Uneven Thrust for Spaceplanes

[solidgeoff]

The spaceplane I'm working on performs generally pretty well, getting into orbit and such, but during the ascent, I find uneven thrust between the two larger Sabre engines in the middle. The one on the right will have a thrust of 500, and the left will have 480 (the numbers change with the atmosphere, but the ratio stays more or less the same). This causes the plane to shift to the left. RCS does a fine job compensating, but the higher I go, the more significant the difference, and it shifts more and more.

Notably, when I switch to rocket mode, the thrust becomes even amongst the engines. I'm assuming it's some kind of air-intake thing.

For those of you who don't use this mod, the big Sabre engines are in the middle and they have the ability to switch from air-breathing jet engines to rocket fuel engines with an action group.

Any thoughts? Picture below. I'm happy to include more if needed.

[Hodo]

The question is what is your slip angle when climbing, are you flying slightly sideways. If so that will affect the air intake of the engines, and cause uneven thrust between them. The easiest way to fix this is to mount your larger SABREs on the main body and this will help some, it will also help clean up the lines of the aircraft. So you won't need as many struts that far out to hold the weight of the SABRE Ms. Also I would ditch those intakes on the ends of the LV-Ns, they are only good at subsonic speeds, use the variable geometry intakes they have less drag and are far more useful at higher speeds.

[solidgeoff]

Wow thanks for the quick and thorough response Hodo. I'll definitely try out some of those recommendations. I'd love to keep the engines out to the side, to keep it looking kinda-sorta like the skylon, but I'll definitely get rid of the air intakes you mentioned, since i went with a "more is better" approach. And aesthetics, for that matter.

Is there a difference between the "DSI Diverterless supersonic Inlet" (the ones I have now, the low-profile ones), and the other two?

[Hodo]

Here is one of my fastest climbing SSTOs.

And here is my workhorse SSTOs.

There are some design differences but not much with mine due to I found what works for me and stuck with it.

And the DSI intake is good but is really not much better than the standard SABRE S intake. I like it for when I want cleaner lines or a sleaker look.

[BubbaWilkins]

The game doesn't assign specific intakes to engines. It takes the sum total intake and then subtracts each engines consumption. The last engine gets starved if there isn't a surplus.

Edited September 24, 2013 by BubbaWilkins

[solidgeoff]

Ah man, I love those spaceplanes. Yeah, I think my biggest issue right now is how the two engines are set up on the wings. I can still go high and fast enough that when i switch to rocket mode, i can get into orbit (with a small payload), but i feel like every change I make just makes it worse. I'm sure we all know that feeling. And yeah, even the slightest roll in one direction or the other dramatically changes the thrust im getting. Though a successful ship, i might have to start back at the drawing board. Thanks for your help.

[solidgeoff]

Thanks for everyone's help. Here's an updated craft I made yesterday. It can take 18 tons up into orbit with a nice margin of error. I still struggle with flying at high altitudes and thrust differences. Fortunately, thing go well if I just switch to rocket mode as things get hairy, and then the plane will fly stable up to orbit.

[Hodo]

Good looking craft. If I may make some suggestions to help with your part counts and weight savings.

You can reduce the numbers of RCS thrusters on the sides of the nose to one per side. And for that matter everywhere where you have more than one in a location you can reduce that to one. It will slow down your reactions a bit but unless you are making a space fighter you don't need to move that fast in space. I try and mount them on the flatter parts of it so they are inline with the aircraft. This reduces drift when use in space. You can also get rid of the solar panels if you have the retractable panels, and the smaller cased ones would work just fine. I personally like to stick a pair of nuke reactors in the cargobay, on one of the walls to generate power so I don't have to worry about being on the dark side of anything.

Not sure if you have your wings strutted if they are needed.

[solidgeoff]

Thanks Hodo. I had a long weekend away and I wasn't able to respond. Your suggestions are definitely good. I have a lot of RCS to counter the problem this thread was originally about. mostly, they are for keeping the thing level at the 20,000m altitude area. Nuke reactors are a good idea. I'll have to try that one out. I guess overall, I don't have too many issues with part counts. My computer can handle quite a bit, and i dont like extendables because they break up in the atmosphere, and I always take forever to land. what might be better for me is using one of the solar panels from Bobcat, the big radial mounted ones, since clearly im not apposed to mods. However, my solar panels spread is clearly not ideal.

Finally, here is a pic of the payload I took up. I sent it on a Dres-and-back mission. On a not-spaceplane note, I was proud of using the stretchy-tanks to fill in that empty space around the nuclear engine, as well as using them as landing gear. Also two mini-satellites that are in orbit around Dres right now. though, I would recommend attaching actual landing gear to them, as the landing was a bit rough.