Space Planes. Why, why bother?

[Osprey]

Hi Folks,

It's fair to say that, despite a number of efforts at scrath building a space plane, I can get them into orbit with enough fuel to circularise, or to go about docking.

I think my design is alright, usually enough fuel to hit 1000m/s at 20km in horizontal flight, then enough fuel to reach 80km LKO, but never enough to circularise. Not sure if it's the design, thrust, flight profile. So anyway, two questions.

1. What are the key rules about space plane construction (I usually try to balance CoL over the CoM, but really not sure if that's right) to get best lift and best stability?

2. Why on earth should I be building this sodding plane given I can reach orbit in under 4mins with no fuss using a capsule? Maybe when budgets come into it having a totally reusable vehicle would be helpful, but the hassle of launching the thing at the moment seems not worth it.

Cheers,

Os

Edited May 2, 2014 by Osprey

[KvickFlygarn87]

First of all, try FAR. Greatly enhances SpacePlane experience. Second of all, AFAIK you want the CoL slightly behind the CoM.

[Tex]

^^^

I also prefer to not use spaceplanes, but many use them as a show of skill or as an RP thing. If, say, you are trying to not leave much debris behind, spaceplanes are incredibly useful to develop. I just never use 'em because I, like you, find them to be quite a bit troublesome.

[Frank_G]

I think spaceplanes are great and fun to fly.

Some tipps i can offer are:

Construction:

- Keep your first designs light and small. Dont concentrate on payload... first bring a plane into orbit, then think about upgrading the design to fulfill some purpose.

- Take a look at the R.A.P.I.E.R engine. Its your friend.

- With FerramAeroSpace installed, as mentioned above, aim for about 4k of delta V. That brings you into almost any desired circular orbit, even to the mun and back.

- Dont fall for the Air Hogging trick... it is not necessary. In fact it increases drag by a large amount, making your plane very inefficient.

- Keep the center of lift slightly behind and slightly above the center of mass. (see picture)

- Keep the center of thrust in a horizontal line with the center of mass. (see picture)

Flight:

- Ascend as fast as possible to about 10000 m, at best at a high angle like 50 - 60°.

- Reduce angle to something between 10 - 20°, make sure you continue to climb.

- The plane will start to build up speed now. Reduce throttle as your plane starts to accelerate faster, this will reduce the amount of intake air needed.

- Watch your intake air, either by right clicking one of your planes intakes or through the resource panel, when it gets close to 0 switch to rockets and close your intakes to reduce the drag.

- When reaching the point where you have to switch engine modes, you should have a flight altitude of about 32.000m and a speed of at least 1.500 m/s.

- Increase the flight angle to 45°, then continue like circularizing orbit with a rocket.

I hope this will help you.

Edited May 1, 2014 by Frank_G

[KerikBalm]

"1. What are the key rules about space plane construction (I usually try to balance CoL over the CoM, but really not sure if that's right) to get best lift and best stability?"

* CoL behind COM. Ideally the intakes behind the CoM too. KSP's dumb aerodynamics model has drag increase with the square of velocity, but lift only increases linearly with velocity. At high speeds, "drag parts" can overpower "lifting surfaces", even if its skewed in the other direction while at low speed and altitude. It took me a while to realize that my planes weren't flying well, and were unstable on reentry, because of the massive amount of drag from open intakes mounted ahead of the CoM. For every ram intake you have ahead of the CoM, put a radial equally far behind the CoM

* At least 3 intakes per jet engine. This shouls be enough to get you going 1,500 m/s at 25,000 meters altitude (assuming you've got a good TWR, you wont get an orange tank doing that with 1 turbojet and 3 intakes).

You can "air hog" with much more intakes (like 8-12), and get into orbit super easy- its not hard to get into orbit from 40k going over 2,000 m/s

* watch your intake air, and throttle back as it gets low to prevent flameouts - if you have just a centrally mounted jet, a flameout isn't such a big deal (so small SSTOs where 1 jet is sufficient are easiest), just reduce throttle until the engine fires back up, but for multi jet designs, the thrust imbalance can send you out of control

* Don't climb too fast past 10-15 km, the faster you go, the more intake air you have, its better to be going close to orbital velocity, than simply close to the edge of the atmosphere.

"2. Why on earth should I be building this sodding plane given I can reach orbit in under 4mins with no fuss using a capsule? Maybe when budgets come into it having a totally reusable vehicle would be helpful, but the hassle of launching the thing at the moment seems not worth it"

For fun - I've made an SSTO that can lift 100 tons to orbit, my computer runs really slow when doing it, takes me a good 15-25 minutes just to get to orbit, the SLS parts get me there much faster, and without the payload studded with strut connectors that I used to anchor it to my SSTO.

So, I built the SSTO cargoplane for the challenge, and now I just use SLS parts to launch stuff up to save time/go easy on my computer, then I pretend it was my SSTO launching it (so I do 1 or two launches to establish the SSTO lifting capabilities, then I build my payloads within those bounds, but just stick them on top of rockets, and delete the debris).

I have no desire to do a SSTO to Duna and back, although I'd guess that my 100 ton lifter could do it if it carried just 100 tons of extra fuel to orbit.

I much prefer: 1 re-suable stage to get to LKO, 1 re-suable stage to get to orbit at my destination and from my destination back to LKO, 1 stage for the descent and ascent from my destination to orbit around my destination (that is reusable, but mostly refueled and left in orbit at the destination, rather than brough back to kerbin orbit).

Its easy to do reusable Duna missions using my SSTO, but I see no reason to take my SSTO out of LKO (once, for giggles, after dropping off a 53 ton payload in LKO, I sent my SSTO on a free return trajectory around the Mun, it barely had enough liquid fuel for that, and needed to use monoprop to fine tune the trajectory to have a nice return)

Edited May 1, 2014 by KerikBalm

[Kasuha]

I don't consider myself very experienced in building spaceplanes but I have very good experience with making my planes fly higher and faster. If you can keep your plane flying at 35 km, you can reach orbital speed (2400+ m/s) using turbojet there and then circularize for almost no dv. Such flight profile calls for about three ramjets per engine, though, so nothing for intake spam haters (although I don't think three pieces make up spam).

[Hodo]

There is two ways of making a space plane.

1- RAPIER/SABRE engines, which have shorter range but are far easier to get into orbit

or

2- Jet engines and small high ISP rockets as your booster stage.

Both are quite effective when you get the design down. The biggest factors to consider when building a spaceplane is you need to factor in your TWR ratio at all stages, unlike a rocket which you can get away with slapping the biggest booster you can on it and blowing it into orbit. Ideally you want to be going well above 1000m/s before you switch over to your rockets and be between 22-30km altitude. I prefer to be going in excess of mach 5 before I switch over, so momentum does most of the work. My airbreathing stage is usually a TWR of 1.5-2.5:1 and my rocket stage is rarely more than 1:1.

I start by climbing as fast as possible to 18-20km before leveling off for speed. At that altitude I try and maintain a positive climb rate but nothing to drastic, usually less than 10m/s. And I try and accelerate as fast as possible during this time on the airbreathing engines. Once I hit 25-28km altitude my airbreathers start to run out of air and are going so fast that they start to lose power. At that point I gradually pull up and increase my rate of climb to about 20-30deg, and switch over to rocket engines, and shutdown the airbreathing engines. I also close my intakes at this time to make sure there is as little drag as possible on the airframe. Once I hit my target AP, usually 100-150km I shutdown the rocket engines and wait till I am around 65km before I maneuver into position for the circulization burn.

[KerikBalm]

Yea, I don't think 3 intakes is spamming... 12 is though

Triple your intakes, and you triple the air you get, which means you can fly in 1/3 the air, and thus 1/3 the drag - when you have a perapsis while still flying on jets... you've done too much intake spamming.

Most of the SSTO designs i see that perform really well make heavy use of intake spamming and part clipping, I don't do either (assuming 4:1 intake ratio is not spaming, and I use more radials than ram intakes, because... as I said, no part clipping for me)

[Frank_G]

I don't consider myself very experienced in building spaceplanes but I have very good experience with making my planes fly higher and faster. If you can keep your plane flying at 35 km, you can reach orbital speed (2400+ m/s) using turbojet there and then circularize for almost no dv. Such flight profile calls for about three ramjets per engine, though, so nothing for intake spam haters (although I don't think three pieces make up spam).

Thanks! I think, i will try that flight profile. I usually use between 1.5 to 2.5 Intakes per engine.

[Monger]

2. Why on earth should I be building this sodding plane given I can reach orbit in under 4mins with no fuss using a capsule?

You are right: currently there is no reason why you should take the long and stony road of SSTOs if you can easily lift bigger payloads with much simpler designs.

I guess SSTOs are kind of the holy grail of space exploration, because they would make space travel cheap enough that anyone can get there. Also, it is one of the few concepts in KSP that are not directly based on existing technology, but rather on the one that is in development just right now. Designing SSTOs feels a little like designing for SpaceX must feel like.

[Lohan2008]

Space Planes are a separate challenge, it is still part of testing out what works for you. 2> You can clock up additional science by flying over difference landscapes.

[Hodo]

You are right: currently there is no reason why you should take the long and stony road of SSTOs if you can easily lift bigger payloads with much simpler designs.

I guess SSTOs are kind of the holy grail of space exploration, because they would make space travel cheap enough that anyone can get there. Also, it is one of the few concepts in KSP that are not directly based on existing technology, but rather on the one that is in development just right now. Designing SSTOs feels a little like designing for SpaceX must feel like.

Oddly enough I get bigger loads in my SSTO, then I could with most of my rocket designs. But I am not a big asparagus launcher guy, as a matter a fact they just don't work in FAR or in RO. So if I want to get anything into orbit of 100+ tons I use my SP-406 design which can easily haul 108tons into a 100km orbit.

[Taki117]

In answeer to your second question: I use a mod called Kerbal Construction Time that adds a build time to things, it takes 3-4 earth days to build a rocket, whereas with my cargo SSTO space plane I can have it ready in 16 earth hours.

[O-Doc]

Most space plane issues are related to piloting skill level. There's a number of principles you need to be aware of o make a good SSTO and get it to space. Brute force is not the answer here. CruzanAK has a great video series which covers all the basic concepts.

[Lunaran]

I hated space planes for the longest time. Still kinda do. As far as I was concerned, there was the VAB, and some other useless building I'd never use. When I tried, I'd just absolutely massacre Kerbals with machines that looked exactly like everyone else's planes but would flip out and explode on the runway without even lifting off, and I'd say "oh yeah, that's why I don't bother with that."

However, as with everything else in this game, it seems like an absolutely impossible task until you discover the magic optimum path, and an hour later you can do it with your eyes closed. Designing a spaceplane you can fly straight to orbit just has one of the narrowest magic optimum paths of anything in the game.

- Design any plane that you know just works as a plane, that you can launch and fly out over the sea and actually steer and then actually line up with the runway again and actually land. Moving a few parts the tiniest bit can make a huge difference, and with enough screwing around and/or green bodies on the tarmac, you will start to get a handle on how.

- Once you've got a plane that works, keep changing the design and going for personal altitude/speed records. Start with a tiny plane and one turbojet so that when it flames out you don't lose all control, and try to get it as close to flaming out as you can without going over. Then change the design a little and see if you can move that flameout line a little higher. With 3 ram intakes per turbojet you can cruise at minimal throttle at, if you're super careful, up to 40km and still be picking up speed. Watch how much liquid fuel you use getting up to that point, and tweak out any big surplus to lighten the craft for the next flight.

- If you're still sticking with it at this point, you'll eventually become good enough at this that you'll find it takes constant nosing down to avoid climbing to an altitude where your jet no longer works. If you use Mechjeb or KER, it'll probably tell you your periapsis is above 70km. Hooray! Now put a rocket on that plane and some asbestos pajamas on Jebediah and you are, in theory, set.

- Use the tiny red rockomax rockets. Dragging a whole LVT30 or 909 up with you is just a waste. If you nail that special spaceplane ascent profile just right, where you skim along with your intakeAir holding at 0.01 until you're pushing 2km/s, you don't need much of a squirt to get up the rest of the way.

- Make an action group that switches the jets off, the rockets on, and shuts the air intakes. If you use the 'toggle' action for all of them and have the first stage activate only the jets, you can use the same button to switch back to air breathing mode when you come back from space, inevitably nowhere near the stupid runway.

Or: don't bother and keep making rockets! Do what's fun. You might come back to it some day in a more patient and curious mood. That's more important than any design tip.

[Jouni]

I guess SSTOs are kind of the holy grail of space exploration, because they would make space travel cheap enough that anyone can get there. Also, it is one of the few concepts in KSP that are not directly based on existing technology, but rather on the one that is in development just right now. Designing SSTOs feels a little like designing for SpaceX must feel like.

Reusability is the holy grail of space travel. SSTOs are just one, possibly misguided way to do it. The SpaceX way might ultimately be better, because they don't have to design the entire launch vehicle to survive reentry.

In KSP, SSTOs are ultimately easy, if you just ignore real-world ideas and concentrate on what works in the game. Most rocket engines can be used to build SSTO rockets, and some of them can even carry reasonable payloads to orbit. Jet engines can be used to increase the payload fraction significantly, but reaching orbit becomes harder and takes more time. Wings increase the payload fraction a bit more and make the ship more fun to fly, but designing it becomes much harder.

[MarvinKitFox]

Hi Folks,

... to hit 1000m/s at 20km in horizontal flight, then ...

There's your problem.

You want 1900m/s at 35km, not 1000m/s at 20km.

Turbojet, several highspeed air intakes, and an adequate TWR = win.

Rapier works, too, but I find Mixx of turbojet and Airspike to be better.

[bsalis]

1. What are the key rules about space plane construction (I usually try to balance CoL over the CoM, but really not sure if that's right) to get best lift and best stability?

There are very few rules really, mostly just guidelines and design concepts that work well. I find that good SSTO Spaceplane design is mostly about balance and proportions.

2. Why on earth should I be building this sodding plane given I can reach orbit in under 4mins with no fuss using a capsule? Maybe when budgets come into it having a totally reusable vehicle would be helpful, but the hassle of launching the thing at the moment seems not worth it.

It's really a matter of personal taste and what you are trying to achieve. Personally I love the design challenge. When I started building them in v0.16, it was a lot harder. Nowadays I only build ones that are special in some way. More than anything I just find them great fun to pilot. With refueling, I have travelled all over the Kerbol system with spaceplanes (yes - even Moho). They are like the automobile of space travel. Which reminds me! I'll repost some pics from a previous "why spaceplanes?" thread....

Rocket

SSTO Spaceplane

I know what I prefer.

[FREEFALL1984]

There is no good reason to be building spaceplanes, same as there is no reason to build rovers, or airplanes, or even rockets, in fact there is no good reason to be playing KSP at all, unless you consider playing KSP to be fun and enjoyable like me, in which case that's the reason we build rockets, airplanes, rovers and spaceplanes... because we enjoy it and relish the challenge.

[kiwi1960]

I try NOT to use space planes to go into orbit, why bother?

BUT.... as a challenge, I do it BUT do not orbit them. Examples....

I built a light weight solar powered electric prop engined plane for the challenge, it can also be taken to planets with an atmosphere and flown around to explore. (if you can stay ahead of the dusk, you need never land it... )

I built a super sonic plane that was able to circumnavigate Kerbal and would have reached its starting point had I not run out of fuel, it was close, I only had a smidgen over 1000k left to go... this is an ongoing project... (ceiling was 19,000 meters and speed was between 1350M/s a 1400M/s)

I build airships which allow me to explore, these have flown all over the planet at slow speeds... cruising speeds.... they have landed at BOTH poles... one after the other... and as many points in-between.... (using solar powered electric engines as well as jet engines....)

I agree totally with bsalis .... space planes are the gentleman's way to get around, why drive a truck when you can do it in head turning style....

Edited May 2, 2014 by kiwi1960

[Claw]

1. What are the key rules about space plane construction (I usually try to balance CoL over the CoM, but really not sure if that's right) to get best lift and best stability?

This is all assuming you're using stock aerodynamics. Some of it also applies to FAR, but there are some more considerations when using FAR.

Key "rules" (or "must do") I would say is...

- The pink Center of Thrust (CoT) should be inline (or very close to it) with your yellow Center of Mass (CoM).

- The blue Center of Lift (CoL) should be slightly above and a bit behind the CoM marker for general stability.

You can stray from these, but you probably need some experience before you try to bend these rules too far.

Most other things are up to your design taste. However, I find the following guidelines are a good "rule of thumb" in general for initial designing.

- Engines: 1 TurboJet per 12 tons- Maximum of 15 tons per TurboJet (even at that, it gets difficult and you need to airhog.)

- Intakes: 3 Ram intakes per TurboJet - Minimum of 1 per TurboJet. Beyond 3 intakes, you need to start doubling them to see much effect.

- Fuel: Around 150 unts per TurboJet (or roughly 1 Mk-1 fuselage worth of fuel per TurboJet) - Minimum of usually 100 units per TurboJet.

- Wings: 0.5-1.0 lift rating per ton - Maximum of 1.0 lift rating per ton. Minimum is a bit misleading because you can build VTOL with 0 lift rating, but generally no less than 0.5 for standard space planes. Less is better if you're going far with it.

Those aren't perfect, and as you get better at design and flying you can stretch them a bit. Generally speaking, if you increase one area you can decrease another. So if you have 8 tons and 1 TurboJet, you can probably get away with fewer intakes, less fuel, or less wing.

And they fit well with my typical ascent profile (which must also be tailored slightly depending on the craft itself).

Altitude - Airspeed

10 km - 200 m/s

15 km - 350 m/s

20 km - 650 m/s

24 km - 1000 m/s

30 km - 1600 m/s

Flight stability takes a bit more art than science, but you need to ensure you have enough pitch, roll, and yaw authority for your craft to do what you want. So it's a good idea to have a design goal in mind when you start, then work your way out from there.

2. Why on earth should I be building this sodding plane given I can reach orbit in under 4mins with no fuss using a capsule? Maybe when budgets come into it having a totally reusable vehicle would be helpful, but the hassle of launching the thing at the moment seems not worth it.

Because you can and you want to. It's virtually like anything else in this game. What good is a space station, or why would I bother sending a probe when a kerballed mission is way more profitable for science?

It seems space planes are much less explored than rockets simply because the design challenges are different. So it's really up to what your goals are in game and what you enjoy doing. I personally enjoy them and have spent way more time on air/space planes than rockets. Some people enjoy the challenge of building a plane capable of delivering 100t into orbit and back. Some people absolutely hate it or are just unsure how, so they choose to stay away.

The flip side is that if/when budgets are introduced there will probably be a group of people who are upset because they will feel "forced" into building planes.

Edited May 2, 2014 by Claw

[panarchist]

There are very few rules really, mostly just guidelines and design concepts that work well. I find that good SSTO Spaceplane design is mostly about balance and proportions.

Rocket

Yeah, but only the trailer behind the cab is going to space today - you're going to lose trailers #2 and #3 through staging. :-D

[Red Iron Crown]

Oddly enough I get bigger loads in my SSTO, then I could with most of my rocket designs. But I am not a big asparagus launcher guy, as a matter a fact they just don't work in FAR or in RO. So if I want to get anything into orbit of 100+ tons I use my SP-406 design which can easily haul 108tons into a 100km orbit.

I'm a rocket guy, and I find it way, way easier to get big payloads into orbit with them, even without asparagus staging or in an SSTO design. I find spaceplanes finicky to get working well (or at all!) and getting big payloads on them nearly impossible. Not a criticism or anything, just an example of how someone's playstyle can greatly affect what works for them.

[Tortoise]

Make sure you have FAR first.

And, reasons for spaceplanes:

-Efficiency

-Reusabe

-Looks cooler

-Land without parachute

-No dumping a bunch of stage debris all over the universe

-Much more

[Zeroignite]

Do spaceplanes suck in terms of design effort and ease of flight for payload lifted to LKO? Yeah. I'd not use them for anything more than light cargo and crew transfer personally.

But yes, building one that works just feels neat. Also, it doesn't take nearly as much mass as some people think... I don't think I've actually built a SSTO plane over 10 tons.