help! why do my ssto's suck :(

[Zeekin]

So after (more-or-less) getting a grip on rocket building, I decided to try my hand at space-plane building, I've built a few designs that I thought would be good, they always make it to ~24000 and when I switch to rocket-engines only go all ninja on me and flip out like crazy, often deciding that it's time to drive butt-first for some reason.

I built this one first:

that one took quite a long time and I thought maybe it was too asymetrical once it's in orbit or something causing it to do backflips, so then I built this one:

this one can basically fly vertically up, I fly it to around 17000, get a bit more level, get to around 24000 and 1400m/s, turn on the boosters, pitch up and BAM doing backflips again....

I've tried various weight configurations, I've tried turning various fuel tanks on or off to try to distribute load (if that was the problem), but nothing seems to matter...

what am I doing wrong?? Help!

[deadshot462]

I'm no expert in SSTO's, but I think a large part of your problem is your plane's size is too big. Start as small as possible with the plane size, then add fuel tanks/wings as needed. I've had the best success in SSTO's when the plane was smaller.

Edit: Make sure you have an action group to do the following at the same time:

-Close air intakes (reduces drag)

-De-activate air engines

-Activate rocket engines

Edited April 26, 2013 by deadshot462

[RoboRay]

Yeah, those are huge. An SSTO design is always going to be a marginal performer, and the law of diminishing returns on added mass in rocketry still applies. So, it's exponentially easier to build a small SSTO than a big one.

You really do not need all that intake-spam, either. More than one per engine helps, but even just one per engine is still possible... you'll just need to use more rocket fuel. For example, my very first successful SSTO spaceplane used one intake per engine.

My current two-seater design does use more, intakes though, so that I only need tiny rocket motors and not as much fuel for them.

But definitely try smaller designs while learning how to make an SSTO. Once you've figured out the narrow band of tolerances they require, you'll find it easier to build a large one.

I suggest not shutting your jets down at the same moment you engage the rockets, though. As the rockets accelerate you, your intakes become more effective and gather more air, enabling the jets to keep thrusting for higher and longer. I control the jets and rockets with separate action groups, to maximize what I can get from the jets.

Edited April 26, 2013 by RoboRay

[Kroden]

See above for some great suggestions. Starting small and working up is a really big help with SSTO.

The most likely problem with flipping for you is that your center of thrust is not inline with your center of mass. This is very important, as the thinner the atmosphere gets, the less your control surfaces will be able to counteract any misalignment.

Also be sure that your center of mass isn't changing too much throughout the flight. As it can more forward or backward of your center of lift, things can get a little chaotic. You want the center of mass to stay on or in front of your center of lift, and not deviate much from either position.

Listen to RoboRay about the action groups - that is key. You should also be able to throttle down with the jets and rockets both enabled to get more use of the jets, while still getting a positive velocity change. When you drop below about 50% throttle, then shut down the jets and power up those rockets!

If you need some examples of successful SSTO craft, I can post a few.

Good luck.

[Flixxbeatz]

I'm no expert in SSTO's, but I think a large part of your problem is your plane's size is too big.

...but what about these things? they're big, yet they can fly.

Am I missing something here?

EDIT: I've also made small spaceplanes, hoping that I'll make it finally. Still the same outcome: either...

-can't penetrate the atmosphere at the right time

-out of fuel during ascent

-out of fuel during orbit

-no fuel to deorbit

-stall spin for no reason during re-entry

I'm officially done with spaceplanes. Might as well make some while waiting for my orbital colonies to arrive at Duna.

Edited April 26, 2013 by Flixxbeatz

[Spatzimaus]

First of all, go read this thread to see other people's SSTOs. You'll get a good idea for what size you can have before scale issues crop up.

But to your specific design, there are five things that can explain what you're seeing:

1> Your Center of Lift (CoL) is too far from the Center of Mass (CoM). Note that the CoM moves rearwards as the plane burns fuel, so the indicators in the SPH need to be tweaked a bit.

2> Your Center of Thrust (CoT) is too far above or below the CoM. It's easy to line them up horizontally, since your design is symmetric, but you should check vertically.

----- Note that the stock landing gear are bugged. In the SPH and VAB they're treated as having their listed mass of 0.5 tons, but in-flight they actually have zero mass or drag. So, your CoM in actual flight will be higher than you thought it'd be, especially if you've got more than the minimum three wheels. When making sure that your CoT and CoM line up, remove all landing gear, and put it back on afterwards. (This only applies to stock gear; if you're using wheels from another mod, like modular multiwheels, then it works just fine.)

3> Your drag is too much. The intakes are well forward of your CoM, so when they're applying a lot of drag, it'd pull your nose away from your velocity vector. Since your nose will be pointing above your velocity for any spaceplane, this'd pull the nose further up and lead to a flip. The solution here is something you should be doing already: create an action group, assign all but a couple intakes to it, and have the group toggle each. On takeoff you close most of the vents to minimize drag on the runway, and don't open them until you're above 10-12km. Once you reach the altitude where your engines need to shut off, close those vents again. (And you won't need them during landing.)

4> It's not enough to just pitch up and turn ON the rockets, you also have to explicitly turn OFF the turbojets once they're almost out of air. Engines don't consume air in a perfectly symmetrical way, so if one set of engines runs out of air (and flames out) before the other, it'll put you into a spin.

The fifth possibility, related to the others, is that you just didn't put on an ASAS system. Or you did and it didn't have enough SAS to compensate for the imbalances in forces from the above factors. Or you went with the Avionics system, which never completely stops any imbalances in forces. (I use Avionics a lot, so I have to get used to continually pulling the nose up/down as the CoM shifts from fuel use.)

I'd guess it's #2, personally. It's the big reason why the most successful SSTO designs are all coplanar, with everything linked horizontally. It's not that you can't make something bigger, but your CoT tends to get out of whack with the CoM and it's harder to line those two up perfectly.

[Zeekin]

Good news! I have managed to get both designs (without any changes!) into orbit (albeit minimal oxidiser left - but I'm guessing I'll get better at it as I figure it out) turns out I needed to make use of the RCS sooner, as long as it doesn't *start* flipping out, it typically wont. My first design you can see there actually worked the best, weight distribution was much less an issue than my second.