Operation KISS - A pictorial guide on fuel consumption and not making massive rockets

[wildkittyv1]

Disclaimer:

I'm not stating that this is the most fuel efficient rocket you can possibly make, and it does have it flaws. This is simply a pictorial walk through of sending a 3 Kerbal pod to the Mun and back while having enough fuel leftover to make small corrections with the final stages and/or possibly make a Minmus trip/return if you're smart about your fuel (haven't tested it to Minmus). This thing is a little bit tight on fuel, but it has plenty to land and return. Also, since the new forums have a limit on photo embeds in the post, I'm just going to upload them to imgur as solo photos and embed a few of them. Let's get started!

I'm sure most of you know what KISS means, if not it's an acronym for Keep It Simple, Stupid. I see a lot of rockets around here and the spacecraft exchange which are massive and contain unnecessary amounts of boosters and liquid fuel tanks. Most of us who aren't engineers or astrophysicists (or just don't feel like working out the math) probably started out with these and have learned over time to make our rockets smaller and more simple. Now, I normally use mods but I wanted to do this with stock parts as an example. This ship has flaws and isn't the most efficient design (I suggest checking Scott Manley/Szyzyg's youtube for that - http://www.youtube.com/user/szyzyg?feature=watch), but it will demonstrate a rather simple rocket design that still looks kinda cool (and functional) and how I got it to the Mun and back.

Alright, here's the KISS Mk I just after liftoff. There's nothing spectacular about the design of this rocket - 6 large boosters attached radially, 3 each to stacks of 3 of the large liquid fuel tanks using the 1200 thrust 2m engines to start us off. Note that the interior LFE (liquid fuel engine) and SRBs (solid rocket boosters) are not lit for the first stage.

http://i.imgur.com/3pZtq.png - Here we have the boosters running out and detaching - currently at 2700m and going 124 m/s

http://i.imgur.com/hNwCx.png - The flaw with the rocket is seen here. Our velocity has dropped to 88 m/s after we dropped the boosters since they provide a massive amount of thrust. I'm sure I could've designed the staging differently, but I just kept it as is to show that even inefficient rockets are capable of getting to the Mun/Minmus and back with fuel to spare. Had I ignited the two boosters left during this stage, this lost in velocity wouldn't exist and the potential dV (delta V - ultimately how far your craft can go/return) is likely compounded by poor staging, but I am not a rocket scientist.

http://i.imgur.com/CUksQ.png - Igniting the interior LFEs and SRBs after ditching the burnt out ones. Our velocity is back up to 260 m/s here just after ignition. This stage, after dropping the SRBs, is going to get us to about 90% of LKO (Low-Kerbin Orbit). I have began tilting my rocket towards the horizon here to begin my gravity turn approximately 20 degrees off from being straight up. I do this maneuver after 10km because it just seems to have worked out for me. There are likely better methods, but this is when I begin.

http://i.imgur.com/oms6v.png - Here, we've ditched the last set of SRBs and are making a bigger dip towards the horizon, we're going 584 m/s at about 25km. My usual goal is to be pointing at 45 degrees off the horizon as I reach 35km altitude.

Here we are at 52km up and traveling 1240 m/s as provided by only our LFE after dropping the SRBs; I have just over one fuel tank left. I am tilted just at/slightly below the horizon because at this point, my AP (apoapsis - furthest point you will be from the body you are orbiting) is at around 90km and I don't want it going any higher. This, also, is likely causing me some loss in dV, but I won't be bringing this stage into orbit with me and will be dumping it so it falls to Kerbin as I don't want to leave any space debris behind. This stage of the rocket will get me to about 95% of an 100km x 80km LKO; I finish off the last 10% with my fuel sipping, low powered LFE.

http://i.imgur.com/CsT36.png - Here's the ship at the PE (periapsis - closest point to the body you are orbiting), around 83km, and completing the orbit - I've barely dipped into the tank.

http://i.imgur.com/wfIcw.png - Beginning of TMI (Trans-Munar Injection). The general idea without any math is to start the burn, full throttle, as the Mun rises over the horizon.

http://i.imgur.com/TzxjT.png - Completion of TMI. I've expended a good deal of the fuel in this tank, but we've still got plenty left to do what we have to, even if looks like we're running out. We don't need that much fuel once we get out to the Mun as long as we're smart about what we do. My Munar PE is around 50km, you can adjust this before you even arrive at your encounter by using small thrusts and watching the orbital map. Hover over the PE marker to see the altitude.

http://i.imgur.com/McJxo.png - Here we right around our PE after having closed our orbit by again firing retrograde. We've used a bit of the fuel we have left, but it's okay because we still have plenty to make our final few maneuvers. My altitude is at 58k because I began firing before exactly reaching the PE, but decided I'd land near one of my other bases, which is the small pink dot you see.

http://i.imgur.com/P6bze.png - On the opposite side of the Mun here after bringing our PE (situated above our desired LZ [landing zone]) down to about 10km. I bring it down this far because it takes just about the same amount of energy to slow down at 10km as it does at 200km altitude, but being closer to the surface of the Mun when we kill our horizontal velocity means that gravity affects us for a shorter period of time and we don't need to burn as much fuel to slow us down during descent.

http://i.imgur.com/74zCE.png - Back at our PE on the light side, roughly over our desired LZ after having expended all the fuel we have except for the lander/return vehicle. Note the KM height on the altimeter. Killing horizontal velocity is a key step in saving fuel and a successful landing. You only have to fight gravity on one vector and can concentrate on keeping your ship pointed relatively vertically rather than worrying about any lateral movement you may have as well.

Here, we've killed just about all of our horizontal velocity and have began descent. We're still traveling at 27 m/s, but a lot of that is vertical speed. We've used a bit of fuel in our lander, but it's okay since we're only going to be making two more burns during the descent and we'll still have plenty of fuel to get home. I'm not as close as I wanted to the other base, but I wasn't really aiming for a pinpoint landing with this trip.

http://i.imgur.com/ow1EN.png - Our first burn during descent. I started around 3500m altitude and burned at full throttle vertically to slow myself down to about 10 m/s descent. Notice how there's not a large difference between the above image's fuel level and this one - it didn't take much to slow down, as I was only traveling around 100 m/s towards the Mun when I began the burn.

http://i.imgur.com/bhnwz.png - Our second burn before slow descent. My velocity climbed to about 40 m/s, and I began this burn around 2200m altitude. Burning at full throttle is (supposed to be if there was no fuel bug) more efficient than a slow burn. It takes practice and experience to know how much a certain engine can slow you down and in what amount of time to maximize this, but it's how you want to do it. It's more nerve-racking, but it's something you should eventually learn so you don't need tons of extra fuel and massive rockets, which leads to bigger rockets since you need more SAS and more fuel to lift them.

Landing! Look at how much fuel we have left. If you compare the fuel levels between this image and the one from just after killing horizontal velocity above, you can see that there is hardly a difference. I hardly used any fuel to land myself because I didn't have to fight gravity as much as if I was coming in at a steeper angle or from a higher altitude. You can increase this effect by bringing the PE even closer to something like 5km over your target LZ before beginning descent.

Now let's get these guys home.

Continued below

Edited August 11, 2012 by wildkittyv1

[wildkittyv1]

http://i.imgur.com/rCxfH.png - Here we are after Munar liftoff and having traveled a bit of distance both equatorially and vertically. I'm burning along the horizon because I don't need to be any higher; you can orbit the Mun at 3km up if you wanted to because the gravity is so low.

http://i.imgur.com/hkcVo.png - Here, I've completed my TKI (Trans-Kerbin Injection) and haven't used much more fuel. At this point, we've reached a velocity where we will escape the Mun's gravity and begin to orbit Kerbin, with our Kerbin PE at around 2.5 million meters and our AP is around 10 million.

http://i.imgur.com/Rl0FI.png - At this point, I've fired retrograde during my Kerbin orbit and have killed enough momentum to ensure I will fall into its atmosphere and land. Look at how much fuel is left in the lander compared to the image from where we began our Munar descent. We used maybe half a tank of fuel to land on the Mun safely, liftoff, escape its gravity, and slow us down enough to land on Kerbin. You don't need a massive lander with a crap ton of fuel to get yourself back home safe and sound as long as you're efficient with it.

Splashdown. The chute didn't quite slow me enough, so the engine and landing gear broke off, but we landed! Had I attached another decoupler so it was just the command pod this wouldn't have been an issue, but this was really my first experiment with the stock parts in .16 since I'm not a fan of their look and size relative to mod parts; I figured the chute could slow a near empty fuel tank and the pod without anything breaking, but I was wrong.

Anyway, that's the end of this little guide and I hope it helped some of you who are tired of building massive rockets to get you to the Mun/Minmus and back home. It's more about your technique than the amount of fuel you bring, and even this ship wasn't optimized with its staging. I could upload the craft if you want to give it a shot, just let me know, but I'll have to make some changes which may affect its performance as my landing vehicle had no ASAS and there was no RCS on the whole ship.

Cheers

Edited August 11, 2012 by wildkittyv1

[pebble_garden]

This is an excellent guide, very well written and illustrated. Thanks so much!

[UmbralRaptor]

I like the use of vectored thrust for control authority. Did you have any issues with uncontrolled rolls?

[wildkittyv1]

I like the use of vectored thrust for control authority. Did you have any issues with uncontrolled rolls?

I only started using the horizontal velocity killing technique a few weeks ago after I was messing around in orbit and trying to figure out the best way to do a precision landing. I was all up in orbit and realized I had messed up and would undershoot my target, so I fired prograde again and it just dawned on me that I could easily control my LZ by how I fired. This lead to killing horiz. velocity and that eventually lead to realizing that if I was closer to the body I wanted to land at, I'd have to fight gravity less; pretty standard progression through logic for someone not entirely oriented with orbital mechanics.

Anyway, as far as this particular craft is concerned, it was a little janky landing it without any SAS at all. I normally use the tiny ASAS module from Kosmos, but wanted to do this entirely stock for demonstration purposes, and I absolutely hate the way the 1m SAS looks and even the ghetto trick of putting the parachute off-center so you can stack on top of it to hide it looks pretty funky to me. I wasn't really spinning uncontrollably, but it was a bit of a space rodeo since it would twist twice as far as I intended as I got down below 50m from the surface while chasing the retrograde symbol. I actually wasted a bunch of fuel because the 2m lander engine is a bit too strong for this style of lander and "bounced" around a bit, which would mean having to aim almost parallel to the Munar surface to kill that velocity, and then try to come down smooth for another landing attempt. I think I got down to about 5 m from the surface like 3 times before finally landing.

I hardly ever use RCS on my landers anymore as I find if you've done the stages prior to descent correctly you don't really need it, but not having RCS or any kind of SAS made this thing a bit of a wild stallion. If I wasn't rotating the lander, it was pretty stable.

[Caroliano]

Great tutorial! I will try this efficient way to land in the moon, managing the periapsis, latter.

You should make the three bottom liquid fuel rockets fire since the start, and add a fuel cross-feed to keep the central tank full when you eject the two others. This should increase quite a bit the efficiency, and lower the gravity loses. I made a similar rocket w/o the solid boosters, kinda like the Falcon Heavy, that can lift an heavier lunar lander than this.

Edited August 13, 2012 by Caroliano
side -> solid

[Panichio]

Nice read! Thanks

I shall revisit my 'Colossal-Titan-Megathrust-Superlifter XVIII'

[wildkittyv1]

Great tutorial! I will try this efficient way to land in the moon, managing the periapsis, latter.

You should make the three bottom liquid fuel rockets fire since the start, and add a fuel cross-feed to keep the central tank full when you eject the two others. This should increase quite a bit the efficiency, and lower the gravity loses. I made a similar rocket w/o the solid boosters, kinda like the Falcon Heavy, that can lift an heavier lunar lander than this.

Yeah, I sort of did it on purpose that way so I could show that even though this is small and inefficient, it still manages to get to the Mun and back with some fuel leftover. I see a lot of posts from people making massive rockets that can't get them where they want to go and they wonder why. It can be fun to build big, but I just threw this together so people would see how fuel gets used during the course of a trip and how little you actually need to get to the Mun, land, and return.

[Caroliano]

Right. The flight tutorial was indeed very good! But I think you should teach about the marvelous of fuel cross-feed too, because it really helps to make smaller and more efficient rockets.

I ended up making a post with my rocket (look at the second post for it): http://kerbalspaceprogram.com/forum/showthread.php/16963-0-16-stock-Falcon-9-%28satelite-launcher%29-and-Falcon-Heavy-%28Minmus-Moon-trip%29

It uses more liquid fuel than yours, but no solid booster (thus is simpler and has less stages), and can power an entire Kerbal -> Minmus -> Mun -> Kerbal trip (image gallery on the link too).

[Felsmak]

That rocket looks good, but I like my rockets with a designated ascent and descent stage for the lander. I like to leave something behind when I leave, you know, as a memory.

[Tigga]

How fast are you re-entering Kerbin's atmosphere? I've a feeling ships might end up a little bigger once re-entry heat is simulated as at the moment you save quite a bit of fuel by not having to do any work to keep your entry velocity low. You can have your apoapsis pretty much wherever you want but so long as the periapsis is <30km or so you'll touchdown safely. Having to shove all the extra fuel onto the final stage increases the lower stages by an annoying amount!

Incidentally I find I can't use ASAS combined with a three man lander either. Just can't get it to look any good. I can sometimes squeeze it in lower down the stack without looking too bad, but I'm really hoping for increased radius versions in .17.

[wildkittyv1]

Caroliano - I could redo it with fuel crossfeeding, but this was really just meant to show the level of fuel consumption after getting into orbit and showing how little you actually need (and my rocket is still oversized - if I had done the fuel crossfeed you could probably get away with not having the interior SRBs and 1/3 of the exterior ones I lit on ignition. I've done pure LFE rockets before but it just feels wrong if I don't have some SRBs on it. I love me some SRBs and am working on a rocket that can land and return from the Mun using nothing but SRBs and timing the burns correctly.

Unsinn - That's fine. I'm not a fan of leaving space debris and I consider landing stages to be that. I have a few designs that leave the landing stages behind but those are purpose built as rescue vehicles and I went with a completely different design philosophy for them. Even if I have leftover fuel in a landing stage I won't go back and redesign. I'll decouple the extra fuel tank about 1km before landing so it has a chance to pickup some speed and explode on Munar contact.

Tigga - for this return I think I slowed down to like 2km/s with the remaining fuel and let the atmosphere take over after that. I'm not sure how reentry is going to work once we have heat shields and the like, but I'm sure it's not going to be overly complex as it seems pretty clear that Squad plans on keeping the game accessible. We'll just have to change stuff as we need to. As for the ASAS thing, I hardly ever build stock rockets anymore since I feel the parts are limiting and design capabilities plateaus pretty quickly with them. I normally use the tiny ASAS thing from Kosmos and the parachute from NovaPunch that lets you stack stuff on it. I know they're not stock parts, but I'm not a purist and stock parts are too limiting in their size and capabilities in my opinion. If I was going to redesign this so I could have ASAS during the landing, I'd have just put it between a set of decouplers from the Munar injection/orbital slowdown stage and the ascent/return stage and dumped it like 50m up from the Munar surface. It would've looked even weirder, but it would've given me a bit more control on the way down.