Looking for launch feedback (1:33 video)

[Drew Kerman]

This is a standard launch rocket design I've used for over a dozen successful launches in various configurations (2 SRBs instead of 3, 2 main tanks/engines instead of 1, etc) and this is the first time it's experienced such a huge structural failure. Standard launch procedure for me is to await the "tower clear!" and begin to point my velocity vector east, following it until I reach 45 degrees then holding my navball there while the velocity vector continues towards the horizon.

This time I wanted to launch into a polar orbit so I immediately began heading south after clearing the tower. The most important thing to note during the launch is at 1:19 you can see I have my pitch at full deflection just to prevent my rocket from tipping over past 45 degrees. I've never had to hold even close to full deflection before to do this during a launch. When the SRBs drop and I lose thrust prior to main engine throttle up, the rocket flips out because I didn't zero out my pitch.

I'm wondering if turning over towards the south was a bad idea, and if I should have began east and gradually increased my velocity vector towards north/south as I ascended. I can't think of any other reason why I had the pitch issue with the rocket.

Another guess is the use of the RealChute nose cones to recover the SRBs. In all previous launches I've used stock Squad nose cones. I usually just use a sub-assembly for the lift stage but in this case I had to remove and replace the SRBs - perhaps I put them on a tad bit angled? Doubtful, since I used part symmetry and EditorExtensions to get them evenly spaced around the rocket. But still, that is the only other change in this launch besides the southward turn over.

Edited July 21, 2014 by Gaiiden

[Taki117]

The issue is that you don't have enough thrust at the point you separate your SRBs. It looks like you are playing with FAR (Hard to tell from your toolbar) which means you should probably start your turn at about 200m, not whatever altitude you started at, it should also be a smooth turn, with very few jumps in pitch. As your SRBs start running out of fuel start throttling up your LFB. Another option (Depending on your TWR for your SRBs) is to have your Liquid engine ignite at the same time your SRBs separate, then you just have to keep your throttle on full and your engine will start at full thrust when it's ignited, which will help you push through the massive wall of air that builds up in front of your rocket because of those SRBs.

[Roastduck]

Basically what Taki said, but I would actually slightly lower the launch TWR by tweaking the SRBs a little. With FAR, I've always felt more comfortable with a launch TWR of about 1.3. As far as the gravity turn, instead of doing it at a specific altitude, as soon as your velocity hits 100m/s, start pitching over about 2-4 degrees, then just follow the prograde marker. Once you nudge it over that first time, it will naturally keep shifting very slightly. Let it creep over, and just keep pointing right down the center of it

[Master Tao]

Agreed – tweak those SRBs down for an initial TWR of about 1.1 to 1.2 (1.3 is fine, too). Between 75 and 100 m/s surface vertical speed, pitchover 2-5 degrees. Follow the prograde vector, adjusting the throttle so that your gravity turn is gentle and aerodynamic. You want to be within 5-10 degrees of horizontal by the time you reach 50 km altitude. At that point, burn full throttle until your apoapsis is a hair over your target.

[Roastduck]

At that point, burn full throttle until your apoapsis is a hair over your target.

You can then pitch down slightly below horizontal (exact angle depends on the launch and craft) and keep burning to start bringing the periapsis up while keeping the apoapsis mostly constant

[Drew Kerman]

The issue is that you don't have enough thrust at the point you separate your SRBs. It looks like you are playing with FAR (Hard to tell from your toolbar) which means you should probably start your turn at about 200m, not whatever altitude you started at ... which will help you push through the massive wall of air that builds up in front of your rocket because of those SRBs.

FAR icon is right there on the toolbar and you can see my altitude is 200m when I begin my turn But regardless, you mentioning the wall of air I need to bust through after SRB drop is one of the things I thought was happening so nice to have some confirmation there. I guess I was just pitched over too far too soon - the SRB hit when it dropped is also something that doesn't ever happen.

I'll try the lower TWR. Tweaking the SRBs for 1.5 has always worked out perfect for me, but perhaps with the new nose cone chutes I need to throttle back some. They are a bit bigger than the stock nose cones.

[Aethon]

Holy cow! So with Far you disregard terminal velocity through the atmosphere?

[Master Tao]

Holy cow! So with Far you disregard terminal velocity through the atmosphere?

No, but terminal velocity is much, much higher for aerodynamic rockets than in stock. At sea level, the terminal velocity for the Kerbal X is over 800 m/s. With the FAR flight data window open, you can follow your terminal velocity to make sure you're never exceeding it.

[Hodo]

Holy cow! So with Far you disregard terminal velocity through the atmosphere?

No FAR just turns the atmosphere into a gas based atmosphere not one modelled on used motor oil.