Better to push or pull payload?

[AlexanderB]

That would work fine if the engines gymballed the other way when on the other side of the center of gravity, which they currently do not in KSP (its a bug that still needs fixing..)

So you need something else to make and keep the top pointing in the right direction. Lots of RCS, usually..

[Tobmaster]

It actually does work quite well. That was how I got awkward pieces up for my space station - basically a mammoth skycrane - but like said before, the gimbaling goes the wrong way.

[Sirrobert]

Is that true? I'm having some issues lifting heavy payloads because right around the time when I start the gravity turn my payload at the top weighs much more than the emptying tanks (and jettisoned stages, obviously) at the bottom. This tutorial on the wiki references pushing upwards on a broom balanced on your hand: it's much easier when the heavy part is at the top rather than the bottom.

Assuming KSP physics and parts rigidity, assuming you could avoid any exhaust damage issues, and acknowledging the backwards-gimbal issue, couldn't a case be made for pulling huge payloads into space?

I'm picturing a massive payload with fuel tanks mounted below it, with engines way up on outriggers connected to the fuel by fuel lines.

I can't test it until later today, but it seems like that should work fairly well, no?

The main problem with pulling is that you can't stage like normal. Ofcourse you can have the final orbit stage a pull setup, while the liftoff stage a push setup, but that'd still make the top very heavy during liftof

[sgt_flyer]

The true interest about pull designs is in space in real life, is that it enable to reduce considerably the ship's structure - most materials are much better withstanding traction effort than compression efforts - thus, much less weight is needed for the ship's structure, so if you use the same tanks and engines, you will have more delta-V in a puller design than in a pusher design. (i'm speaking about absurdly big space only ships here, not reentry capable crafts )

[John FX]

One more thing, if you put your intakes in the front of your COM then (if you have as many as me) there will be a point about 30-40km where they just drag the front of your craft round 180 degrees and you are flying backwards (in my experience). Intakes and engines at the rear? so stable you don`t need aerofoils...

Obviously this only applies to jet stages.

[John FX]

Is that true? I'm having some issues lifting heavy payloads because right around the time when I start the gravity turn my payload at the top weighs much more than the emptying tanks (and jettisoned stages, obviously) at the bottom. This tutorial on the wiki references pushing upwards on a broom balanced on your hand: it's much easier when the heavy part is at the top rather than the bottom.

Assuming KSP physics and parts rigidity, assuming you could avoid any exhaust damage issues, and acknowledging the backwards-gimbal issue, couldn't a case be made for pulling huge payloads into space?

I'm picturing a massive payload with fuel tanks mounted below it, with engines way up on outriggers connected to the fuel by fuel lines.

I can't test it until later today, but it seems like that should work fairly well, no?

You`ll need some winglets at the top if you are top heavy, remember large mass at the ends has more inertia so your gimbals will be less effective and you will flip. Try that maybe before a pull into space design.

[magnemoe]

The true interest about pull designs is in space in real life, is that it enable to reduce considerably the ship's structure - most materials are much better withstanding traction effort than compression efforts - thus, much less weight is needed for the ship's structure, so if you use the same tanks and engines, you will have more delta-V in a puller design than in a pusher design. (i'm speaking about absurdly big space only ships here, not reentry capable crafts )

Added benefit if you want distance because of radiation from the engines.

This is main concern as most deep space engine ideas have low trust but might generate a lot of radiation.

[nyrath]

Beyond things that KSP models, I have to wonder what the effects might be of putting your payload "downwind" of your propulsion. This sounds like it would have less-than-ideal results, especially in the case of nuclear thrusters.

It is possible to mount your thrusters so instead of firing their exhaust directly backwards into the payload, you instead have three or four thrusters firing at an angle away from directly backwards. The vector sum of the thrusts is still forwards.

The good news is that can prevent the possibly radioactive exhaust plume from hitting the payload. The bad news is that the effective thrust is reduces by the cosine of the angle off from backwards. So for example, if they were firing 30 degrees away, the effective thrust would be about 87%.

Having said that, a solid core nuclear thermal rocket like the Kerbal nuclear engine has a non-radioactive exhaust. Unless the engine is undergoing a nuclear melt-down or something catastrophic like that.

See "Helios"

http://io9.com/5928996/the-50+year-history-of-nasas-revolutionary-sky-crane

Edited July 9, 2013 by nyrath

[ecat]

Both push and pull can be made to work in game, after some experiments I concluded it's best to do both...

Pushing from the bottom.

Mass of payload acts on the centre of the pusher, outer engines are free to flop around which limits maximum design thrust...

Having the engines push a frame the frame pull the payload.

Engine force acts directly up through the columns and is evenly distributed by the frame, mass of payload acts directly through the centre of the frame, some additional side bracing required to prevent payload from moving off centre...

Attaching the payload to both the top and the bottom of the frame.

Forces on payload reduced and are distributed between top and bottom couplers. Bottom attachment points constrain inner engines ensuring engine force acts directly through the columns...

Some of my other test designs would provide better examples but I don't have recordings of the those test. I suppose that final design is really pulling from the top and pulling from the bottom. I would have liked to try a full on pull/push design but tests ended when part count made lag unacceptable

Warning:

Test designs may not be applicable to real world physics

Edited July 10, 2013 by ecat