Maintaining attitude with a Spaceplane

[Leeksoup]

Hi everyone!

I\'ve been playing around with this game for a couple weeks now and have been having a blast. Also, it\'s fun getting weird looks from nearby people on the street as you describe your new rocket plans to a friend...

Anyway, I\'ve successfully gotten to orbit, I\'ve gotten to the Mun, and most recently, I managed to not just get to the Mun but have landed on it and gotten back in one piece! I figured my next step was to emulate some of you fine people and put stuff in orbit, and since I\'d been wanting to make a Spaceplane anyway...

But I\'ve been having some problems with it. It gets to orbit no problem, and in fact it\'s quite handy in the atmosphere. (Handles like a brick wall in space, but happily there\'s lots of time for maneuvers there.) This is a heavy design, with lots of fuel, wings and engines, so I was expecting it to be hard to fly, but no, actually it\'s quite nice... so long as its nose is pointing up. As soon as it hits 90 degrees, it is *impossible* to get above that. Not even RCS pointing straight down can help. As a result, I can never shed enough speed on reentry to make a safe landing.

I attached my .craft file, so you can look at my design if you like. (It\'s called the Avalanche- slow to start, but try stopping it...) Thank you!

[Katalliaan]

What mods are used in this one? I went to try it to see what the problem is, but I\'m missing parts.

[Leeksoup]

Ah, sorry.

Um... I\'m not sure which mods are in it, I downloaded a lot. C7, definitely, and DownUnder. Whichever mod gives you the Bertha engines, also, and the reinforced struts.

[Goddess Bhavani]

For a basic attitude hold \'autopilot\' just orient the craft and hit the SAS button. All you need is an ASAS module to manage RCS and airfoils for you.

[Leeksoup]

For a basic attitude hold \'autopilot\' just orient the craft and hit the SAS button. All you need is an ASAS module to manage RCS and airfoils for you.

Oh yes, doing that works as long as my nose is pointing up. It\'s when I\'m coming back down that everything goes horribly, horribly kerbal.

[Katalliaan]

So, some poking around in the .craft leads me to think (some of) the mods that the OP can\'t remember are the KW Challenger pack, NovaPunch, HSTW Amphibious Gear, and some pack that prefixes its parts with 'YM'. Slowly getting to the point where someone can try to find the problem by flying the thing.

[Goddess Bhavani]

I see it as an aerodynamic issue - can your aircraft\'s re-entry stage fly 'straight and level'?

I reckon Leeksoup, try and fly a more simple design first. The usual problems with spaceplane designs are as follows:

1. Centre of gravity too far front / back

2. Centre of lift too far front / back from centre of gravity

3. Asymmetrical thrust

4. Unbalanced thrust (too many engines at the wrong place will destabilise the plane)

5. Insufficient / unbalanced lift

6. Insufficient / overpowered control surface authority

Almost anything will launch fine pointed straight up, but without subsonic low altitude flight testing I won\'t bother trying to recover anything. Work the bugs out stage by stage before you try a complete orbital launch and recovery for easier troubleshooting. For an unpowered gliding ship, you also need to do glide tests to make sure its recoverable.

It\'s very hard to diagnose the issue without at least a picture but for a quick fix - you may want to try getting a single engine space \'fighter\' type plane up and down again first. Try using RCS and ASAS to hold re-entry attitude and transition to fixed wing flight at about 20km altitude.

If the aircraft flies decently well at low altitude then just add more thrust without upsetting the balance of the design. It\'s easy to reach orbital altitude now anyway because C7\'s \'jet\' engines do not lose power as they go higher. (more realistic service ceilings are being looked at in the future)

[Leeksoup]

Thank you for your checklist, Pandora! I\'ll definitely go through that later.

At the very least, I took some pictures of the Avalanche. Maybe you can see what\'s wrong?

[White Owl]

At first glance, it looks like too much mass for not enough wing area.

[Goddess Bhavani]

Seconded wing area issue. Though I must say the ship was very well put together!

Those liquid fuel stages are VERY heavy.

The wing area looks like what I\'d use for a 3 engined 1 meter hull supersonic interceptor carrying... no more than 6 missiles. That\'s considered a very light fighter by some standards.

However the Avalanche currently has 4 2 meter fuselages and two huge LF engines with fuel tanks. Definitely need a much bigger wing area to fly it. You may also have the heavy liquid fuel engines at the back weighing down the back of the ship too much making it fall to the ground tail-first regardless of how much lift you have.

Also, for this kind of mass, the ship is way too short to fly like an aircraft. Length in terms of KSP C7\'s spaceplanes like a car\'s wheelbase - the longer it is the higher the stability you have and the more horsepower you can cram into the hood without upsetting the handling. Currently the craft has something like a turbo Hayabusa engine in a Smart car. It probably just spins round and round the moment you up the power.

Looks stable for vertical launch as you have described, because vertically it is well balanced.

Transforming it into a multi-plane would probably work to keep the sleek and well put together profile but... it\'s much easier to stick a pair of mega wings (the huge ones) to the side if you want to keep the LF engines... or if you get rid of the LF stacks you could transform it into a delta winged spaceplane easy enough!

So at this point Avalanche II would either have to be:

1. Tri-rocket delta wing light spaceplane (using C7 engines)

2. Huge liquid fuelled cargo lifter (retain liquid fuel rocket engines)