Little more spaceplane assistance..

[Nightwraith]

So I've been tweaking this design since I last asked for help. This is what I currently have.

It flies in atmosphere wonderfully. The transition to the rocket engines is killing me so far. I think I have the problem narrowed down, my center of thrust (rocket engines only) is ever so slightly above my center of mass. Its nosing the plane down. RCS is strong enough to counter act the rotation, but its really, really, really hard to control when in the upper reaches of the atmosphere. I hacked gravity and turned infinite fuel on just to see what it would be like in total vacuum and its not as bad. I would still be range limited by rocket fuel, not RCS...so thats good, haha.

I would still like to get it balanced though. Any ideas on getting that accomplished? The CoT indicator doesn't really work for me in the SPH because its taking the turbines into account as well....All the RCS tanks on back section there are mostly to attempt to shift CoM up a little bit. It helps some. Not enough though.

[Sirrobert]

Would it help if you removed the jet engines, than adjust the rocket engines, than add the jet engines back on?

[g00bd0g]

Bring your rockets down or or point them, i.e. COT down. You can temporarily remove the turbines so you can see what the rockets COT is.

[Kasuha]

The engine looks horizontal so you just need to have it at exactly the same height as CoM.

You can try to add some ballast somewhere or you can just rotate the rocket engine slightly using Shift+QWEASD. Assuming the difference will get better after a 5 degrees-step rotation.

[Hodo]

OK you have a couple of options here.

You can add a couple more ASAS modules to the craft, you have room before the rocket engines and before the turbine engines.

And/Or you could add a couple of RCS thrusters just in front of your canards, one on each side, and one at the bottom and at the top center. So you have 4 of them right behind the cockpit.

The other less attractive fix is, move your center of mass up a hair with more ballast on the top of the craft. You may need about a ton of ballast on the top to bring your CoM up enough for your rockets, but this will throw your CoT off from your jets.

Last option is to move your jet engines to the side of the hull, this will bring them inline with the rockets, and fix your CoT vs CoM issue. But will make the rear of the hull very cluttered.

[Nightwraith]

Dang you guys are quick. I never thought to just remove the engines to get a look at CoT. Here is what I'm dealing with.

Never really gave ASAS modules a thought. I might try that. Also good points with adding ballast, I worked pretty hard to get the thrust from those turbines balanced.

[Hodo]

Ok here is a tip, I use this process for building VTOL SSTO space planes.

After I have everything on the craft where I want it, I then place a light or some other small lightweight object over the CoM marker to mark where it is. Then I remove the rear firing engines and point the VTOL engines down, if they already are not facing down. This way I now know where the CoT is in relation to the correct CoM.

I move the VTOL engines or ballast as needed to get the CoT in line with the object that I placed as a marker for my CoM. Once i have them lined up, I put it back together and do a test flight. 90% of the time it works as I intend it to... 5% of time, it needs some fine tuning, the other 5% it is a spectacular failure.

EDIT-

I notice there is a small movement of your CoM up as you remove the engines. Try moving the bottom engine pair to the top of the wing. It may line everything up.

[Nightwraith]

Yeah, I orginally had the bottom pair on top of the wing, but that was causing the nose to pitch down -really- strongly around 13,000 meters. Couldn't reliably climb any higher than that until I moved two of them down.

I'll try the light thing, should help get everything fine tuned.

[Hodo]

Yeah, I orginally had the bottom pair on top of the wing, but that was causing the nose to pitch down -really- strongly around 13,000 meters. Couldn't reliably climb any higher than that until I moved two of them down.

I'll try the light thing, should help get everything fine tuned.

What speed were you going when this happened?

And do you have FAR?

[Nightwraith]

I am using FAR, speed was around 400m/s.

[Hodo]

I am using FAR, speed was around 400m/s.

Ok you are hitting "Mach tuck".

Switch out the stock canards with a pair of smaller winglets from B9, like the 4x2.2m ones, maybe a bit larger. Turn off their control in FAR, by clicking on P,Y,R in the FAR control menu in the VAB/SPH.

[Nightwraith]

I wondered if it was something happening while 'trans-sonic'. So the canards shouldn't be providing any pitch or roll authority? Why is that? (trying to learn)

[Hodo]

I wondered if it was something happening while 'trans-sonic'. So the canards shouldn't be providing any pitch or roll authority? Why is that? (trying to learn)

Canards tend to stall out before the wings, this is a good and bad thing. So adding a simple non-control lifting surface at the front of the craft, to bring your CoL a little closer to the rear of your CoM will help maintaining its control at super sonic and faster speeds.

Like this monster I use for heavy lift jobs. It is a SSTO, and can haul 72 tons into orbit. It took a few days of work to get it right but it works great now.

Notice how the CoL is JUSt behind the CoM.

[Nightwraith]

Thanks, I'll give it a go.

Just hope I have enough oxidizer on board to get me to orbit.

[Hodo]

Thanks, I'll give it a go.

Just hope I have enough oxidizer on board to get me to orbit.

The best approach to getting a SSTO into orbit is climb at best possible speed to 15km, then reduce your climb so that you are accelerating laterally faster than you are climbing. So by the time you hit 25km you should be going mach 4+.

Ideally you should be on jet engines till 30km and around Mach 5+. Then switch over to rockets and climb as best you can, usually around 100m/s or better.

[Nightwraith]

That's an awesome SSTO.

Balance not bad, eh? Feels good in flight, going to see if I can get it into orbit.

[Hodo]

That's an awesome SSTO.

Balance not bad, eh? Feels good in flight, going to see if I can get it into orbit.

Now that looks good, and should work just fine. You have mechjeb and it is great for the information it provides. I personally use Kerbal Engineering Redux but not much difference in the information they provide. So you shouldn't have any issues.

[Nightwraith]

Appreciate your help. Yeah, MechJeb is nice for the info it provides. I don't like it to fly for me though, makes the game kinda lame.

[Nightwraith]

Almost got it up there. Fell about 20m/s short of a 80k orbit. Nearly lost it around 30k though, I think a little too much lift on the nose. Did everything I could to keep the nose down and it kept climbing. Had to kill the turbines way before I wanted to. Going to try some smaller stub wings and try again.

[Hodo]

Almost got it up there. Fell about 20m/s short of a 80k orbit. Nearly lost it around 30k though, I think a little too much lift on the nose. Did everything I could to keep the nose down and it kept climbing. Had to kill the turbines way before I wanted to. Going to try some smaller stub wings and try again.

Here are some quick fixes I came up with on your design. This will most likely fly, and make a VERY low 70-75km orbit. The problems I am seeing is this,

You have .94:1 Thrust to weight ratio on the rocket engines, I am assuming you are using the LV-T45s, which have a 200.0kn thrust rating. You have plenty of power with the turbine engines totaling in at 2.11:1 ratio. But you only have a max of 1793d/V total on the craft for fuel. The jet engines you have chosen are heavy and die out around Mach 3.5. I would suggest using the turbojet engines, which don't lose power till around mach 4.5.

I also rearranged your engine layout so there is no issues with the CoT vs CoM, no matter the engine in use.

Here is the quick build I did off your pics you posted and 15minutes of work. There are some changes I did not do in this build that I would do differently. Move the RCS tanks on the top to the sides of the craft, in line with the CoM. This would help bring the handling more in line and remove the nose pitch issue. I have not tested it myself but it is very close to one of my most successful SSTO spaceplanes, the SP-311 Trident.

My last piece of advice is design the craft to do one thing or the other, if you want it to haul crew, make it a crew shuttle it will be easiest at first. Or if you want it to haul cargo, make it a cargo plane. Trust me it is REAL easy to over complicate a craft with to many ideas at once. There is an old adage "Jack of all trades, master of none."

Good luck, and have fun.

Edited November 6, 2013 by Hodo

[Gusius1]

So I've been tweaking this design since I last asked for help. This is what I currently have.

https://lh6.googleusercontent.com/-JCmcR75Jjdw/UnlnPuBiLdI/AAAAAAAADNg/PsWB7jEurZc/s800/screenshot6.png

https://lh5.googleusercontent.com/-Lygb59QG3Pw/Unlo1bMSWqI/AAAAAAAADOA/bnYLH3H5oK4/s800/screenshot7.png

It flies in atmosphere wonderfully. The transition to the rocket engines is killing me so far. I think I have the problem narrowed down, my center of thrust (rocket engines only) is ever so slightly above my center of mass. Its nosing the plane down. RCS is strong enough to counter act the rotation, but its really, really, really hard to control when in the upper reaches of the atmosphere. I hacked gravity and turned infinite fuel on just to see what it would be like in total vacuum and its not as bad. I would still be range limited by rocket fuel, not RCS...so thats good, haha.

I would still like to get it balanced though. Any ideas on getting that accomplished? The CoT indicator doesn't really work for me in the SPH because its taking the turbines into account as well....All the RCS tanks on back section there are mostly to attempt to shift CoM up a little bit. It helps some. Not enough though.

Dude, where did you get this design? I drew a shuttle design for my art class, and it looks JUST like this lol.

[Wyre311]

http://forum.kerbalspaceprogram.com/threads/96406-The-Arcadia-series-and-the-Zeus-Ships?highlight=Arcadia

I can give you my ships.. PM me your email and I will send you the files.. I scan my computer regularly and always use antivirus.

[Claw]

http://forum.kerbalspaceprogram.com/threads/96406-The-Arcadia-series-and-the-Zeus-Ships?highlight=Arcadia

I can give you my ships.. PM me your email and I will send you the files.. I scan my computer regularly and always use antivirus.

Thanks for the kind offer. However, this question is over 5 months old. It's likely the user has figured out a solution and/or moved on.

Cheers,

~Claw