Mainsail trouble

[Gods315Pawn]

I recently unlocked the Mainsail engines and large tanks, and so I've been experimenting with them. I'm trying to build a design for lifting. I started large scale and watched time and again how everything falls apart for various reasons. I scaled back. This morning I was working with a single orange tank and mainsail with four large solid fuel rockets for beginning support. It then went to Asparagus staging with the thin long tanks (I really need to learn the designations). The payload was a three man crew replacement for my space station. Every time I tried this, the moment I decoupled the solid fuel boosters the Mainsail pushed its way up through my asparagus staging in wanton destruction. Sometimes physics wouldn't notice right away and they would be sharing space, but it happened everything. I added struts. That didn't help. I added more struts. Still didn't help. Is there something I'm missing?

Here is a picture of the rocket in question.

[Tex_NL]

The sudden change in mass in the lower half of your craft causes immense stress on the connection between the orange tank and your mid section. When that connection snaps the lower section will overtake and rip through the upper half.

Easiest solution: Throttle back just before you drop your spent SRB's and throttle up after you've dropped them. Either that or MOAR STRUTS.

[Kasuha]

The mainsail has certain thrust, but as it eats fuel from the tank, the whole rocket is becoming lighter and the acceleration and the force it applies on ship connections grows. The simplest thing you can do is to throttle down a bit.

Another thing to do is to press F3 after such thing happens and check in the log which part snapped first. Then you can try to reinforce that with struts. Because adding struts randomly where you feel they could help may sometimes even make the ship weaker. The most likely point to give way IMO is the connection between the Jumbo and the SAS module. You might try to strut around that.

Here, just one idea:

Edited January 4, 2014 by Kasuha

[Gods315Pawn]

Duly noted. I will test those ideas out and see what happens.

[zekes]

See, most people think of Mainsails as overpowered and make everything break, but in fact they are the most realistic engines in the game. You have to design your rocket right or it will break apart. Example in Case: The T-9-III. The T-9-III-6S (6 sat) is the heaviest legit ship I've ever made. 935 tons on the launchpad, 192 tons for the upper stage plus payload means the massive first stage is 743 tons! All up she carries 79 thousand liters of Oxidizer and 64 thousand liters of fuel! Even with the 15% reduction of power 12.6 million Newtons of thrust! This is facilitated by the shape and where the tanks are attached. Plus struts help. The First stage looks like one big stage but it is actually 5 separate pieces attached to the 2nd stage (which is in an asparagus formation) so that the power is not forced on radial attachment points, but actually pushes the second stage (and the rocket) skyward.

[capi3101]

It's likely the Mainsail is outputting more thrust than what you need for your remaining payload. The advice you've got so far - throttling back - is sound. What you need to do once you've separated the last set of boosters (assuming you don't pancake) is look at your gee meter. If you're above the green zone, you need to throttle back even more (and consider swapping out with a Skipper if you want to launch the design again). If not, throttle back up - put your gee meter right at the top of the zone. That'll put you right round that 2.0-2.2 TWR you're looking for when you launch.

[BlackBicycle]

Rightclick on the mainsail and limit thrust to less than a hundred percent. Less power - less jerk.

[Skorpychan]

The large reaction wheel is a weak link. Replace it with the narrower one, above the Rockomax Adapter.

To counteract the increased weight, use the Rockomax Adapter 02 instead of the regular one.

[doginneed]

Also, the orange tanks have been known to handle stress poorly, try just using the white tanks

[KvickFlygarn87]

By the way, the second you activate that LV-N you lose those tanks. First of all, might wanna move them up a little. Also, lose the engines on the top stage; they add a truckload of weight and eat fuel like I eat chocolate bars.

[canuckster]

I've often found that too many struts can be the cause of ship instability . I wouldn't strut the boosters to each other, nor would I strut the boosters to the upper stage, I'd suggest putting some cubic struts on the orange tank and string struts to the upper stage from there. I mean, once you lose the boosters there is very little holding the two stages together.

[Tex_NL]

Of course there is another option besides more struts: Kerbal Joint Reinforcement. It really works wonders when it comes to connection strength and stability.

[canuckster]

Here's how I built it, I didn't quite understand your upper stage so I just put the assumed weight there instead. I managed to get this all the way to Mun. It becomes slightly unstable around 6000m but the instability is lost once the boosters are jettisoned, I think it's just a problem those boosters have, they cause all my ships to rotate and I never use them.

Edit, a Mun flyby that is, if I had an engine on the upper stage I assume I could land it.

[Blue]

Every part has a certain stress limit, and if that stress limit is exceeded, the part fails and blows up. The weakest points in any spacecraft should be located either where parts are less numerous or structurally integral (such as the payload), and more robust parts located so that they're staged closer to the launch initial, where strength and toughness is most important.

Also, take into account the full mass of your upper stages. Those 6-symmetry tanks with engines mounted above the large orange tank would weigh the equivalent of slightly more than 1.5 orange tanks-- and that's not even including the stages above it. That's an awful lot of weight that has to rest on top of that one lonely 1.5m decoupler at the bottom of your nuclear engine.