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Super-micro Eve lander, and its development process


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Motivation

For a long time, I've been interested in ultra low mass missions, and even flown a few myself. It's incredibly satisfying to iterate on a design and watch it get smaller and smaller, often beyond what I believed was possible originally.

One of the most difficult challenges in stock KSP is getting a Kerbal from Eve's surface to a low Eve orbit. The high gravity, dense and unoxygenated atmosphere, and large planetary size all combine to make Eve more difficult than any other planet (with the possible exception of Jool, which doesn't really have a surface). Low mass Eve missions are very interesting - Bradley Whistance's 9 ton Eve mission was a big influence in how I play KSP.

The Design Process

At the beginning, I stuck to a few design principles common in low mass craft:

-The craft would be controlled through a command seat. The command chair is much lighter than any command pod - not only does this reduce mass of the craft directly, it also requires less propellant to launch into orbit. However, it's not very aerodynamic, which is a bad thing when dealing with Eve's atmosphere. Fortunately, a command chair in a fairing is still much lighter than a command pod, plus the fairing can shield fuel tanks and engines from drag. So I also decided to use a fairing on the lander.

-I would use a propeller to get above the majority of the atmosphere. Not only does the atmosphere present issues with drag, it also reduces the efficiency of my engines. With a propeller, not only do I incur less drag, I also start burning the rocket engines from a higher altitude (meaning I have more energy and thus use less delta-v), and the burn can be done at a higher efficiency. Note that propellers aren't the best for Kerbin because jet engines are lighter and can give a bigger speed boost, but jet engines don't work on Eve.

-I also want to use the EVA pack to finish circularization. The EVA pack with the extra fuel cylinder is about 900 m/s of delta-v, which is too much to pass up and more than makes up for the additional EVA propellant mass by lowering the fuel requirement.

I've divided the various iterations of the lander design into categories so there isn't a big text wall. The initial lander design is 5 tons and gets improved from there.

Category 1: Ducted Fans

Spoiler

9xGp6jR.png

Iteration 1: 5.0 tons.

This was my first attempt at a small Eve lander. A small ducted fan brings me above most of the atmosphere, a cluster of Spark and Twitch engines puts me into space, and a two stage spark powered chair finishes orbit. I didn't use the jetpack at this point, and there's a lot of other things that don't really make sense / aren't optimized, but this is still a decently small lander.

IeKUi3a.png

Iteration 2: 4.8 tons.

I add wings and control surfaces to help with landing. Instead of the last drop tank, I begin to use the EVA jetpack. Since mass is reduced, I can also remove the Spark engines on the first stage. I remove the solar panels as well, since ascent is quick enough to do on just battery.

oxmAIiI.png

Iteration 5: 4.4 tons. I didn't include the intermediate versions because they're not really interesting, and because for a while I was trying to optimize for minimum cost instead of mass.

I switch to a Twitch instead of Spark engine for the upper stage, and reduce the fuel because I had sufficient margin. I also remove the service bay and change the ducted fan blades to a more mass efficient option. The large parachutes are replaced with drogue chutes, with the remainder of braking coming from autorotation of the ducted fan. I also reduce battery capacity and wing area since they weren't needed.

O98eDVM.png

Iteration 8: 3.3 tons. The previous couple versions were variations on this theme.

I realized that I don't actually need a reaction wheel on the top stage. Instead, I can get a lot lower mass by using an Oscar tank instead of a Baguette, and using Spider engines for control through their engine gimbal. This lets me reduce the fuel load of the first stage. The reduced mass of the rocket means I can remove a Twitch engine and still be at a good TWR.

At this point the mass of the Eve lander is 3.31 tons. This is a huge improvement, but we can do better still.

Category 2: Basic Fins

Spoiler

eXpPMfM.png

Iteration 11: 2.5 tons.

I replaced the ducted fans with basic fins, which don't drop off their lift as much at transonic speeds. This means I can spin the props faster and generate more lift, letting me climb higher. The fins also weigh much less than the fan blades.

The higher burn start means I don't need as much fuel on the rocket stage, and I can take off another Twitch engine because I don't need the thrust.

yUi3Oep.png

Iteration 12: 2.3 tons. 

I remove the parachutes and wings, because I can land vertically with just the propellers. One thing I didn't realize was that I needed to add solar panels, since the basic fins were a lot slower on the ascent and the batteries wouldn't last long enough. I had been testing with infinite electricity enabled.

QjKlH6U.png

Iteration 13: 2.1 tons. This is the version of the Eve lander that was used in both my low mass Eve mission and grand tour - I didn't optimize it at all between those points (and for a while after).

I remove some fuel on the lower stage, so I can remove another Twitch engine also. This is too little thrust, so I fire the Ant and Spider engines at the same time for a bit extra. For some reason, KSP thinks the thrust of those engines is never blocked by anything, just like the ion engine.

Category 3: Basic Fins, Part 2

Spoiler

KSEX5Ai.png

Iteration 14: 2021 kg.

I don't make any changes to the rocket here, but I increase the prop diameter and rearrange the blades. This lets me go higher and reach Eve orbit with more margin remaining. I also switch out the solar panels for lighter solar panels.

HWalZht.png

Iteration 15: 1897 kg.

I remove a fuel tank and don't change anything else about the rocket. I did find a bit better of an ascent profile here - instead of going all the way out of the atmosphere on the first stage, I focus a bit more on building up horizontal speed and then fire the second stage within the upper layer of Eve's atmosphere for a bit better efficiency (drag is minimal at 80+ km)

eupISPq.png

Iteration 17: 1796 kg.

I switch a fuel tank for a smaller one due to the improved ascent profile.

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Iteration 18: 1735 kg.

I remove the upper stage engines and move one of the lower stage engines onto it. This hurts TWR and delta-v, but simply eats into the margin the previous design had. Iteration 19 is a version of this with a very slightly heavier fairing that occludes all three engines, which has more margin on orbit at the cost of a bit of mass.

Category 4: Fairingless

Spoiler

This one actually came as a bit of a surprise to me. I hinted at it earlier by building up speed with an unshielded chair rocket inside Eve's atmosphere, and again by finding that it was a bit better to leave the engines exposed to drag instead of having a large fairing, but didn't connect the dots.

Thanks to ZG Alpaca and Ultimate Steve for the idea.

elUXbNn.png

Iteration 20: 1384 kg.

I keep the same engines but remove some fuel tanks now that I don't have to haul the fairing around. I also have the kerbal sit upright so it produces less drag. This was a big drop in mass!

nmYA8u5.png

Iteration 21: 1347 kg. 

I remove one of the solar panels as I really only need one to power the propellers. I swap out the Twitch engines for a Spark engine because I don't need the extra TWR here, and add slightly more fuel to compensate.

Egn6daX.png

Iteration 22: 1216 kg. 

I realized here that my previous two stage design was really only because I had a fairing, and now that I don't have it, it's more efficient to drop engines and fuel tanks whenever I don't need them. I made the lightest upper stage possible, consisting of just one spider engine and dumpling tank. The next stage is one oscar tank and a twitch engine, followed by a donut tank and another twitch engine. This keeps TWR reasonable and dry mass at a minimum at all times, while also dropping the draggiest parts first. (Drag is still important here, even at the 40+ km altitude where the propellers top out. It's just not enough to warrant a fairing anymore).

LkOybC0.png

Iteration 23: 1156 kg.

This is the same as the previous rocket with fuel margins tweaked. I don't like using partially empty fuel tanks, and there's a lot of dry mass on the table.

resnpPO.png

Iteration 24: 1126 kg.

This is the final version (For now). I swap the Donut tank with two Dumpling tanks, because the reduced fuel load and the reduced dry mass cancel out nicely. This design has a little bit of EVA propellant remaining in orbit, but I can't think of any way to take advantage of this without messing with partially loaded fuel tanks, which I don't really like.

The final result

Spoiler

1126 kg is way less than I ever expected was possible for an Eve lander. As far as I can tell, this is actually the lowest mass ever for a stock Eve lander.

I don't use any "magic wings" or other aerodynamic exploits here, but it still beats anything that does. I also don't take advantage of EVA construction, which could save a bit of mass and drag.

I was happy enough about this final result that I made a video about it:

 

 

 

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37 minutes ago, Nazalassa said:

What's the module under the upper dumpling?

It's a Mechjeb unit because I'm not good enough of a pilot to fly this craft consistently - the ascent profile needs to be very precise with a lander this small. It doesn't really provide a benefit to the craft otherwise, in fact it just adds drag, but it's a good band-aid solution for my lack of piloting skill.

I also use Mechjeb for aero and TWR info in a nice compact display, so it's not just used as an autopilot. 
 

Edit: The ascent profile is roughly as follows:

-Go up to 43.5 km altitude on propellers. The propeller stage can go higher than this but it takes a really long time. 
-Light all engines and pitch down to 55 degrees pitch. I found that this was a good angle to get out of most of the atmosphere and also build up vertical speed. If drag weren't an issue, I would want to go closer to 30 degrees, but drag is an issue so I want to get out of the atmosphere quickly. 
-Drop fuel tanks as they empty. I don't draw fuel from upper stages as the engine dry mass isn't worth it. 
-When apoapsis reaches 65 km, lock prograde because there's enough vertical speed from the second stage. 
-When the second stage detaches, pitch to 30 degrees and hold it - the Spider engine has really low TWR so this is necessary. 
-Finally, circularize with the jetpack. I hold the translate forward and upward keys at the same time, which loses a bit of efficiency but buys time to gain horizontal speed. This is important because the jetpack also has very low TWR, so it needs a lot of centripetal force in order to maintain altitude. 

Edited by camacju
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