Difference between revisions of "Tutorial:Basic SSTO Design"

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This tutorial explains how to build a [[single-stage-to-orbit]] (SSTO) craft that is able to launch into space and return without jettisoning any parts.
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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.  
  
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Hello, I am Suzaku a dedicated SSTO enthusiast. In the ideal world all spacecraft are launched via mass driver or any kind of launch system that does not rely on costly fuel and materials in order to reach orbit, however until that is implemented into KSP craft will have to rely on internally contained power for the launching of spacecraft into orbit and beyond.
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= Powerplant =
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Any craft that hopes to get anywhere needs engines. For SSTO craft there are two popular options.
  
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So lets get started!
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'''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.
  
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== Bare Essentials ==
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==Jet Engines==
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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.
  
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SSTO are by leaps and bounds far harder to construct than a vertical launch space vehicle in KSP. Not only do you need to concern yourself with Center of Gravity, Center of Thrust, and structural integrity, now you also need to focus on Center of lift, aerodynamic drag, Angle of attack, and Horizontal structural integrity.
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==Liquid Fuel==
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'''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.
  
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Lets get started.
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=Stability=
  
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Just to note, this tutorial does not cover the vertical take off vertical landing SSTOs, only the Horizontal Space Plane type SSTO.
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'''The one of the most important factors to how well an SSTO will perform is it's 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 affect on the flight characteristics of SSTO craft.
  
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== Why build an SSTO ==
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==Center of gravity==
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One is [[Center of gravity]]. This is where the mass off your craft is most concentrated on your craft. As a result of burning heavy fuel, SSTOs can have an unstable center of gravity which can hinder their ability to get into orbit, inducing craft spin-outs and otherwise catastrophic stability failures.
  
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SSTOs are hailed as the future of Spacecraft, not only are they far more cost effective than multi-stage rockets, they are also safer and capable of landing and taking off at traditional runways.
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==Center of lift==
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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 gravity.
  
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== Is it impossible to build an SSTO? ==
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==Center of thrust==
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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 gravity and lift.
  
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Nonsense, many people say it is in version 0.16, but I think they don't understand the nuances of building an SSTO and then blame it on the game. SSTOs are capable of reaching orbit, the Mun, Minimus! and beyond! (I even broke orbit of the Kerbin Sun with a SSTO)
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The bottom line: Ensure your center of gravity is always ahead of your center of lift, and your center of thrust is inline with your center of gravity.
  
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== Symmetry ==
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=Additional Systems==
  
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Spaceplane SSTOs by nature are not symmetrical, the tail, landing gear, etc. all will change the aerodynamic drag profile of an SSTO. Many people enjoy building a SSTO that looks similar to a typical aircraft, however many fail to consider that aircraft are designed to thrust and operate inside of an atmosphere fighting against gravity and therefore the center of gravity for such craft does not need to be in line with the center of thrust. If the center of gravity is not in line with the center of thrust, the vehicle will either spin constantly when attempting to thrust outside of the atmosphere or will have a tendency to pitch up and or down.
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These are common features of SSTO crafts (however the Command Pod is a must, like always).
  
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== Liquid Fuel or Solid Fuel? ==
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== Command Pod ==
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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.
  
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There is basically only once choice for a type of fuel for a SSTO. Liquid. A SSTO is exactly as it is described, a single stage to orbit craft which must not jettison any part of it in order to reduce operating cost and increase passenger safety. Anything once drained is dead weight on an SSTO and therefore power to weight must be put into consideration. Solid fuel boosters can be used for take-off, however it must be noted that they are quite heavy and do affect your center of gravity quite a bit. And then there is the matter of Jet Engines. They use, as opposed to Liquid Fuel and Oxidizer, Liquid Fuel and Intake Air. Now, although more efficient and just as powerful as Rocket Engines, (At least in KSP) Intake Air cannot be obtained in space. So you use a R.A.P.I.E.R. Engine. These hybrid engines are slightly underpowered as a jet and more-than-slightly underpowered as a rocket, they can run either as a jet or as a rocket. These may prove quite useful for SSTO vehicles.
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== Takeoff Gear ==
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"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.
  
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== [[Center of mass|Center of Gravity]] ==
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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.
  
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There is nothing else more important in SSTO design than center of gravity. As your vehicle consumes fuel it cannot jettison stages into a smaller and smaller stage that has its center of gravity maintained at a close to center position. The center of gravity for a SSTO will rapidly move forwards and backwards as fuel tanks are expended and this applies without changing the aerodynamic profile of the craft and therefore the fuel tanks must be positioned in such a way that they will not rapidly shift from the front to the rear after a tank is drained.
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A parachute is useful for SSTOs as it removes the need to land. Simply deploy to return to the ground safely.
  
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== [[Center of thrust|Center of Thrust]] ==
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== SAS ==
  
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The same that applies to all other spacecraft, however once again center of thrust should be in line with the center of gravity for any spaceship, even SSTOs.
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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.
  
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== [[Center of lift|Center of Lift]] ==
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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.
  
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This is the Killer issue for most players when it comes to making a SSTO. Most players have the tendency to design the coolest looking ship they can possibly design and then try to fly it. Some even try to copy real-life craft and attempt to fly it and then blame the game when the craft fails miserably. Once again, remember you are building a SSTO out of rocket parts which weigh 4 times more than aircraft components. There is no guarantee that the parts used in KSP are of the same mass of the parts used on the craft you are trying to imitate and therefore don't expect too much out of copying a craft.
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== RCS ==
  
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Traditionally the center of lift should be slightly in front of your center of gravity for lower speed craft or slightly behind it for higher speed craft. If the center of lift is in front of the center of gravity it will promote the tendency to pitch up and enhance maneuverability, a positive attribute for low speed low altitude craft.
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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.
  
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For high speed craft if the center of lift is on the center of gravity (a very bad idea; we'll explain why in a second) or in front, it will cause aerodynamic instability. Typically a SSTO should have its center of lift slightly behind the center of gravity as it provides aerodynamic stability during high speeds as well as the resistance to turning on its axis. Center of lift can be changed by aerodynamic resistance as well.
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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.
  
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== Angle of Attack During Takeoff ==
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== Aerodynamics ==
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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.
  
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Angle of attack is nothing more than the angle of the lifting surfaces in comparison to the surrounding winds (like riding a bike and getting a breeze where there was none). The greater the angle the greater the lift that will be generated as the vehicle is moving forwards off the runway. The energy required to lift the vehicle off of the runway if the wings are perpendicular to the runway or at a negative angle is exponentially greater than if the wings themselves were generating some lift as the vehicle moves forwards as the craft will be relying on nothing but the lift generated by the far smaller control surfaces.
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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.
  
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On a side note, if the angle of attack is too steep and the craft does not have enough thrust to take off it will generate instability for the craft where one wheel might lose contact will the runway and cause a tendency to yaw to one side. Also, if the craft too great of an angle of attack for the engines to compensate then the vehicle will stall and fall back down.
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==Sustenance==
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Depending on your conditions, you will need to power your SSTO with electricity. These are supplied by batteries and solar panels.
  
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The angle of attack can be adjusted on any craft by moving the rear landing gear so that it sits higher up into the fuselage than the forwards landing gear.
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== Putting the pieces together ==
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Now that the basics of SSTO design have been covered, it is time for you to assemble your first SSTO.
  
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== SAS ==
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Remember to consider everything above to create a working SSTO.
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==Troubleshooting==
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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).
 +
 
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''"It doesn't take off."''
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You're missing something there...probably a brain but also experience reading this article.
  
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SAS is almost required for any spaceplane. A computer that can stabilize your craft faster than you can react is invaluable for a vehicle with a constantly shifting center of gravity.
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''"It spins out on the runway."''
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-This is due to bad landing gear placement, most likely. The front landing gear  will need to be symmetric, you can accomplish this by using Angle snap (it will be obvious which is aligned and which is not). The rear landing gear should be just under and to the rear of the man wings. If not, a bad center of gravity/thrust.
  
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== RCS and Vectoring Engines ==
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''"I keep blowing up my engines when trying to lift off"''
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-Be slightly gentler on the stick, or change the layout of your engine modules to be slightly forwards.
  
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RCS is useful in space only, however due to pod torque mechanics it is not entirely necessary for smaller SSTOs unless the need to dock arises
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''"It flips out whenever I move/start flying.''
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-This is likely a result of a poor center of gravity/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 gravity (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 gravity ahead of the center of lift.
  
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== Advantages of a SSTO ==
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''"I can't reach orbit"''
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This is the result of not enough fuel or Delta-V. Also, check your 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.
  
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* 1/100th the operating cost of a traditional multi-stage spacecraft
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''"I hit the runway when landing and died."''
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* The ability to land at any runway and take off after a simple refuel
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-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. When you contact with the tarmac, apply brakes. If you die again you were either to fast or your center of gravity was off (see a trend?).
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* No required re-assembly or hoisting vertical required after each mission
 
  
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== Disadvantages of a SSTO ==
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''"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.
  
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* Must keep all original components from launch
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''"Is there any way to make it better than it already is?"''
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* Poor thrust to weight ratio when compared to typical multi-stage rocket designs
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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.
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* Cannot be built too heavy due to horizontal structural weaknesses
 
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* Extremely complicated design process
 
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* Must be designed to operate as an aircraft as well as a spacecraft
 
  
 
[[Category:Tutorials|Tutorial:Basic SSTO Design]]
 
[[Category:Tutorials|Tutorial:Basic SSTO Design]]

Revision as of 10:26, 25 November 2014

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

The one of the most important factors to how well an SSTO will perform is it's 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 affect on the flight characteristics of SSTO craft.

Center of gravity

One is Center of gravity. This is where the mass off your craft is most concentrated on your craft. As a result of burning heavy fuel, SSTOs can have an unstable center of gravity which can hinder their ability to get into orbit, inducing craft spin-outs and otherwise catastrophic stability failures.

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 gravity.

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 gravity and lift.

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

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.

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." You're missing something there...probably a brain but also experience reading this article.

"It spins out on the runway." -This is due to bad landing gear placement, most likely. The front landing gear will need to be symmetric, you can accomplish this by using Angle snap (it will be obvious which is aligned and which is not). The rear landing gear should be just under and to the rear of the man wings. If not, a bad center of gravity/thrust.

"I keep blowing up my engines when trying to lift off" -Be slightly gentler on the stick, or change the layout of your engine modules to be slightly forwards.

"It flips out whenever I move/start flying. -This is likely a result of a poor center of gravity/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 gravity (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 gravity ahead of the center of lift.

"I can't reach orbit" This is the result of not enough fuel or Delta-V. Also, check your 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. When you contact with the tarmac, apply brakes. If you die again you were either to fast or your center of gravity was off (see a trend?).

"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.

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