How can I place engines in front of the centre of mass of a rocket?

[Uberdude9001]

I've been trying to design a more powerful heavy lifter for a while now. I have come to the conclusion that it would be most efficient to place radial engines on the top of the first stage. This would allow me to run a large number of stages with only one set of engines. I'm having difficulty with this design because the engines vector their thrust in the wrong direction. This causes ASAS to stop working, making the rocket difficult to fly.

I have seen people get away with similar designs. How do they do it? Optimally this is done without disabling thrust vectoring.

[Tokay Gris]

Hm... how about heavy RCS-usage?

I also had that problem... I wished for a "reverse gimbal" option in the engines...

Because that seems to be the problem: They turn the wrong way.

[Uberdude9001]

I'm considering using RCS or a group of low power but vectoring engines at the bottom. How much RCS and mono propellant would I need to keep a 1000 ton rocket stable during all stages of ascent?

[Temstar]

Sometimes there are good reasons for puller configuration rockets but:

I have come to the conclusion that it would be most efficient to place radial engines on the top of the first stage

Is not one. The thing is you want to get rid of your engines during ascent, not holding onto them. As your rocket get lighter you gain excess TWR. By ditching engines you trade that excess TWR for more delta-V.

[Eric S]

To answer the original question, mostly by disabling gimballing on engines above the CoM, and having enough control authority elsewhere to keep the ship under control.

[Markus Reese]

The best way is having a main thrust engine up high above center of mass, then smaller steering engines below mass. Turn off steering on main drive then it should work fine. It worked in old version before we had large engines and the heavy thrusts did not vector. A sample idea is have a single small engine spine good for once in space and not needing the heavy thrust. Place large engine up top, then your fuel tanks also on spine and connect to that.

[Slavechild]

I've been trying to design a more powerful heavy lifter for a while now. I have come to the conclusion that it would be most efficient to place radial engines on the top of the first stage. This would allow me to run a large number of stages with only one set of engines. I'm having difficulty with this design because the engines vector their thrust in the wrong direction. This causes ASAS to stop working, making the rocket difficult to fly.

I have seen people get away with similar designs. How do they do it? Optimally this is done without disabling thrust vectoring.

Searching for ways of improving my "rocket-first/fuel-last" setup I found this post with not much of an answer, here's a few screenshots of my ship! (top 4 screenshots).

http://steamcommunity.com/profiles/76561198013090083/screenshots

The only mod is Mechjeb (0.22) with half of my tech tree unlocked!

[Sirrobert]

Searching for ways of improving my "rocket-first/fuel-last" setup I found this post with not much of an answer, here's a few screenshots of my ship! (top 4 screenshots).

http://steamcommunity.com/profiles/76561198013090083/screenshots

The only mod is Mechjeb (0.22) with half of my tech tree unlocked!

WTF does that have to do with anything in this thread?

[Kasuha]

I've been trying to design a more powerful heavy lifter for a while now. I have come to the conclusion that it would be most efficient to place radial engines on the top of the first stage. This would allow me to run a large number of stages with only one set of engines. I'm having difficulty with this design because the engines vector their thrust in the wrong direction. This causes ASAS to stop working, making the rocket difficult to fly.

I have seen people get away with similar designs. How do they do it? Optimally this is done without disabling thrust vectoring.

If you want the ship to not fall over with engines at the top and without disabling thrust vectoring, your only option is to add enough torque to overcome whatever damage will the inverted thrust vectoring do to the ship's steering. Small engines at the bottom are probably not an option since you want to stage that bottom off.

But in my opinion the best approach is to have the engines at the bottom at one tank length and have the rest of tanks on radial decouplers, connected by fuel lines. Something like in the picture below.

Then you might find out that as long as you already have so many tank bottoms available, you can add some engines there too to modulate thrust during ascent. And voila, you've discovered asparagus staging.

[Vanamonde]

WTF does that have to do with anything in this thread?

It's guy's first post. Could we be a bit friendlier about it, please?

[nadreck]

Actually a very valid reason to have engines above the CG running is to have base engines supplemented by engines you are not throwing away with the base. You get a better overall delta-V if (with no extra weight penalty) you can use all the crafts engines at launch (like an asparagus design). My NERVA landers have their engines available during ascent phase, though I don't see a point in turning them on below 30,000M on Kerbin as their 240Kn really only contributes significantly when the last boost stage is running.

[Dispatcher]

I've succeeded with jet engines radially attached to rockets; typically 2 or 3 stages up from the initially fired stage. I have the jets fire first and warm up a few seconds, then fire the initial stage engine and boosters. I jettison the jet assemblies when they are no longer needed (and often that is when I fire the engine of the stage on which they are mounted). It takes experimentation to obtain optimal results.

In essence, the jets pull while the engines below push.

[EtherDragon]

"Engine First" designs are certainly possible, unfortunately, they mostly rely on disabling the Gimbal on the rocket engines that are in front of the center of mass. Try using an in-line SAS near the top of your stack, that should give you enough control authority to overcome the lack of gimbal on your lifting engines.

[Kasuha]

My NERVA landers have their engines available during ascent phase, though I don't see a point in turning them on below 30,000M on Kerbin as their 240Kn really only contributes significantly when the last boost stage is running.

You should notice that at 1200 m above Kerbin Nervas are already the most efficient engine in game. Even though it's a small contribution it's still the cheapes contribution of all engines running.

[robsr3v3ng3]

I would avoid using engines mounted at the top as your main thrust, and here is why.

A rocket is unstable if the centre of thrust is below the centre of mass. This instability is what gives you ease of control, a very stable craft will fly in a straight line more easily, while an unstable one will try to tip over as you thrust in atmosphere.

However, if you put the centre of thrust too far above the centre of mass it because too stable because the effect of dragging the mass behind you makes it very hard to actually turn because it is too stable.

The best analogy I can think of is imagine trying to push or pull a trailer. If you pull it, to steer you have to pull it to the side (similar to having a gimbal that could be as much as 90 degrees!). But if you push it and you start pushing only a few degrees to the side it starts guiding itself in the right direction.

As mentioned by others it's best to ditch excess engines as your craft losses weight because then you avoid the situation where you're carrying twice as much thrust as you need and running them at 50% (this is more of an issue below 12km).

If you need extra thrust the best option in my opinion is stick on some asparagus stages (with engines on). N.B. If you're talking about huge amounts of thrust you may need to make these stages using Rockomax engines (or a quad coupler with small engines on) to have a decent effect. If you need better stability position these so that the centre of thrust is closer to the centre of mass (but still below it).

Please note: I am not saying don't build a ship with radially mounted engines near the top and that it won't work. I'm just saying it is easier to put extra asparagus stages on which are ditched first. If you need extra fuel at low altitude it can be held by the asparagus stages with engines. If you need extra fuel but not extra thrust (as you shouldn't go much faster than 250m/s below 12km) then use asparagus with just fuel tanks and ditch them to save weight.

[Slavechild]

WTF does that have to do with anything in this thread?

"How can I place engines in front of the centre of mass of a rocket"

That is a rocket with the engines in front of the centre of mass that works..... but reviewing your other posts I can understand why you struggled with the concept.

On a lighter note Kasuha has the right idea, standard asparagus staging is the best way to go but can be tricky with career mode limiting modules.

[Slavechild]

However, if you put the centre of thrust too far above the centre of mass it because too stable because the effect of dragging the mass behind you makes it very hard to actually turn because it is too stable.

I find this to be an aid if you're not wanting to orbit Kerbin at all, all my probes are sent up and out of the Kerbin system using this but usually the payload will have a small asparagus rocket and then the probe.

[robsr3v3ng3]

I find this to be an aid if you're not wanting to orbit Kerbin at all, all my probes are sent up and out of the Kerbin system using this but usually the payload will have a small asparagus rocket and then the probe.

Fair enough. You do get a more efficient lift if you take off, do a gravity turn, then increase your apoapsis rather than just go straight up.