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Personal experience with heavy rockets and FAR.


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Recently I have been trying to move larger payloads into space with FAR; it's been pretty successful so far and I would like to speak of a few things i noted as I experimented. I generally do trial and error for most things and I am not exactly an aerospace engineer, but a few things seemed to stick out and while I think i sorted them out I was wondering if anybody was willing to further comment on these things, or give me the sciencey stuff behind them.

Notes:

1. The length of the rocket you are using along with the weight and location of the payload are much more important than stock. Adding too much weight to a rocket that is too tall tends to increase the chance of flipping while maneuvering in atmosphere. On the other hand if the rocket is too stubby the same thing occurs, kinda have to find a length sweet-spot depending on the payload weight(or add tons of sas).

2. I generally find adding wings to anything that doesn't need to use lift counterproductive, yes you gain some movement control, but lose a lot of stability if you don't plan on sitting the VAB for an hour tweaking your COM, COL, and COT every time you change payloads. I prefer to stick with SAS or Mono unless absolutely necessary as they are useful in pretty much any location with any level of atmosphere at any speed and are pretty easy to slap on to something.

3. When dealing with really heavy payloads, you can still do early gravity turns without much effort, the key to doing them early in my experience is to keep under mach-1 until in the turn, then throttle up past it, no idea why, but as long as I am under mach one, doing sharp turns is much easier without the risk of mid air gymnastics.

4. Sometimes when possible I put my payload at the bottom of the rocket, another not sure but it works; putting the payload on the bottom and mounting rockets on the sides of the tanks seems to allow for a lot more control providing you can side mount the rockets. this tends to not work as well when you have to mount any rockets bigger than 1.25m due to the increase in drag over a traditional dart design. I'm guessing it puts the center of mass somewhere more pleasant, but again not sure.

Well those are the few noteworthy things I have found crashing rockets; If anybody wishes to add to, correct, or explain why these things are happening please go ahead; it's always nice to learn something useful. Good luck and happy trails of smoke :)

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Point 4 is probably the centre of drag being way rearwards. Fitting stabillizing fins is a good idea,and you won't have to re-edit the launcher if they're just control fins at the base - fitting wings to something that isn't a plane is probably not a good idea. Fairings are an astoundingly good idea too.

All my lifters for anything that is too heavy to use one stack look like this:

9450160088_e1e4dc35c8_c.jpg

Short second stage with the payload on it, twin stacks for the first stage feed into the second stage, they'll run out somewhere in the upper atmosphere after providing stabilizing crossbracing for the trip through atmospheric pressure. Flexing payloads aren't such a pain when they can't cause aerodynamic upsets. The fairing is hanging around in the shadows there, I'd not send it up like that. Could probably just stick a nosecone on it, though.

Scales fine too:

9501018406_a056a3db65_c.jpg

It does help to have access to large parts, of course.

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I read this but both together would be to long

Thanks for the tips, I use procedural fairings as well so top drag should be a non-issue as mentioned though I shy away from stabilizer winglets, when I said wings I meant anything shaped like a wing including fins, from my understanding IRL rocket stabilizers do not have lift due to lack of curvature on the surface

pof-clean-airflow.jpg

That shape is what causes lift as it changes the velocity of the air between the sides causing a difference in pressure resulting in what we consider lift. The stabilizers on rockets do not curve so they shouldn't generate lift, but unless FAR distinguishes between the functions rockets with winglets will become nose heavy in turns due to the upwards pull of rear mounted winglets, I always test my rockets for horizontal stability and the ones with rear winglets have a tendency to do flips.

Standard Medium Lifter- Still doing some tweaks and considering switching to Liquid boosters for more throttle control, but it handles well and is pretty flexible on payloads as long as they don't take up to much space, still doing weight test as I go along, although I have managed 40 tons so far.

U1jMXYr.jpg?1

P.S. Sorry for the slow response, power outage.

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A tail fin is symmetrical ( or it wouldn't be much use as a tail fin ) - given that my aircraft don't all yaw in one direction all the time then FAR is at least treating some tail parts as symmetrical aerofoils and they're safe to use on a rocket - the fully moving winglets/canards should also be safe. If you arranged a bunch of wing shapes around the base of a rocket it should want to roll rather than tumble - I vaguely remember that FAR picks one wall of the VAB as "up" for setting up wing orientation though, but don't quote me on that.

What usually tumbles my rockets is a combination of excess flex and excess speed/twr - the centre of drag gets offset too far by the flexing and starts pushing the nose round which causes progressively more drag as more and more surface is exposed to airflow, and round it goes. That's what led me to shorter stages under the payload and parallel designs, and also low launch TWR. I still find small control surfaces make a lot of difference though.

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