Kerbal Space Program

Struts are your friend. Its basically impossibly to build a large rocket without them. The games' physics engine only calculates stuff like forces and torques around a single connection point between parts (they did this in order to keep the math the game was doing down). The consequence is that any small torque is going to cause things to wobble, which will get more pronounced on larger heavier objects. I always connect all my fuel tanks with a couple of struts - this increases the number of connections and therefore the physics engine dampens down the movement a great deal.
 
So...the rocket I designed yesterday didn't fare so well. First time I've had SRBs explode on me. I don't think the game liked me trying to mix the LV-T30s and LV-T45s in the one stage. Lander section probably would've worked had I been on the Mun; didn't do so well trying to land on Kerbin. Luckily I made that part detachable and was able to get the drogue chute deployed in time; ol' Jeb walked away from it okay.

Redesigned the rocket with T45s alone and was doing well until I switched over to the orbital map to calculate the orbit burn cutoff point. I switched back when I noticed the apoapsis falling off rapidly; most of my freakin' rocket had disappeared. I think it was the Kraken...

Third and final launch attempt of the night didn't make a sustainable orbit; ran out of gas in the third stage with a periapsis of only 40k. This was the engine I had intended to make the mun transfer burn with. I think I may have shot for a LKO that was too high.

Still got a mean wobble going on at takeoff. I've got the tanks connected with struts at this point; turns out you can't do the same with SRBs.

Going back to the drawing board this morning. Gonna see if I can get a rocket that has the same capabilities but isn't quite so tall; from what I've read, that's probably a big part of the problems I'm having.
 
Made it. Sorta...

made_it_sorta.png


Yes, those are my landing struts laying off to the side, right next to the smouldering wreckage of my descent/return stage. Next thing to do, obviously, is to learn how to land...
 
Attempt number two: got my Mun periapsis too low and didn't have enough time to correct it. Plowed into the Mun at over 300 mps. Jeb wasn't as lucky this time...

At least the rocket design works. Still wondering if there's anything I can do to improve its maneuverability.
 
Since the mun has no atmosphere, you're gonna need to execute some sort of breaking burn while still in orbit of it to control your descent. You want to probably be at about 30,000 meters and then fire retrograde until your course line intersects the mun where you want to land. Then as you descend, burn to keep your speed under control - you want to stay under 100 m/s once you pass through 10,000 meters. Once you appear to be under 500m from the ground (remember, your altimeter shows absolute altitude, not terrain, so unless you have one of the mod parts that acts as a radar altimeter, you need to sort of wing it), increase your deceleration until you are going no faster than 8-10 m/s for the last 100 meters of the descent. Right before touchdown, give it a little gas so you come down as soft as possible; in the range of 1-3 m/s is good. Anything more than 10 and you're probably going to break something on touchdown.
 
Yeah, that was the problem last time - lowered my periapsis to 2000 m. Not smart when there are parts of the Mun with elevations to 1500. And I'm still just flying the demo - no radar altimeters to help.

It'd be nice if there was a tutorial in the demo that would let you practice Mun landing...well, I guess there is; it's the "To the Mun - Part 1" training scenario. It just goes over what you need to know to get in orbit around the Mun, though; it doesn't give you any pointers on actually landing. Is there a tutorial that covers that topic in the full version?

I've heard that you can take off from the Mun and make it back to Kerbin solely on RCS thrusters...

Also tried my hand at the Impending Impact scenario last night. Avoided the Mun but botched the angle of the gravity assist and ran out of gas with my Kerbin periapsis at about 2 million...
 
Hey, is there any way to tell if you've killed your lateral motion or not? Last two times that's been what broke my ship. 0/4 on landing now but I still have a 75% survival rate.

I also seem to have shut shadows off; I should probably turn those back on on account of that's the only real way you have of figuring out just how high up you still are. I'm still convinced I could've made that last landing if I'd been able to tell where I was in relation to the ground.
 
That's one small step for geek...actually, strike that; that freakin' ladder didn't go all the way to the ground......

screenshot5.png


Only took me seven tries to get it right.

Not a perfect landing, either; I still had to upright the capsule, but at least I DID land without anything breaking.
 
I was just glad I was able to do that one honestly; I had already come to the decision that if I mucked up another landing I was going to start savescumming until I finally got it right. Didn't have to.

Now to see if I can do it again...
 
Well, got to the Mun again. Another mucked up landing - broke off two landing struts and an RCS block, but got it down safe. The busted block proved fatal; I couldn't keep control even with RCS shut off, went on a wild trajectory on launch, was able to get to lunar orbit but didn't have enough fuel left for the return trip.

I may be starting my braking maneuver too soon and burning up too much fuel there. What's a good altitude at which to start?
 
You want to aim for an intercept periapsis with the Mun of about 50 km. Start your braking burn right before you hit periapsis. You should have to expend a delta V of about 300 m/s to get into orbit of the moon. Any orbit works - even a very hyperbolic one can be brought down near the surface with a couple correction burns later.
 
No, I mean on the actual descent; I've got what looks like a pretty typical descent setup (FT-400/LV-909). I usually close to my final parking orbit with my third stage engine and jettison it right after I've begun the final approach. I've been making the burn to begin slowing my lateral velocity and taking me down to the surface at 8,000 m. By 4,500, I've got almost no lateral velocity and my descent is down to a crawl - I can sit there and burn up most of the fuel in the tank. What I'm asking is if I should I be starting later (like around 5,000 m)?

Also figured I should calculate the safe fuel reserve level - I haven't done that yet but that shouldn't be hard to figure out.

EDIT: Okay...get a delta-V map of the Kerbin system...the latest one's only about a month old or so. Okay...it says you need 640 to launch from the Mun, 210 to orbit and 860 to get back to Kerbin. You can aerobrake to land on Kerbin; I've done that before no problem. So that's 1710 m/s of delta-V to get back. Divide that by the minimum Isp of the LV-909 = 300, divide by 9.81 m/s... take the inverse natural logarithm... 1.787896 is the ratio of "full mass" to "dry mass". Invert that... 0.559. There are 180 units of fuel in the tank total, so multiply that by 180...100.677. So that's the amount of fuel that can be used up safely and still have enough to return to Kerbin. So subtract that from 180...79.32 is the point of no-return. If you add a 5% fuel reserve for screwups (9 units), you get 88.32, which meshes with my observations so far.
 
Ah, okay, sorry I misunderstood that. For your descent, its a bit of a guessing game. I would actually suggest quicksaving before you start descending, and find an altitude where you can start a short, full power burn that will have you slowed right before the surface, and then you can tweak it from there. That's how you save the most fuel.
 
Right. I've tried 5000 twice; worked once, had to abort the second one (proved my reserve fuel equations were right, that's for sure). It really is a matter of knowing where the deck is, isn't it?

I've made 10 moonshots now; have successfully landed 3 times and have returned the pilot to Kerbin twice. Last trip was an abort but I got Jeb back.

A couple of screenshots:

screenshot27.png

This is the mighty Fireball 7 (also known as the Flaming Death Trap 7). 8300 m/s of delta-V in the design with a first stage TWR of 2.3. I did switch the positions of the RCS fuel tank and SAS module to below the SM stack decoupler after I took this picture; got tired of the capsule pitching over after it splashed down...

screenshot26.png

Jeb looking at Kerbin. I hit the deck at a mere 3 m/s and still managed to break off a strut.

screenshot23.png

Splashdown at sunrise. Makes me wish I had more firepower in the graphics department.
 
So....I learned today that there's a radar altimeter in every capsule in the current version. You'd think they'd put an important gauge like that somewhere on the staging screen; I know tonight it saved my bacon (started at 4000; deck was at 2000 - if I hadn't known about the altimeter I'd've plowed in at over 200 again). You have to go IVA (C-key) to get to it and it only works during the last 1000m, but it is extremely useful.

3/12 now on getting to the moon, putting a guy on it and getting him back safely. 4 successful landings now and one abort. Success rate is going up.

Was disappointed to find a flaw in my delta-V calculations for the Fireball 7 tonight; she only has 7,563 m/s and the first stage TWR is 1.71. I had discounted the mass of the SM/LM portion of my rocket on the third stage. She still gets there and back just fine, it's just that I don't have as much margin for error as I thought I did.
 
You should be damned impressed with what you've done so far. I have several mods on mine, but I'm working on building another Munar mission - complete with rover. I'll post the results.
 
Broke down and bought the full version. I was just having too much fun with the demo.

Had my first fatality on Kerbin re-entry yesterday. Linkage broke between the chute and the capsule. Only difference between the design of what I have been flying and last night was an ASAS as opposed to a SAS. I don't like ASAS...

Trying to decide what to do next. I've got Mun landings down at this point. Even landed withiut tipping over last night.
 
capi3101 said:
No, I mean on the actual descent; I've got what looks like a pretty typical descent setup (FT-400/LV-909). I usually close to my final parking orbit with my third stage engine and jettison it right after I've begun the final approach. I've been making the burn to begin slowing my lateral velocity and taking me down to the surface at 8,000 m. By 4,500, I've got almost no lateral velocity and my descent is down to a crawl - I can sit there and burn up most of the fuel in the tank. What I'm asking is if I should I be starting later (like around 5,000 m)?
Click to expand...

You know, I took engineering-level physics in college, you'd think I'd have realized a lot sooner that it'd all be dependent on the mass of the craft ergo Newton's second law. Mass ain't a constant in this case because of how rockets work, of course, but it won't change that much over the course of a burn, will it?

5,000 is a good altitude to begin the descent burn from for my little Fireball 7 lander (which is 4.67 tons fully loaded); with my Apocalypse 7/Fiery Death Trap 7 landers (7.16 tons/6.97 tons fully loaded, respectively), 5,000 is essentially a suicide burn and that's been borne out in their performance; I've smacked the moon hard twice and have had to pull up the remaining times.

There is a way to figure this out. Unfortunately, I smell a diff-eq...

EDIT: Actually, looking at the numbers......I have a hunch that I should begin the descent burns at 7,000 for the Apocalypse/Death Trap.

EDIT: 7,000 worked out pretty well.
 
You must log in or register to reply here.
Back
Top