SSTO issues. Noob alert.

[Al-Rabban]

Greetings and I guess I'll say hello to my fellow Kerbonauts.

I have the full version of the game for a month or so now and have had plenty of success and failures in kerbalopolis. I have however only recently discovered the online community here on KSP Forums and wish to join. Unfortunately my first post happens to be one of distress. Let me start from the beginning.

I have after a few recent successful rocket launched decided to try my luck with space planes and so far my luck has been less than desired. After about two dozen or so attempts at various designs I created my first successful prototype plane that successfully took off the runway and after a few terrifying loops and barrel rolls landed in one piece.

Behold Aero Prototype I

After learning how to fly I am able to take this prototype to just over mach 1 at about 10km. I deemed this a success and proceeded to phase two. Transforming the prototype plane into a SSTO. Another two dozen or so attempts I have been left with this.

SSTO Prototype I

The issues lay somewhere within this design. I can take off, incline to 45 and reach 10km. I level out at a 15 degree incline and speed up to about mach 3.5 ( +/- 1000m/s ) at around 19km. At this point my air intake is sitting between 0.30 and 0.20. I then using the custom binds alphas toggle off my turbo's and their intakes while toggling on my spike.

Now at this point I plan to increase my incline to a solid 60 to rapidly gain ground but instead before I can even toggle off my ASAS to do so my prototype does either, back flips, sideways cartwheels, or in the worst situations both which end with me desperately attempting to regain control followed with deploying shoots and bailing.

I thank you all ahead of time.

[numerobis]

Is that a nacelle up front? If you happen to have left it open, you are generating huge drag for little benefit. And if you leave it open and close all the radial intakes in the back, you'll have huge drag up front and not much in the back -- disaster!

Generally you should use ram air intakes for all you intake needs; they are by far the most effective. In the usual case they produce 2.5x as much air as a radial intake, but only one tenth the drag. Engine nacelles are by far the least efficient. With 4 ram air intakes for two engines, you should reach nearly 2km/s.

[tavert]

Another thing to note is the manned aircraft cockpits have less drag than most other parts in KSP. Due to the way KSP's drag model currently works, for the best stability you want low-drag parts up front, and high-drag parts (ram & circular intakes, SRB's, and oddly some of the nosecones, tricoupler, etc) towards the back. Your planes look very cool, but having the cockpit in the back like that hurts your stability. Also aerospikes are low-drag parts as well, so you may find replacing them with one of the conventional rockets will be more stable (just be careful you don't hit the nozzle on takeoffs/landings).

[bsalis]

back flips

Probably a couple of reasons...

1. You have all the rocket fuel at the front with the dry mass of cockpit and engines at the back, so the CoM is moving behind the CoL

2. The CoM is all the way at the back end. From experience that seems to make flipping a problem.

sideways cartwheels

The CoD is probably in front of the CoM.

To keep the design basically the same, try putting control surfaces behind the cockpit.

[Jason Patterson]

The instabilities that the others have mentioned would tend to result in backflips, but a flat spin has to be because of a left-right imbalance, either in thrust or drag. If the radial intakes on one wing were staying open unexpectedly, that would be enough to cause this problem, especially once you've switched to rockets only (and lost thrust vectoring as a result.)

It might be worth your time to clear out all of your action groups and set them up again, especially if you've adjusted the position of a wing or something like that. The game sometimes makes it look like an action group remains intact after moving/duplicating a part, but it doesn't actually copy the action group setting to the symmetrical parts, only the original.

[Al-Rabban]

Is that a nacelle up front?

Yes, and an aeronautic nose cone from B9 Aerospace mod. (prefer to use stock but I love that nose cone!)

Generally you should use ram air intakes for all you intake needs; they are by far the most effective.

I have found this but they are ugly XD. I do use them but I like to add in the other styles for visual flare!

[Al-Rabban]

Due to the way KSP's drag model currently works, for the best stability you want low-drag parts up front, and high-drag parts (ram & circular intakes, SRB's, and oddly some of the nosecones, tricoupler, etc) towards the back. Your planes look very cool, but having the cockpit in the back like that hurts your stability.

Seriously, how could I sacrifice coolness for functionality I have played with the idea for having the cockpit upfront but out of my original trials in getting airborne I found for me at least that this style makes it easier to fly.

[Al-Rabban]

1. You have all the rocket fuel at the front with the dry mass of cockpit and engines at the back, so the CoM is moving behind the CoL

In my rocketing career I have more recently discovered asparagus staging and have been implementing them on all my designs but minus the staging would it be possible to drain fuel from the back forwards?

The CoD is probably in front of the CoM.

I remain diligent on my use and knowledge of acronyms for this game but CoD has caught me off guard. What is that?

[Al-Rabban]

The game sometimes makes it look like an action group remains intact after moving/duplicating a part, but it doesn't actually copy the action group setting to the symmetrical parts, only the original.

Interesting, I thought I had done this effectively but I'll give it another go.

Thank you all for your help thus far, I will re-evaluate my design based on your feedback and let you know of my result.

[tavert]

In my rocketing career I have more recently discovered asparagus staging and have been implementing them on all my designs but minus the staging would it be possible to drain fuel from the back forwards?

I remain diligent on my use and knowledge of acronyms for this game but CoD has caught me off guard. What is that?

There are a few things you can do to manipulate how fuel drains. If fuel tanks are mounted inline with "fuel cross-feed capable" parts between them, then those tanks get treated as one unit and drain starting with the furthest tank away from the engines. If tanks are placed radially, side-to-side, then they don't get drained unless you start connecting fuel lines to them. Decouplers are one of the few things I know of that aren't fuel cross-feed capable, so for inline tanks you can try putting decouplers between your fuel tanks and routing fuel lines around them to manually control fuel feed order, but it would make your plane less rigid.

CoD = center of drag

[Al-Rabban]

routing fuel lines around them to manually control fuel feed order, but it would make your plane less rigid.

I'll keep that in mind as I am rebuilding my SSTO Prototype. BTW, is there anyway to toggle the CoD? I don't recall seeing a button for it.

[tavert]

Sadly no, you have to sort of guess at it based on the drag ratings and locations of parts. If every part has the same drag rating, then the CoD is at the CoM. Otherwise it'll be a bit off...