How to calculate a sub-orbital ballistic trajectory and build a proper rocket for it?

[Suseika]

Hi all, a newby here. I've been playing KSP dedicatively for 3 days, not a lot of experince but I'm loving the game.

I find it most challenging and interesting to fulfill those stupid difficult and cheap contracts that ask to do something extremely stu... specific in the atmosphere, like activating a huge engine made for the initial lifting at 20 km altitude. I've found a lot of great tutorials on KSP, but the topic of properly figuring out such low trajectory (max altitude, speed and landing point) was somehow omitted by tutorialmakers.

The atmospheric drag makes things very complicated. For example, on a "fly above this point" mission I tried ascending to 15 km and from there targeting the destination point, but the once atmosphere became dense enough I started to spin rapidly and got completely off-course.

So, can you provide the equations related to getting to a certain speed/altitude in the atmosphere, or just general tips on how to choose the right parts and control the vehicle?

[Bryce Ring]

Hi,

I like to do empirical testing in sandbox mode. In my case it may not be Empirical due to it being mostly testing with no Science behind it

Try Scotts videos on YouTube for great advice.

I'm not sure why you entered a spin. talk more of what parts you used and who was the pilot, this may help.

PS welcome, I'm am also new to KSP.

Edited January 17, 2015 by Bryce Ring

[Ival70]

That's great that you enjoy those contracts, I think most people just decline them due to the low reward. I've never really needed to use equations for them before. I usually just eyeball them (it's the kerbal way), or use space-planes, but that can take awhile to get a feel for. But now that you bring it up, I'd like to know as well. I think the problem is that the KSP aerodynamics model is unique, and I don't think equations for other models are going provide the level of precision your looking for (could definitely be wrong here). A quick search surprisingly didn't bring up anything, which I suppose explains why you made this thread. Hopefully someone reading this knows the equations offhand.

As for tips, well, one thing to keep in mind is to ensure that as your ship uses up all of it's fuel, the center of mass stays in front of the center of lift/drag, otherwise it will constantly want to flip around. Another way to combat this is to use an excessive amount of RCS or SAS torque.

In the meantime, if you are not opposed to mods, I know there are several that give you this information. Off the top of my head, these come to mind:

Mechjeb; one of it's many functions displays a marker designating where you will land, taking into account the rotation of the planet as well as the atmosphere.

http://forum.kerbalspaceprogram.com/threads/12384-PART-0-90-Anatid-Robotics-MuMech-MechJeb-Autopilot-v2-4-2

Atmospheric Trajectories; also provides a landing marker accounting for planet rotation and atmosphere, and it additionally provides a trajectory line in map view.

http://forum.kerbalspaceprogram.com/threads/104694

[UmbralRaptor]

Write a script to do a bunch of physics simulations, or a bunch of empirical testing. There are no analytic solutions once you consider the various drag effects. Although if you *want* a few differential equations that throw in all the terms...

[ghpstage]

Unless you really like differential equations I would advise trial and error!

A few tips.

For altitude record contracts, a single RT-10 Solid Rocket Booster (SRB) placed under a command pod with a parachute can clear the 5, 11, 22 and 33km records. You just need to tweak the fuel quantities to avoid hitting 11km on the first flight, and adjust the thrust limiter to get the rest. With a little A pair of SRB's can get you through the 56km record and reaching space contract with the limiter set low.

For parts tests, avoid using SRBs near the test altitudes as the high drag and inability to change thrust makes them difficult to control. Liquids with their in flight throttle are a lot easier to handle!

Additionally, the terminal velocity values in the wiki could be useful as they will sometimes line up perfectly with your test requirements. When they do you can overshoot the altitude and drop onto it to achieve your requirements easily, and sometimes at a lower cost.

For surveys, again liquid engines are much easier to control. Also useful to know is that 'above' an altitude, like your 15km, includes any altitude above it, meaning it can be done while in orbit.