Is it technically possible to build huge rockets that don't wobble? Is KSP realistic?

[xcorps]

Depends what you mean by absolutely minimise. Define a limit and design down to it. Generally the idea is to try and be a bit more clever than simply throwing more metal it. You can play around with geometry. You're normally trying to minimize the amount of materials used to stay within your specs.

http://en.wikipedia.org/wiki/Strength_of_ships

Has some good information regarding this portion of the discussion.

[78stonewobble]

Depends what you mean by absolutely minimise. Define a limit and design down to it. Generally the idea is to try and be a bit more clever than simply throwing more metal it. You can play around with geometry. You're normally trying to minimize the amount of materials used to stay within your specs.

Yeah, offcourse... but obviously you can't put in too many crossbeams in a cargo ship .

[Camacha]

True, KSP doesn't model anything like buckling, which is quite a low energy failure mode.

Which reminds me of this video:

Which resulted in the following damage. Not a good day to be a pilot, or an airline.

Yeah, offcourse... but obviously you can't put in too many crossbeams in a cargo ship .

It also rather depends on what you intend to carry Shipping oil is a different matter than hauling containers.

[Triarius]

A lot of people don't really understand why low density alloys are useful in airplanes aside from "it's lighter". I am an engineer who has spent a good deal of time working with aircraft structures. I am also a pilot and work on my own airplane. If you look at the BEST available alloys for steel, aluminum, and even titanium, you will find out that the strength to weight ratio is surprisingly similar for all three materials. In general aluminum has about 1/3 the density, and 1/3 the strength, and 1/3 the stiffness of steel. Titanium alloys are in between. So, given a similar strength to weigh ratio, why is aluminum better? Looking at an aircraft skin for example you have a sheet surface supported intermittently by ribs underneath and loaded by a distributed aerodynamic pressure. This aircraft skin is primarily loaded in bending. I won't to through all the math, but the strength and stiffness of a beam loaded in bending depends on the thickness of that beam CUBED. So, if you replaced a steel wing skin with an aluminum wing skin, and used the same mass of material, the aluminum will be three times as thick. Using the more "flimsy" material has cost you a factor of three in both strength and stiffness, but tripling your section thickness has gained you a factor of 3^3. The skin is, in fact, 3^2 as strong at resisting the load meaning that in this application it's 9 TIMES STRONGER (but not, unfortunately, 9 times lighter). Note that this advantage is not gained in every application. Struts, flywheels, fasteners and lots of other parts are still made from steel in many areas of an aircraft.

Interesting concept, but the first objection that comes up in my head is "That would probably be quite heavy"

Rockets and airplanes are designed to be light and stiff, in that order of priority. A certain amount of flex is expected, and in some cases, desired. That means that "but that would be heavy" is, by itself, usually enough to rule out solutions that have it as a disadvantage.

The wings of basically every single modern passenger airplane are actually designed to flex a certain amount in flight. Wings that don't flex would weigh a lot more, which would mean taking either less payload or less fuel.

That's actually most of the reason that airplanes and rockets are built with aluminum alloys, Al/Li alloys, and titanium alloys, instead of steel alloys.

It might take more volume of those alloys to get the same strength as steel, but it will still end up weighing less overall.

Basically, the goal is "As light as possible while still remaining safe". Most of the "remaining safe" part is handled by making the parts a little bit stronger than absolutely required, and then manufacturing the part to be as close to exactly that strong as possible. Strength takes mass, after all.

For example, a part that is required to take a 1 kN tension load will be designed to fail at 1.25 kN tension, giving a 25% safety margin. In extreme cases such as safety critical parts, there can be 200% safety margin or more. Usually that's only seen when failure of the part will result in catastrophic failure of the entire device, and multiples of the same part won't help or can't be done (space constraints, failure of any one of them results in same catastrophic failure no matter how many of them there are, etc.)

Also, if you make a structure TOO rigid, it will fracture instead of bending. That's NEVER a good thing. Bent parts can be unbent. Fractured parts can only be repaired by welding or use of a patch panel. Both of those options are weaker than an unbent part. Of course, a bent and unbent part will be weaker than a part that wasn't bent in the first place, but that can often be solved by reapplication of the heat treatment used on the original part, if any. If there was no heat treatment originally, it should be annealed to remove any work hardening that happened.

-edit-

The dampers used might not be big and heavy, but each Space Shuttle Main Engine actually has damper bellows in the LOX feed line to prevent oscillation of the thrust produced, commonly known as "pogo oscillation"

Pogo is a resonance problem, which is why dampers work so well. They don't actually get rid of oscillations, they just change their frequency to one that nothing else on the vehicle resonates at.

The F-1 engine also had problems with pogo, but it's designers were able to work that out by modifying the combustion chamber geometry.

The Atlas missiles that were converted for launching Mercury and Gemini capsules had pogo issues too, IIRC.

Oh, and don't forget the center engine on the 2nd stage of Apollo 13's Saturn V. That was pogo induced by the flexing of the structure that the engines mounted to, and actually if that engine hadn't shut itself off, Apollo 13 might have ended much sooner than it did.

Edited March 3, 2015 by Triarius

[PB666]

Long story short: Every object in real life "wobbles" to some degree.

But no rocket wobbles as much as these in KSP do. You'd see catastrophic collapse long before any rocket would bend to the degree it happens in KSP.

I have built rockets that are taller than the VAB using the following factors (0.5, .75, 1, 1.5, 2, 3, 4) (13 stages) there is a secret to building up, it will work until 1.0 comes out.

Without stabilizer bars they bend over like a fishing pole, imagine that velocity ^ 2 * atm pressure = scalar fish, the fast your go, the the stronger the fish the more it pulls over, the more acceleration you put on the craft the more the fish weighs the more it bends.

1. Make XL MGS in all scale factors

2. Create Double size strut connectors (8 fold more massive and double the maximum length and quadrupel the strength)

Set the XL MGS relative to each sections size 3 fold symmetry minimum. On every other successive stage the go to the opposite do the same reducing the size of the XL MGS as you go. Interconnect the XL MGS forming triangles each MGS has 2 struts going upward.

Use Mech Jeb, place controllers at several points down the rocket, when launching use the lower most controller, before you decouple, change to the next higher controller.

One can also increase the breaking torque.

One can also combine parts for example create an engine tank combo (Hard)

Avoid using two tanks where one will do, create tanks of various size exactly to fit the need. (e.g. use the scale function to compress or extend tanks)

One can also increase the fuel density in a tank and reduce the vertical component (cheat)

Provide the RCS for each stage of flight by the section that is to be decoupled, then deblock RCS fuel for the next section. (extra weight, but you get rid of unneeded cannisters and keep the weight up top down).

Use my jet powered launch pad (essentially modify a 200 fuel tank and make it thin and wide attach jets and feul and lift the rocket up to 33, 000 meters, then launch and avoid the atmosphere, go up to a comfortable altitude (say 300km) and make your turn. This platform circumvents drag because you can travel essentially IAS 20 kts. (10m/s)

With the exception of launch clamps, on can using XL MGS and strut connectors create a stablized launch pad on top of a poly-jet.

Use small ports instead of lauching side mounted gear. Gear (batteries, RCS tanks, Xenon) can be attached later. three solar panels on the payload suffices to power a craft. The stock xenon tanks are lousy anyway, mod a large tank and attach it later, with ported tanks, when they empty, decouple and save the weight.

- - - Updated - - -

Which reminds me of this video:

Which resulted in the following damage. Not a good day to be a pilot, or an airline.

It also rather depends on what you intend to carry Shipping oil is a different matter than hauling containers.

The Japanese get their moneys worth out of those planes, wouldn't be surprised if some of that was stress from previous landings.

ANA is my favorite airline, but they have some overworked old mares in their livery.

Edited March 5, 2015 by PB666

[StrandedonEarth]

What? you mean this rocket wouldn't stay together in real life?