remember than manned ones have crew and unmanned have electricity. The Mk1 Cockpit is an example of a manned command pod, while the Probodobodyne HECS is an unmanned command pod. A probe will require a fairing or cargo bay to maintain aerodynamics.

Takeoff Gear

"Takeoff gear" is being used here since it is possible to launch a craft using the TT18-A Launch Stability Enhancer and land using parachutes.

But for conventional SSTOs the Small Gear Bay is what you'll be use to be able to take off. Remember to retract these wheels right after takeoff and to extend right before landing.

A parachute is useful for SSTOs as it removes the need to land. Simply deploy to return to the ground safely.

SAS

On most SSTOs with a command module or similar central pod there is going to be an ASAS. Since the standard cockpits are supplied with their own SAS and a decent one mind you, it is mostly unnecessary to add additional SAS to your SSTO, in the form of reaction wheels. However, for larger craft it may be important to include external SAS to help maneuver with the extra mass, and you can always disable external SAS at your whim.

SAS is a stability augmentation system, which basically means it helps your craft fly straight. In order to accomplish this, it could hijack the circuits to your control surfaces but it also provides "torque", which would be applied here as "movement". This helps your craft maneuver in vacuum, even without RCS, and where conventional control surfaces are ineffective.

RCS

RCS is required for maneuvering in a vacuum, a must-have if you plan to actually perform tasks in orbit. Otherwise it is unnecessary for simple orbiting/deorbiting. However, on heavier and/or less stable crafts, an RCS coupled with SAS can make flying much easier.

RCS is activated the with the "R" button with stock controls, first and foremost. RCS is fueled exclusively by Monopropellant tanks. When your monopropellant stores are depleted your RCS will fail to function.

Aerodynamics

Specifically Wings and Control Surfaces. These are what allow the SSTO to function as an atmospheric craft. The Delta Wing is a solid choice, with a large surface area handy for attaching additional objects to, as well as being rather symmetrical and easy to work with.

However, the Delta Wing is fixed and cannot provide any kind of maneuvering (it only makes your plane fly). In order to turn in-atmosphere, you are going to need control surfaces. The numerous elevons, rotating winglets, and canards are all considered control surfaces. Control surfaces have the distinction of being to pivot or otherwise react to controls, providing lift in certain directions. This is contrasted with regular winglets, which are fixed and cannot move.

Sustenance

Depending on your conditions, you will need to power your SSTO with electricity. These are supplied by batteries and solar panels.

Putting the pieces together

Now that the basics of SSTO design have been covered, it is time for you to assemble your first SSTO.

Remember to consider everything above to create a working SSTO.

Troubleshooting

Answers likely questions people may run into when designing SSTOs. However, if you even try to build an SSTO you will need a basic understanding of Kerbal Space Program physics, so check there as well if this can't answer your question(s).

It doesn't take off.

Make sure your landing gear is placed such that your craft can achieve a positive angle of attack while it's on the runway. Typically you'll have either a nose wheel and two wheels slightly behind centre of mass that act as a fulcrum (your elevators push down on the tail and lift the nose, rotating around the rear wheels), or a tail wheel that's placed higher than two other wheels (which are ahead of the centre of mass) so that the craft has a positive angle of attack even when stationary on the runway and will naturally lift off at the right speed without pitching upwards.

It spins out on the runway.

Make sure your landing gear is placed symmetrically and your center of mass and lift are both in the center of the craft. You will also need horizontal aerodynamic stability, which is classically accomplished with a vertical tail fin. As with launch stability of rockets, you may find your pilot has too much control authority in the yaw dimension and is overcompensating - try using a different part as a tail fin.

I keep blowing up my engines when trying to lift off

Be slightly gentler on the stick, change the layout of your engine modules to be slightly forwards, or make your landing gear taller so your tail won't hit the ground when you pitch up.

It flips out whenever I move/start flying.

This is likely a result of a poor center of mass/lift. If this occurs just after liftoff, check these in the Spaceplane Hangar; you will be most likely find your answer there. If this occurs late-flight, it is a result of either something breaking off or fuel moving around and changing the center of mass (probably the latter). This is a relatively easy to fix problem. Before entering the atmosphere again, move your fuel forwards to hopefully place the center of mass ahead of the center of lift. Use fuselages rather than rocket fuel tanks for your jet fuel since these drain evenly.

I can't reach orbit

This is the result of not having enough Delta-V, or just poor flying. For most SSTOs, a safe procedure is to: Take off from the runway, make a 45-degree angle climb. You should know at what altitude your jet engines burn out, so shut them off and close intakes. Immediately start your main engine(s) and keep climbing till you reach your desired orbit height. Burn towards the horizon until your periapsis is at least above 70km.

I hit the runway when landing and died.

Coming in for hot landings (usually above 80 m/s) will cause your aircraft to either bounce off the runway or blow up. Slow your descent by throttling back when around a kilometer from the runway, then nose up slightly and deploy landing gear and flaps (right click your ailerons and elevators and deploy them - this will generate extra lift and drag). When you contact with the tarmac, apply brakes. If you die again you were either too fast or your center of mass was off. If you can't slow down enough, try adding air brakes and deploying them before landing.

How can I make landing easier?

Just burn retrograde when in orbit until your trajectory is intersecting with the planet. Have a parachute or two on standby and deploy them when you think you're close enough to the ground (they will activate for you but will induce significant G-force on your craft). You will probably not land where you wanted.

Is there any way to make it better than it already is?

Chances are if you have already reached this point it is unnecessary to create a new craft for the purpose of this previous craft. Your SSTO is no longer a prototype and is finally a complete, but better yet working design! You can always modify the design to suit a particular need.