How to build a proper space plane?

[Monger]

I have tried to build a space plane for some time now, and since 0.21 introduced new SAS controls I though I'd give it a new shot...

My goal is to create a plane that can make it into a stable orbit (let's say, 100.000m), carry a little bit of payload (say, a second Kerbalist who wants to make it's rendezvous with a low-orbit space station), and return safely to the Kerbal space center with a little fuel reserve.

I am aware that Squad already has included a ship that can achieve an orbital flight, the Aeris IV. But it is a tiny plane, with barely enough place for the pilot.

I tried to start from this design, and basically scale it up. But I am failing every time, and I'm not sure why: if I add additional fuel tanks, I can barely lift the plane. If I then enlarge the wings, I can rise to a certain height, but with the second phase (i.e. non-air breathing engine) my ship seems to heavy to get enough thrust in the outer atmosphere, i.e. my fuel runs out before I can achieve a stable orbit. Adding a second engine would make the ship even bigger - up to a point where it breaks while accelerating on the runway for unknown reasons.

So, can anyone give me a few hints how to basically build a space plane that can actually achieve something useful?

Edited July 31, 2013 by Monger

[Stochasty]

There are some counter-intuitive rules of thumb for building effective spaceplanes, but once you learn them getting a plane into orbit is rather easy. I'll try to lay them out:

1) When considering the jet engines, thrust is your enemy, not your friend. The reason for this is that the max speed obtainable by your jet engines is almost strictly determined by their operational ceiling, which is determined by the ratio of intakes to engines. Adding more engines doesn't help, since you have to just add that many more intakes to feed them. You always want to shoot for the fewest amount of engines necessary to do the job. (The hardest stage of a spaceplanes ascent is the climb from 10km to 15km altitude. Add engines until you can manage that climb, then stop adding engines and start adding intakes.)

2) Corollary to 1), carry only the jet fuel that you need. One of the easiest ways for your design to fail is for you to accidentally add too much jet fuel. Any fuel unspent during your climb is dead weight in orbit and will limit your operational range.

3) In contrast to jet fuel, you want to carry as much rocket fuel as you can lift. Every extra unit increases your operational range.

4) You should not need much in the way of rocket engines to finish off your orbit. A proper ascent trajectory has you level out near the operational ceiling of your turbojets so you have time to accelerate as much as possible before you ignite the rockets. Done right, the ascent on jets should put you at near-orbital velocity, and you should not need much thrust at all to circularize. It is entirely possibly to design a spaceplane capable of orbit using only turbojets and ion engines, although I typically use nukes because I can't stand 15 minute long burns.

5) Use only the ram intakes. Everything else is added drag for paltry intake air. The radials might seem like they add a lot, but once you are at altitude they are far less effective than the ram intakes.

6) If you still can't make it work, try building a spaceplane using aerospikes first. It's quite possible to build an SSTO spaceplane without using jets at all (although you won't have much in the way of payload); one aerospike, about six of the long 1m tanks, and enough wing surface to get you airborne should suffice for a one-man pod. Once you know what it takes to make orbit without jet engines, you can start adding jets to the design. You'll be amazed at how much easier it is to make orbit, and how much extra fuel you have left.

[MBobrik]

Judging from my admittedly limited experience, it is quite simple

- use turbojets

- spam ram intakes. 7 per engine will allow you to gain orbital velocity just on jets.

- don't forget to throttle down to avoid flameout. It is counter-intuitive, but it allows you to climb higher and thus fly faster even if you decreased the thrust

- if your apoapsis is still in the atmosphere when you have to fire the rocket, fire it without shutting off the jets. You will get speed boost and thus more air allowing you to fly on the jets longer.

- don't forget action groups for the jets and the rocket.

[Monger]

Thanks already for the hints. I will get back here after I tried them thoroughly. But I still have two questions:

- How do I spam ram intakes? I need a lot of cylindric tubes, don't I? And the ram intakes should face towards flight direction, right? Can I build them directly after each other, or do I need to leave space for air intake?

- The Aeris IV closes all air intakes when igniting the rocket engine. Is there a special reason for this? Less drag, or... ?

[longhornchris]

Well, first I'd suggest getting FAR because the modeling is more realistic which for me made the design more intuitive. It does, however, punish you for mistakes, etc.

Next, having Mechjeb is a huge benefit, if only for information. The surface and orbital information tabs help planning maneuvers, timing burns, and flying a 'clean' ascent. The TWR info in the hangar will keep you from over-designing the bird. The 'Utilities' tab is nice as the 'Prevent Flame-out' will automatically manage the throttle to keep you from having a flame out.

TAC Fuel balancer is also crucial - as your CG will move as fuel burns. Because the forward tanks burn first your CG will move aft if not balanced... this can be a very bad thing.

Finally, having some parts mods will help just to build bigger planes. Taverio’s Pizza and Aerospace v1.4.1 has bigger landing gear, which make takeoff much easier (tailstrikes suck). Its also got some extra wing parts making it easier to get more lift w/o having huge wobbly wings made of lots of parts. B9 Aerospace has a ton of great spaceplane parts too. Having the different part sizes (mostly fuel tanks) helps get the right fuel/oxidizer ratio - as a good spaceplane needs extra jet fuel compared to oxidizer.

Now, for the design. As others said for stock parts Ram intakes are best. If you get Taverio's or B9 then their 'spike style' intakes work equally well. I generally go for a jet-only TWR around 1.5, any more and you've got too much thrust (and thus 'dead weight'), less and you will be have trouble at altitude/speed when the thrust starts to fall off. I've found the rockets being at 1.5 at 'launch' works well for me to reach orbit from max jet altitude.

For fuel/ox ratio, remember the rocket fuel tanks are 'balanced' while the fuel tanks are just fuel. I've found 4 parts rocket fuel tanks to 1 part jet fuel tanks works about right, but usually I fly at 3-1 because of tank selection. This also provides me extra jet fuel for landing.

Other things to remember in the design - center of lift (COL) needs to be near, but behind the center of mass (COM). Remember that as fuel burns off the COM will move, even with the fuel balancer. I try to make sure I have equal fuel fore and aft of the COM so that, when balanced, the COM doesn't move much. If the COL is ahead of the COM the plane will be unstable. If the COL is too far back, however, you won't have sufficient pitch authority - so its all about finding a good balance. Also, if possible have an odd number of jet engines and place the center one last. The last engine placed is the first to flame out... and a center engine flameout won't put you in a flat spin. Finally, don't forget RCS, you may need it to maneuver once in space.

Before you leave the design, make sure you assign action groups. I typically put Rocket toggle on '0,' Jet and Intake toggle on '9.' Closing the intakes cuts your drag, so its worth doing once you switch to rockets.

Now, from a flight perspective, the key is patience above about 15Km. MechJeb's surface tab provides rate of climb info (so does the GUI, but its on a log scale and small). The key is to climb slow enough that you gain speed and altitude together as air intake rate is speed and altitude dependent. I've found 50m/s or less vertical works fairly well. Gain altitude too quickly and you won't have sufficient airspeed to drive the air intakes, flameout, and have to try again (if you don't get thrown into a spin). You can squeeze more from the engines IF you throttle down (or use mechjeb auto-throttle) to keep gaining speed at altitude.

One you've squeezed the jets for everything, its time to engage the rockets and climb again. MechJeb's orbital information tab is useful here - when both your apoapsis and periapsis stop growing you've maxed out... this doesn't necessarily happen when you stop accelerating. There's a small window where the added speed gained by the rockets will provide extra air for the jets - but be prepared to shut them down quickly. Once on rockets there is no need be in the atmosphere so pitch up some to get out of the air and drag. I typically go to about 30 deg nose-high. Downrange speed raises your orbit (apoapsis and periapsis), while vertical speed only really gets you out of the atmosphere. Climb to steep and you will need a longer burn to circularize. Burn until your apoapsis is high enough - 75Km to 80Km works nicely. Then kill the throttle.

For drag minimization, pitch back down so you are nose-on your velocity vector. You may need to tap the throttle to maintain apoapsis as drag will slow you down. Once you clear the atmosphere, setup a maneuver node at apoapsis and raise periapsis accordingly. I like 75Km to 80km because it give me time between clearing the atmosphere and hitting apoapsis to get everything lined up. If you gained enough speed down low, it shouldn't take a huge amount of delta-V to circularize.

[Stochasty]

- How do I spam ram intakes? I need a lot of cylindric tubes, don't I? And the ram intakes should face towards flight direction, right? Can I build them directly after each other, or do I need to leave space for air intake?

Cubic octagonal struts are your friend. You may have to abandon some of your sense of aesthetics (or work really hard to hide the intakes) but you can put intakes just about anywhere if you are creative. They do generally need to be pointing forward, though.

- The Aeris IV closes all air intakes when igniting the rocket engine. Is there a special reason for this? Less drag, or... ?

Yes; less drag.

[espm400]

I've just got a couple of points:

- It wont help you with your designs, but one of the most helpful things I could suggest to anyone who's working on their first SSTOs would be to get either a gamepad (I use a simple 360 controller) or a joystick of some kind. The amount of control you gain from one of those is nearly immeasurable.

- As longhornchris mentioned, another must is FAR (Ferram Aerospace Research). No more will your nose be pointed 40* skyward at 20,000m.

- I'm also with Stochasty on the fewer engines side, although not for the same reason. For my small and mid-sized SSTOs I tend to either use one jet engine or two in an over/under setup. That way I can push them as far as possible and not have to worry about flame-outs causing a flat spin.

- If you're not adverse to mods, I would also suggest picking up B9 Aerospace, if only for the fact that it affords you so many options and expands your inventory greatly.

- Finally, I've never really been a fan of spamming intakes. Partially because with FAR they add too much extra drag, partially because I think it looks terrible, but mainly because it's a bit too (I hesitate to use the word) 'cheaty' for me. To each their own though, it is a sandbox afterall.

[Jean Deaux]

I'm with you buddy, I've been playing with planes for quite a while now with only VERY limited success. After all the playing, I've come to the conclusion with KSP and planes, I just hate, hate, hate, HATE, HAAAAAAAATE space planes! The ONLY success I've had at flying one is with the Lazerz mod, and that's so buggy now nothing else works when it's loaded for me so planes are just not an option for this player. Best of your luck on your attempts though!

[Monger]

I'm not entirely there yet, but I have made a lot of progress - thanks to you. I have improved my plane designs by reducing weight (thanks a lot for the octostruct hint), and optimized the air intakes. I can reach easily 40km height now, and up to 2200 m/s with a single jet engine (and no mods, of course). However, I still need to integrate a proper rocket engine. I thought about adding the nuclear drive, but I guess it is a little to heavy for my little plane.

Nonetheless: I can almost smell space now.