Fairing Stability Problems

[jakeb1993]

Okay, so rocket 5.0

Don't think it is going to fly though as the CoM is way too low. I replaced an orange tank with a dedicated Poodle for the trip to the Mun. Just need to see if this gets into orbit.

EDIT: I had to fight it slightly on ascent, but it held and when i cut the engines, it didn;t flip. Sadly i reached 75,000 with only 758 Dv left in the orange tank, and that was supposed to make my orbit, so i had the 2000Dv tank for Mun transfer, orbit etc. At least it didn't flip

Edited March 29, 2016 by jakeb1993

[jakeb1993]

22 minutes ago, Harry Rhodan said:

With a 13t payload you can get around 5000m/s with one orange tank and a Skipper on top of two orange tanks, a Mainsail and two Kickbacks.

Just tried it, only has a TWR of 0.75, doesnt take off. If you combine Mainsail and Kickback stages, Dv becomes roughly 4400

[Harry Rhodan]

38 minutes ago, jakeb1993 said:

Sadly i reached 75,000 with only 758 Dv left in the orange tank, and that was supposed to make my orbit,

You do turn on your ascent, do you?

20 minutes ago, jakeb1993 said:

Just tried it, only has a TWR of 0.75, doesnt take off. If you combine Mainsail and Kickback stages, Dv becomes roughly 4400

 

[jakeb1993]

I seem to get different numbers to that, I have given up on the Skycrane and using the planet base module, just going to build a large lander for 5 kerbals and put it on the ground. That will be my base for the contract. Even if it did work, i would have to launch a second one, land in the same place and dock it. And considering i can't even get this up into the atmosphere correctly, i don't think i am going to be able to do the rest anytime soon. 

[Plusck]

Well, I went back to your original design and applied most of what has been suggested to get it to work.

Here is the craft file: test craft

And here are the pics;

This launch was a touch too aggressive. The Twin Boar is terrible at overheating, and it blew the solar panels at about 40,000m. The craft file has been changed to put a couple of solar panels on the second booster stage, and a few inside the fairing (which gets ditched in orbit.

Total dv: 7,000 m/s, leaving 3800 m/s or so in orbit with this ascent profile.

I didn't see the "13t payload" info until just now, so obviously you'll get less than with my fake payload (only 11t).

I disabled the second boosters' adaptor tank fuel and ox in the VAB, so you need to correct that after cutting the engines. I also reduced the Twin Boar's thrust to 90% in an attempt to limit overheating: that didn't really work...

And finally I put the engines' gimbal lock on the "brake" action group, so pressing "B" should toggle betwee locked and unlocked (although in fact the craft is not wobbly, even using prograde hold, so it isn't really necessary here).

TWR is so high that I had to throttle down at about 25km altitude, again to try to limit heating.

If you don't get these sorts of numbers, something else is wrong.

Edited March 29, 2016 by Plusck

[N_Danger]

 I have this in my craft file, Similar to Harry's, lifting 14.6 tons of an over designed muner lander. T/W starts out low esp. the fourth staging group and its a long ride up.

But was able put it into a 112 x 94 orbit 

If you need to, test your lifters in sand box over and over. This took about 10-15 launches and then tweaking the build or the flight profile after each one for me to it get acceptable.

[Stoney3K]

7 hours ago, Plusck said:

Precisely the problem! Also your fuel tanks drain from the furthest to the nearest, compounding the problem.

The two aerodynamic command pods (Mk1 and Mk1-2) are both pretty heavy, which helps.

If you have severe difficulties with flipping, you can lock the fuel/ox tanks at the top of the rocket and then unlock them shortly before the engines run out of fuel.

Asparagus staging and/or drop tanks help - a fuel line from the top of one stack to the top of the next will draw fuel from the bottom of the source stack up (but not perfectly: for some unknown reason fuel usage is not exactly intuitive and apparently [though I haven't really tested this myself] in a three-tank column with a fuel line drawing from the top, the top two tanks will empty at the same time once the bottom one is empty).

And if all that fails, the only real solution is fins at the base of the rocket, plus lots of torque (a number of medium-sized torque wheels work better than one large one in the current version of the game, also for unknown reasons. I don't mean stack them together - rather put one on the top of a couple of side stacks and one on the main stack), plus carefully following prograde (SAS doesn't always get this right, so manual adjustment to follow prograde while using only "stability assist" may be better than "prograde hold"), plus not going too fast in the lower atmosphere (accepting gravity losses as the price to be paid for your light and draggy payload).

Oh and plus RCS (or Vernors for larger rockets), too. On difficult, big rockets Vernors work very well.

The easiest starting point is to use inward feeding using fuel lines: Since tanks drain from the furthest away to the closest (seen from the engine), you can abuse that mechanism to keep your CoM higher up.

Put a small fuel tank at the top of your core stage and have the side boosters feed fuel towards that using a fuel line. The core will empty that tank first, causing the fuel in the boosters to flow upwards and top up the tank, shifting the CoM upwards.

Once you dump your empty boosters, you're already far enough up to have less effect from aerodynamic drag, and you've got a full core tank of fuel at your disposal.