SSTO help

[1bdI]

I just can't seem to get the hang of SSTO's.

Some I have been able to get into orbit, but with so little fuel left that it is pointless.

Not sure what I am doing wrong, but I think it somewhere in my flight.

Usually what I do is climb fast to get out of the thick stuff, then once I hit 13k or so, decrease my vertical climb speed to around 10 or so and let my speed increase. Using turbojets and rocket engines design I can usually hit around 20k, with speeds around 1000. However as soon as I cut the engines and switch to rockets the whole plane drops out of the sky and my speed drops off dramatically. By the time I manage to start climbing again I have burnt most of my rocket fuel.

Using sabres I can just switch modes and avoid the speed decrease, but my designs usually don't have enough fuel left to do anything useful once I attain orbit.

Also I can't seem to get above 1000 mph or so at 20km , without the plane becoming very hard to control. Usually my intakes are starving at that point and the plane starts skidding left and right. I dislike designs with tons of air intakes, so I just usually use no more than 4 or so.

Help!

[John Crichton]

[Tank Buddy]

You shouldn't be using your rocket engines until you can no longer fly on turbojets. You should have adequate air intakes to support flight up to 25 km, where your speed should be anywhere from 1300-1500 m/s. There you can pitch up and turn on your rockets. Be sure to turn off the turbojets if you have more than one.

[Hodo]

Here is one of my simplest SSTOs.

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It can climb to space stupid fast and has fuel to do things afterwards. It isn't a long range SSTO but it works and it isn't one of my best.

The typical climb profile for all my SSTOs is this.

1-climb at 45deg till 15km alt, or air required drops to 300%.

2-bring climb down to 15deg and accelerate at best speed till Mach 5.0 or engines will no longer produce thrust.

3-stay on jet engines until either starts to slow down or lose altitude switch to rockets at no lower than 25km altitude.

4-nose up to 30-40deg climb to desired orbit altitude.

5-circurlize orbit.

If you want to check out some of my craft to see if you are doing anything wrong or different here is my forum thread on all of my SSTOs and the mods needed for them.

http://forum.kerbalspaceprogram.com/threads/56945-Hodo-Creations-SSTO-craft-designs-and-files-for-those-who-want-them

[Jouni]

With one turbojet and one ram air intake in a single-kerbal spaceplane, you should be able to fly at least 1900 m/s at 33 km before flameout (and continue even higher by manually reducing throttle). The idea is to climb to 10 km as quickly as possible, and then turn closer to horizontal, so that by 20 km, your climb rate is around 30 m/s. With more intakes per engine, you can afford a higher climb rate during acceleration. When you are about to reach the altitude where flameout occurs (you may need some experience with that particular design to find the altitude), ignite rockets, pitch up to 30-40 degrees, and shut down jet engines.

If your spaceplane becomes hard to control, you may have center of lift ahead of center of mass. A picture of the plane would be helpful.

[capi3101]

General rules of thumb for spaceplane design -

*1 turbojet for every nine tonnes of payload (that's everything besides wings, jet fuel tanks, jets and intakes)

*200-250 units of liquid fuel per turbojet. That's actually a little more than you need but it's more forgiving of a bad ascent profile.

*3 Ram air intakes per turbojet. Since you don't like airhogging, I won't recommend more than that, though I will say the higher the ratio of intakes to jets the more payload you can sustain.

*1 pair of swept or delta wings per six total tonnes of craft.

Flight profile - Once your design has actually made it off the runway and assuming it climbs (an initial problem with many of my designs), you want to ascend with your prograde vector around 45 degrees or so to 10,000. Then start gradually levelling off your ascent through 15,000 or so - you want your rate of climb to be not much more than 10 m/s (use the gauge to the side of the altimeter) at that point, and especially so once you're up to 20k. Keep it level and continue to ascend. You'll be able to maintain full throttle up until you've got about 0.03-0.04 Intake Air per engine; after that, if you've got more than one jet, you'll want to start throttling back if your plane begins to yaw and you're not telling it to. A few notches will do the trick. Continue throttling back as needed until a) you're no longer accelerating, or you run out of gas. Only then do you fire up your rockets - if you've still got jets, keep them running, though; they might not be outputting a lot of thrust at that point, but they're still offering some. If not, or once you've throttled back almost all the way to zero, kill the jets and throttle up. You shouldn't have to burn the rockets for very long at all if you've done things correctly.

[Jouni]

As you can see from the answers, there are many different ways to build spaceplanes. I personally prefer to use more engines than capi3101, maybe something like 1 turbojet for every 7-10 tonnes of total mass. If something doesn't have enough thrust to climb vertically, it belongs to the local gliding club, not to my space program.

If you are carrying crew or cargo to LKO, that extra thrust pays off, as ascent is quicker and easier. On longer trips, jet engines are just dead weight, so ultimately capi3101's approach becomes more efficient.

I went to test how much faster a spaceplane can safely climb with intake-to-engine ratio 3:1, so I put some extra intakes to my 7.5-tonne light spaceplane with one turbojet. Thrust was at 100% all the time, because this is my space program, not a gliding club. At 20 km, my climb rate was 200 m/s. That felt a bit too much, so I started turning towards horizontal flight. At 26 km, the rate was still 100 m/s, getting down to 50 m/s at 28 km and 30 m/s at 28.5 km. After that point, the climb rate was steady for about a minute, until the plane started approaching orbital velocity. At around 30 km, the climb rate started rising again, even though the nose was pointing horizontal. The rate became 50 m/s at 34 km and 80 m/s at 39 km, at which point the turbojet flamed out. After a few short burns from the rocket engines, the plane was at a 120 km orbit, having spent 130 fuel and 38 oxidizer.

[1bdI]

Thanks for the replies.

Having some altitude and speed data points will help me out alot. Capis tips are also very useful, but I don't use mechjeb, or whatever program that is in hodo screenshots that gives you the stats on your plane as you build it out, so I was never really quite sure of how many engines to add versus weight, and things like that. I suppose I should, though it would be nice if that was included in the game at some point. I also have been experimenting with B9 parts, still not sure how everything works with those, there are ALOT of parts in that mod.

I did get one up to LKO last night with enough fuel to at least get the space station and refuel. I had to through on alot of intakes though. I also tried a simple single turbojet on a tiny plane, but got nowhere near 1900 km at 33k even with more than 1 intake.

[WhiteKnuckle]

Get FAR.

With FAR, pretty much anything you can get off the ground (not easy) will be able to get into space with the proper accent profile.

[capi3101]

Capis tips are also very useful, but I don't use mechjeb, or whatever program that is in hodo screenshots that gives you the stats on your plane as you build it out, so I was never really quite sure of how many engines to add versus weight, and things like that. I suppose I should, though it would be nice if that was included in the game at some point.

That's Kerbal Engineer Redux if you're interested. In the stock game you are limited to having to manually calculate your weight as you go (actually, as you go along you can hold down your assembly with launch clamps and occasionally take it out to the runway to see how you're doing in mass - just go to the map and hit the knowledge tab to have it tell you your mass; not exactly an optimal solution but it should help).

[Hodo]

Thanks for the replies.

Having some altitude and speed data points will help me out alot. Capis tips are also very useful, but I don't use mechjeb, or whatever program that is in hodo screenshots that gives you the stats on your plane as you build it out, so I was never really quite sure of how many engines to add versus weight, and things like that. I suppose I should, though it would be nice if that was included in the game at some point. I also have been experimenting with B9 parts, still not sure how everything works with those, there are ALOT of parts in that mod.

I did get one up to LKO last night with enough fuel to at least get the space station and refuel. I had to through on alot of intakes though. I also tried a simple single turbojet on a tiny plane, but got nowhere near 1900 km at 33k even with more than 1 intake.

Honestly the two best mods you can run is FAR which replaces the stock KSP aerodynamic model with one that is actually an aerodynamic model. The stock KSP aerodynamic model was made by a Snork, because they have no clue what air is or how it reacts, it is closer to a hydrodynamic model if anything.

The plugin you see in that picture is Kerbal Engineering Redux, which is an information plugin and that is it. It is handy for so many things but MechJeb does the samething and adds an autopilot function. It is ultimately up to you, I am currently testing MJ and I like it for somethings and hate it for others.

With FAR and B9 you can build an SSTO space plane pretty easily, but flying it right is the challenge.