FAR: What flight profile is appropriate?

[Ice30]

I have been using far for a little while now and i can never work out what flight profile i should be using. Based on the forum page saying:

I tried to fly my standard gravity turn into orbit and my rocket flipped out and then broke apart. What happened? Is this realistic?

The standard KSP "gravity turn" is pretty much the worst way to fly a rocket with FAR installed. The 10km vertical climb followed by a 45-degree pitch over maneuver (note: this is not a gravity turn) causes the rocket to become unstable because it can cause the drag and lift of the upper sections of the rocket to overpower whatever stabilizing elements might be at the bottom of the rocket; this can also happen in real life. Do a proper gravity turn; start it low in the atmosphere and keep aiming prograde through the atmosphere. This should allow you to control your rocket even with the most finicky of designs.

I tried this, and sometimes it works but other times my payload (which is encased in dynamic farings) just topples over when executing a low altitude gravity turn (at whatever angle even when following the prograde circle). Interestingly I have found the only way to not get my payloads to topple over so often is to launch until i'm on the edge of the atmosphere and then pretty much turn 90 degrees, which does not seem realistic at all. Have other people experienced this? What am I doing wrong?

Edited May 4, 2014 by Ice30

[TechnicalK3rbal]

As soon as you get over ~100 m/s vertical, tilt east about 10 degrees, wait till you get to about 200 m/s, turn off sas, and just let it go. You shouldn't need to steer it at all until you're doing your orbital burn.

[Mipe]

I find the payload needs to be generally sleeker and lighter than the launch rocket. As rocket loses fuel, the center of mass creeps upward and if your payload is heavier than the dry mass of launch rocket, it'll topple over. If it is wider, the drag will make it topple faster (dart effect). You can counter this with some fins, but they reduce the efficiency at cost of increased stability.

So, basically, the heavier the payload is, the heavier the launch stage needs to be (in order to keep payload pointing up when going dry). The launch stage also needs to be wider than the payload (i.e. have more drag).

[BudgetHedgehog]

I find the payload needs to be generally sleeker and lighter than the launch rocket. As rocket loses fuel, the center of mass creeps upward and if your payload is heavier than the dry mass of launch rocket, it'll topple over. If it is wider, the drag will make it topple faster (dart effect). You can counter this with some fins, but they reduce the efficiency at cost of increased stability.

So, basically, the heavier the payload is, the heavier the launch stage needs to be (in order to keep payload pointing up when going dry). The launch stage also needs to be wider than the payload (i.e. have more drag).

Incorrect. The CoM should always be as high as possible to make it more stable. Drag should always be as low down as possible to increase stability further. It's similar to how planes work - if the CoL is too close to the CoM, it's unstable and likewise, if the CoT is too close to the CoM in rockets, it's also unstable. If you want an easy to imagine example, try balancing a broom on your hand. You'll find it's easier when the head is at the top because the CoM is further away.

So the payload needs to be low drag and high mass, the lifter stage should be as light as you can make it and have more drag. Easiest way is to put fins as low down as possible.

As for the launch profile, you should turn about 5-10 degrees once you reach about 80 m/s. Basically, don't point your rocket outside the prograde circle, at least not until you're out of the thick atmo. Try and keep your time to AP at about 40-50 seconds. If it goes higher, point down more. If that means you're pointing down a lot, throttle down instead. But really, the perfect gravity turn should be executed without SAS on at all - once you have a good starting angle and speed and as long as you ride the throttle, the rocket will turn itself and it'll be glorious.

Could you post a picture of your rocket? There might be something about the design which makes the turn more difficult than it needs to be.

[Mipe]

Huh. I'll consider that as I design my next lifter. I've been having issues with flipping rockets, perhaps my line of thinking is faulty after all.

[Hodo]

One other consideration that most people who play with the stock KSP dont consider, your Thrust to weight ratio (TWR).

With FAR you do not want a stupid high launch TWR, it is generally best to stay between 1.2-1.4 TWR on launch. Because if you accelerate to fast, you generate a great deal of pressure in front of your rocket basically acting like hitting a wall at the lower altitudes.

I generally start my gravity turn around 1000m, or 100m/s which ever happens first. I never turn more than 5deg at a time, I try and keep the prograde marker with in the flight path marker till I get into the higher atmosphere.

[KvickFlygarn87]

Here are some hints:

• Have a starting TWR of MAX 1.4
  
• Start at around 80m/s
  
• Stay at the outer edge of prograde until you're almost horizontal
  
• If necessary, pull prograde up
  
• You can basically point anywhere above 40km
  

[draeath]

I'm assuming if your TWR is a bit higher, you can just ride the throttle proportionally lower?

[xrayfishx]

How I fly my rockets:

TWR 1.5-2

Start gravity turn at about 25m/s

45 degrees at 8-10k

60-80 degrees by 20k

almost horizontal by 40-50k.

I usually make my rockets so that they have 3300dV. More for lighter rockets, less for heavier. This is enough to get to LKO and then deorbit the last stage, leaving no debris. Also this leaves a nice little margin for error.

[BudgetHedgehog]

I'm assuming if your TWR is a bit higher, you can just ride the throttle proportionally lower?

Yes indeed, but then you're carting around a heavy engine when you could be fine with a lower thrust and lighter one instead

[StrandedonEarth]

So to move the center of lift/pressure farther back, add fins at the bottom to counter the fairing at the top, just like feathers on an arrow. They don't even need to be active fins.

[Ice30]

Thanks for all the replies, I will try out your suggestions and let you know how I go after my exams this week

[draeath]

I got this to work. In my case the only steering I really had to do was below 200m/s, and after that I just had to touch it now and then to keep my inclination in check (antenna and KAS attachment thingy gave it a minor asymmetry). Past that 200m/s I only had to ride the throttle to keep the apoapsis about 50-60 seconds away.

Freakin' beautiful! Thank you for this information!

This technique actually reminds me of the technique used to change altitudes in large (757-like) aircraft - you don't touch the pitch, you use the throttle. If you throttle up, the plane will nose up and climb up until the point where it's in "balance" again, at which point it will naturally level off.

I imagine it works similarly because your angled rocket body forms an airfoil at those speeds, and as such it would behave like... airfoils do.

[HeadHunter67]

Thanks for the tips - I'm considering starting a new build with FAR/DRE and maybe TAC Life Support. It will help to know how not to kill all my Kerbals.

[draeath]

FAR+DRE are REALLY REALLY good to have. (grab "deadly-reentry continued") - TAC is decent as well though from the last time I touched it (still on .23, and been doing unmanned) it just did oxygen and carbon dioxide - no food, temperature etc.

[Ice30]

Okay so I have done a few tests and I realized that it's much more difficult to control your pitch with a heavy payload and the best way I found of controlling it is to look at the flight data on FAR which gives a decimal readout, then keep the change in pitch at a very low rate so you can pitch back when you are approaching the edge of the prograde marker. Additionally, having a fat and tall payload makes things even harder as the surface area of the payload is much greater than the surface area of your rocket once the main boosters are decoupled, so the payload needed to be made slimmer to increase the aerodynamic control. After learning all this I am saving about 900m/s delta v by launching my rockets this way instead of just going straight up and turning 90 degrees. Cheers everyone!