SSTO Tips

[Yobobhi]

I've been building SSTOs, and have made a slightly successful model, that isn't an SSTO, but is very close with 2 recoverable SRBs giving it the extra kick it needs. What would be an optimal fuel-engine-wing combination for an SSTO, and what would be an optimal platform?

[Alshain]

Well, first things first... what kind of SSTO are we referring to in this case... plane or rocket?

[Hodo]

As Alshain asked is it a "rocket" or a "Plane"??

The other question is it stock install or with mods?

But ultimately a SSTO craft of any type you want to have enough delta V to get to space and orbit without dropping anything from the craft.

Make sure you get above 10km as fast as you can, then start to build up as much horizontal speed as you can.  This is best with a turbojet engine or a RAPIER engine in airbreathing mode.   After you get above mach 4, or around 1200m/s I would turn to climb up at 35deg angle or better till I hit target AP.  

[bewing]

Since he said the word "wing", it's gotta be a spaceplane.

MK1 designs are the easiest to get to orbit, but hard to get back down again because of heating problems -- especially in the crew cabins. They land nicely because they are very light and small. You need to use interstage fairings if you want to carry cargo. Once they are in orbit, they can go anywhere easily because they are small and light. You have to scale your missions down to small numbers of kerbals and a minimum amount of equipment.

MK2 designs are pretty, are OK at handling heat, are OK for cargo capacity, and have horrible drag issues -- so they are very hard to get to orbit. They are quite easy to get back down again to land. They land OK, because they are not very heavy.

MK3 designs are extremely heavy, are good at handling heat and cargo -- and you stick so many giant engines, and such a gigantic amount of fuel in them that they get to orbit despite the fact that they are big fat ugly bricks. Good luck with the landings.

2.5 meter designs look clunky, but are probably the best overall for performance -- except for the fact that there are no LF-only 2.5m fuel tanks.

 

There are many debates on the forums about wing area. If you are trying to get to orbit with a lot of fuel remaining, then wings help you lift all that fuel to high altitude very efficiently, and they pay for themselves that way. BigS wing parts also have internal fuel tanks. So you get free fuel storage without having to add extra tanks. It takes a long time to fly a plane to orbit using the wings for lift. Some people don't like to wait that long, so they put little tiny wings and a big engine on the back and pretend that the plane is a rocket, and just blast their way up to 70km. This is fast, but it does not allow you to lift nearly as much weight. An additional problem is that all the stock wings are fairly small. The FAT wings (which are the biggest) have very low heat tolerance, and tend to snap off under high loads. If you try to "tile" a bunch of stock wings to make a bigger wing, it ends up looking silly most of the time. If you go with using plentry of wing area, then a good idea is to put enough to keep your high speed AoA below 2 degrees (preferably adding inclination to the wings to bring the AoA to 0).

So I always either go for MK1 or 2.5M designs, and use a medium amount of wing. I like my SSTOs to be able to fly to Minmus, refuel, and then go anywhere. They carry kerbals and scientific equipment. I never carry cargo.

 

Edited April 14, 2017 by bewing

[FancyMouse]

43 minutes ago, bewing said:

Since he said the word "wing", it's gotta be a spaceplane.

<joke>Not necessarily. I put big delta wings to function as tail fins for my huge rocket where real tail fins aren't enough.</joke>

[Alshain]

1 minute ago, FancyMouse said:

<joke>Not necessarily. I put big delta wings to function as tail fins for my huge rocket where real tail fins aren't enough.</joke>

Not a joke, Winglets are still wings.  He could be asking how much wing you need to balance the rocket.  That's why I asked, nothing in the OP really suggests one or the other.

Edited April 14, 2017 by Alshain

[Wanderfound]

56 minutes ago, bewing said:

Some people don't like to wait that long, so they put little tiny wings and a big engine on the back and pretend that the plane is a rocket, and just blast their way up to 70km. This is fast, but it does not allow you to lift nearly as much weight

Seventy tons of payload in a fifty ton aircraft:

Different flight styles, similar abilities.

 

[Foxster]

A few SSTO thoughts...

Minimise drag. You spend (relatively) a lot of time in the atmos with an SSTO spaceplane and fat parts, multiple stacks, sticky-out bits, no incidence on wings, unneeded intakes, excessive wing area, etc will all rob you of the chance of making orbit. 

Very often the answer to getting an SSTO craft to orbit is not "moar boosters". Think instead about reducing engines, fuel, wings, crew. 

Tune your fuel carefully. Don't get to orbit with an excess of LF or O that is no use to you. If you are building a simple Rapier-based SSTO then get to orbit with a ratio of 1.2:1 of O to LF remaining, plus perhaps a little extra LF for landing assist. 

Rapiers are your friend and the easiest engines to use for an SSTO. Get the hang of building SSTOs with them and once you have that sorted then you can think about adding things like nuke engines.  

If overheating is an issue on re-entry then tune the CoL v CoM so that they are fairly close at that point by shifting fuel about but leaving CoM just ahead of CoL. Then, during re-entry, come in belly first with the nose near vertical. This will create lots of drag, slowing you down quickly. 

[AeroGav]

My rules of thumb -

 

One jet engine per 30 tons of launch weight.  On a design with 3 or more jet engines, consider making one of those jets a whiplash, the other two can be rapier.   The whiplash's good low speed performance helps get the rapier supersonic, where it really begins to shine.  This trick allows you to lift more for the same engine mass than an all-rapier design.

You need about 60kn of close cycle power per 15 tons of weight.   One Rapier puts out 180kn.   A terrier or a nerv gives you 60kn.

About 100 units of oxidizer per 30 tons of launch weight.   You only need enough ox to boost you from air breathing top speed (1500 m/s) to about 1750 m/s.   Your NERVs should then be able to do the rest.

Wing - I'd say one lift rating per two tons of takeoff weight is a good middle ground.    You don't save much dry mass cutting it finer than that.   It does depend on whether it's a pure chemical or nuke equipped spaceplane.   A nuke equipped spaceplane will have much higher dry mass,  so cutting back on wing won't  add much delta V.    It will also have quite weak TWR, so it will need good lift:drag ratio to be able to climb in the upper atmosphere. Finally, wings hold liquid fuel, which a nuke spaceplane can put to use.  A chemical SSTO only needs enough for the air breathing phase of flight, any more than this is dead weight.

 

Weight/Balance/Trim Drag -

Get RCS Build Aid and CorrectCoL.

Try to arrange the engines and other heavy parts so that the plane is balanced without any fuel or cargo on board.  Ideally, the cargo should be in the middle and you should have equal amounts of fuel at the front and the back, so that CoM does not move much in flight.  This makes it easier to fly an accurate flight profile, than if the plane's balance is constantly shifting as the fuel burns off.  Many people just cover up bad handling characteristics by locking the nose angle with SAS, but that makes it impossible to make small adjustments with pitch trim etc. and it can lead to nose-off prograde angles building up the longer you leave the controls untouched.  OTOH if you do press one of the WASD keys the airplane pulls a 15g manuver and looses a bunch of speed.

Finally, if your fuelled / empty CoM are far apart, you are either going to be uncontrollable on return from orbit, or you're going to have to move CoL very far aft of CoM to guarantee stability.  The excessive stability at the beginning of flight, means your control surfaces will be working overtime just stopping it from impersonating a lawn dart, generating high drag.

 

A quick word on drag -

This is a very big subject.  A lot of people react with incredulity when I tell them you only need one jet engine per 30 tons , but if you minimise drag this is quite possible.

The most important thing to remember is that the game does not look at the shape of your craft the way we, or real lift aerodynamics do.   The game is most concerned about your craft's tree structure.  It wants to see every stack start and end with something aerodynamic.  When two attachment nodes join each other, it is vital that both be the same diameter to avoid excess drag.  Ie. Don't put a 1.25m engine on the back of a mk2  fuselage, you must use a mk2 to 1.25m adapter first, either the bicoupler or the single engine mount as you see fit.

In the real world, most of the drag of a high performance airplane comes off the wing (and engine intakes).  In KSP,  well over 90% of the drag in most designs is coming off the fuselage itself.   Fitting a smaller wing may make your plane look like a high speed design (x-15, f-104 starfighter) but will force it to fly lower or at higher AoA to get enough lift, which will increase fuselage drag greatly.       Fuselage drag is so severe you really have to use wing incidence to get a worthwhile mk2 ssto, since that enables you to keep the fuselage near prograde angle. 

 

 

Edited April 14, 2017 by AeroGav

[blakemw]

For those starting out to plane-based SSTOs I recommend to begin with very thrusty spaceplanes because they are more forgiving.

My first successful spaceplane SSTO.

From there you can work on getting the engines to lift more weight into orbit, by making better use of wings, smarter ascent trajectories and improving the ratio of lf to lf/ox so as little unburned propellant remains as possible. The little plane pictured above probably has a runway TWR of about 2 which is ridiculously high, in contrast the lowest viable runway TWR is about 0.3 (through 0.5 is more comfortable), this provides enough power to get enough lift to get into flight, the RAPIER enjoys a high mach multiplier which means as the plane goes faster it generates more thrust, so by flying horizontally over the ocean at fairly low altitude you can gradually build up speed until TWR ramps up allowing the plane to go supersonic, at that point it should have plenty of thrust to climb up to higher altitudes. (note you can also just end up flying horizontally until the plane has burned enough fuel that it can break the sound barrier: in that case you should've just launched with less fuel, which also makes a great way to tune a spaceplane, if it can't break the sound barrier, remove a small amount of fuel and try again).

Note that rocket-SSTOs, based on Twin-Boar or Mammoth, are much more practical and generally economical than spaceplane SSTOs. You should only build plane SSTOs because you enjoy it, not to get stuff into space.

Edited April 14, 2017 by blakemw

[GoSlash27]

I concur with what @Foxster and @AeroGav said. The key is reducing drag rather than adding thrust, so always look for what you can do without instead of what you need to add. Once you get supersonic, it's relatively easy to make orbit.

Best,
-Slashy