Kerbal Space Program

[NeonSamurai]

Originally Posted by Gargamel

Returning from orbit is easy. Burn retro till your orbit drops below like 50k. Then eventually you land. Some practice runs will add precision.

Umm... burning retrograde will have you end up back on the mun (or waste a ton of fuel flipping your orbit around 180 degrees)

I think you mean burn prograde against the mun's orbit (ie burn in the direction opposite from the mun's orbital path).

[Gargamel]

Originally Posted by NeonSamurai

Umm... burning retrograde will have you end up back on the mun (or waste a ton of fuel flipping your orbit around 180 degrees)

I think you mean burn prograde against the mun's orbit (ie burn in the direction opposite from the mun's orbital path).

Err... ummm... I must have missed a good portion of the discussion. I thought he was having trouble landing from LKO.

[Red October1984]

Originally Posted by Gargamel

Err... ummm... I must have missed a good portion of the discussion. I thought he was having trouble landing from LKO.

It was the LKO thing....not the Mun yet. I don't have a rocket designed that will make it to the Mun and back yet.

[NeonSamurai]

Ah I got it wrong then I could have sworn we were talking about the mun and back.

Ya returning from orbit is exactly as Gargamel stated

[Red October1984]

Originally Posted by NeonSamurai

Ah I got it wrong then I could have sworn we were talking about the mun and back.

Ya returning from orbit is exactly as Gargamel stated

I've done orbits of Kerbin several times now....

Need to work on doing a fly by of the Mun.

[Gargamel]

Honestly, if you're hell bent on learning to do this by hand..... go get mechjeb.


Wait wait wait.... Yes that sounds counterintuitive.

But you can learn far more by watching MJ fly your ship, than you could from watching a video of someone else fly not-your ship. Ok yes, their video would explain more theory, but MJ using manuever nodes will teach you how it does what it does.

So Quicksave your flight, let MJ do that Hman transfer to wherever it is you want to go, then quick-load, and do it yourself.

Eventually you'll learn how to do all the advanced stuff intuitively. But watch a lot of the tutorial videos out there. Learning the theory behind orbital mechanics is 90% of the battle.

[NeonSamurai]

I'm not sure that would work, as last I heard he did not have maneuver nodes unlocked yet.

Also FYI Mechjeb does a lot of things wrong (or not at all optimally), and can teach many bad habits because of this.

Anyhow crash course to going to the mun:
Launch into a 90 degrees east circular orbit, 80-100km up. Fly prograde when the mun rises above kerbin's horizon, watch for an interception with the mun on the map and cut your engines when you have an intercept. About half way to the mun, make a course correction so that you will fly around the mun about 20-30km up. For a fly by you want to decelerate as your craft swings round the moon just enough to send the craft back to kerbin, but not enough to form an orbit around the mun. Once out of the mun's sphere of influence make another course correction to achieve an aerobraking capture. You want the orbit to hook around kerbin, with a periapse of about 20-40km depending on how quickly you are coming in.

[Gargamel]

Yeah, thats a good how to. I did that once lol.

I forget people play career. I'm almost a pure sandboxer. I've been playing the latest dev builds of MJ, and there's some very sweet features they are doing a lot better than before. There's also quite a few that it's failing on. My latest version doesn't seem to do landings in atmospheres very accurately. And the rendezvous autopilot is a pain.

But the porkchop (what the heck is that?) for interplanetary transfers is amazing. I can pick the exact amount if Dv or time I want to expend to transfer. And the ascent guidance is really nice. By limiting acceleration to 20-25 m/ss, I can extend what was initially computed at about 3k dv into about 3.5k dv, enough to get to orbit.

[NeonSamurai]

Porkchop plot
http://mars.jpl.nasa.gov/spotlight/porkchopAll.html

Is MJ doing the porkchop plot or are you using a different mod like this one https://kerbalstuff.com/mod/268/Tran...ndow%20Planner
Or just the old website one.

MJ tends not to be so great for a number of things (at least when I last used it which was several versions ago). It tends to be good to idiotic with auto landings, its control inputs are super twitchy (all or nothing control inputs) which can be a huge problem for launches in realistic atmospheres, and it can be down right terrible at plotting intercepts. Worst was for planetary intercepts where the program would sometimes decide to wait for a few game years to make the transfer.

But maybe some of this has changed as it has been a while since I used it last.

[Gargamel]

Cause it looks like a porkchop....... duh. LOL.


Oh yeah, it's launch guidance is pretty darn good. You just need to turn off a few of the options that you think you need (like corrective steering). And like I said, if you limit your thrust to about a twr of 2, then it will really extend the dv of a vehicle.

[NeonSamurai]

Maybe it has been improved then. I'm still pretty out of date with KSP as I'm still trying to finish my interstellar quest game from 23.5, so I wouldn't know how well MJ performs in 1.04.

In 23.5 launch guidance was pretty horrible using a realistic atmosphere (FAR) as its mid flight course corrections were very sharp and would frequently cause my rockets to flip out of control (even with fins and gimbaling engines). Turning off corrective steering didn't help a whole lot either as MJ's control imputs were still mostly all or nothing. The only thing that did help was carefully editing the ascent path by hand, which I had to do anyways as the default profile was too aggressive and inefficient in FAR anyhow. Limiting TWR didn't work in FAR either (and probably still doesn't) as MJ couldn't properly read it when using FAR. The reason MJ got away with the sloppy aggressive controls, was because stock KSP's atmosphere was like flying through sludge.

As for thrust limiting, personaly I don't do it using throttle control (MJ or by hand), I do it in the VAB (kind of in the same manner as real rockets, as most rocket engines do not have much if any in the way of throttle control). I intentionally set the engines (or add enough fuel/payload) so that the rocket's starting TWR is about 1.1-1.4 (including solid fuel boosters) depending on how aerodynamic it is. It works really well in a more realistic atmosphere as it keeps the rocket slower in the lower thicker atmosphere, but the TWR picks up as the rocket expends fuel and stages, so that in the upper atmosphere (35k+) when I am accelerating near horizontally, to orbital speeds, my TWR is typically 2.0 or higher.

This is also in line with most real space craft designs. Most real life rockets typically have a launch thrust to weight ratio of 1.05 to 1.3 (1.2 is the average) with a full payload, and an upper atmosphere (~60km+) TWR of around 2-3 (or higher if unmanned as going beyond 2.5 gets very uncomfortable because of the g forces).

I've often seen on the KSP forums people claiming that the ideal thrust to weight ratio for terminal velocity is 2 in ksp. They are wrong. The reason why, is that the value for terminal velocity is not a static number and depends highly on the aerodynamic profile of the rocket. A rocket with high aerodynamic resistance would have a much lower terminal velocity value compared to a rocket with low aerodynamic resistance.

[Gargamel]

Yeah, I was in the same boat, had a bunch of .23 craft I was having too much fun with. 1.0.2 was my first upgrade since.

In the old stock aero, MJ was great, it would limit the throttle to only travel at terminal velocity, as to not waste fuel. When using a liquid core with boosters, it would sometimes turn off the main engine until the boosters died off, which was great for extending dv.

In 1.0.2 (haven't upgraded to .4 yet), MJ has issues with the aero, so if I limit it to about a TWR of 2 until booster seperation, it has the same effect. The controls are much more subtle though, and there is an AOA limiter function. Depending on the build, I'll have that anywhere from 5'-12'. Mix that with a good ascent profile (I like about 35%), starting my gravity turn at 50 m/s so the fins will have good control, I can limit the orbital insertion burn to about 100-300dv.

I've also found the new aero makes planes feel more realistic (as realistic as KSP could be called). Canards are a rarity on my planes anymore, unless I want high maneuverability.

[NeonSamurai]

It's actually a good idea to manually limit your SRBs thrust from the VAB as they can push craft past terminal velocity. I also find it more fun to try to build like real rocket scientists do, which is why I manually thrust limit everything; even going so far as figuring out the TWR values for each stage and configuring them appropriately based on their purpose.

I think it was possible even in the older MJ versions to AoA limit. But that doesn't help much when the problem is a lack of control dampening. In FAR it is very easy to get into an unstable flight profile if you are too aggressive with the controls. MJ did (does?) not have any kind of control dampening so that it doesn't fling the controls around, as it is that kind of behavior that can quickly have your rocket flipping end over end in flight.

My gravity turn starts almost immediately when using FAR and MJ. As I posted before, I found this worked really well in FAR. The reason is that the gravity turn is very smooth and gradual, it also slowly flattens out as the rocket climes into the upper atmosphere, where rockets can become very unstable in FAR due to the decreasing effectiveness of fins. This profile is also one of the most DV efficient profiles I have found for MJ in FAR that is also very stable too.
Turn Start Altitude: 0
Turn End Altitude: 60
Final Flight Path Angle: 1
Turn Shape: 75% (I sometimes tweak this up to +/- 10 depending on the craft)

Personally I'll be sticking with FAR when I do change versions. I like the increased design complexity and general difficulty. If you wanted to up your game, you should give it a try sometime. It's a whole different experience.


So Red October1984, did you take a crack at the Mun flyby yet?

[Gargamel]

Hmmmm... I'm going to try that, limiting my SRB's. While keeping the throttle limited via max accel, that will really increase my effeciency. I'm assuming reducing thrust on a SRB reduces fuel consumption?

[NeonSamurai]

Yes it does, the SRBs will burn for a longer period of time when thrust limited. You need Kerbal Engineer to help estimate your total TWR as the thrust limit setting is percentage based. You can find it in by right clicking the SRB on the craft in the VAB. Make sure to check each SRB setting. Also keep in mind that the TWR will increase as the srb's burn their fuel.