Low Mass Moon - REALISTIC MODE

[Astro826]

Pt. 1/2
I've recently gotten my hands on the steam version, so of course my first mission is to tackle the extreme. So now I present my 341.49 ton Moon mission in realistic mode.

Rules:
-Realistic mode
-No cheats or glitches
-No exploits such as lithobraking or atmosphere skipping
-Pure Vanilla and noclip (QOL mods are okay)
-No ion engines

Design:
The rocket consists of seven stages, progressively getting smaller until the final stage with a 5 ton fuel tank. To save mass on engines, every engine is given space to run simultaneously, and fuel transferring is used so every tank has full fuel once the lowest stage runs out. In total, the rocket has 555 tons of thrust, giving a (correct) thrust/weight of 1.625.


Ascent:
Before takeoff, the Smart SAS mod is used to set the heading to 1 degree from vertical. As soon as the rocket reaches 23 m/s, SAS is set to prograde. During the first stage burn, fuel is transferred to the other four fuel tanks with engines, keeping them full. SAS is invaluable for making this possible - I've been barely able to transfer fuel to 3 tanks while manually performing an ascent, but with great difficulty. SAS also allows the gravity turn to be highly consistent and always efficient via this method. After the first two stages run out, the rocket pitches up to maintain altitude, since it takes a long time to reach orbital velocity in realistic mode. Upon reaching orbit, only 3 stages and one engine remain.


Lunar Transfer:
To reach the moon with the least amount of delta V possible, the transfer is set to meet the moon at its periapsis. Normally, this is actually the worst place to encounter the Moon because it increases the capture burn needed. However, a capture burn is not done here. Instead, a lunar gravity assist is performed to put the craft on a trajectory that just pokes outside of the Earth's sphere of influence. Due to the precise time of year, this is calibrated such that the rocket quickly re-encounters Earth on a similar trajectory. But by spending this small moment in solar orbit, the periapsis of the Earth flyby can change. Once it is on a flyby with the periapsis slightly above the Moon's orbit, a tiny burn captures the craft in high Earth orbit.

Now in this strange orbit, a miniscule correction is performed to achieve a glancing encounter with the Moon's sphere of influence in a few orbits. Because of this glancing encounter angle, the moon actually comes close to capturing the rocket without a burn, reducing the relative velocity to the moon even more. Unfortunately, this cannot be achieved while also having a periapsis close to the moon. Instead, this flyby is adjusted such that I get more, closer passes to the moon which also have very low relative velocity, resulting in this eldritch horror of a trajectory.

(Continued)

 

[Astro826]

Pt. 2/2
Lunar Capture and Descent:
Because of this extreme and convoluted route, the delta V needed to capture into elliptical lunar orbit is reduced from around 130 m/s to almost none, with no cost except that to time and sanity. Upon nearing the lunar periapsis, the rocket faces retrograde using Smart SAS and begins the capture and descent burn. Soon, the trajectory is suborbital, and the 6th stage runs out, leaving the final stage to complete the mission. to avoid hitting the surface, the rocket pitches up a few degrees while the last of the velocity is reduced. Once the rocket is falling the final few meters, it is hand flown to touchdown on a crater rim. Due to the upscaled terrain, this is actually above 5km, making the camera act quite strange for the descent.


Lunar Escape:
Like the descent, the ascent from the Moon is performed as horizontally as possible to reduce gravity loss, making it frighteningly close to the terrain. This is however much easier to do precisely on ascent, so low lunar orbit is safely achieved. Then, the spacecraft waits for the right time to escape the moon to reverse the journey. With 1.6% fuel remaining, the trajectory slightly pokes out of the Moon's SOI such that it re-encounters the moon, similar to the Earth escapes earlier in the mission. This trajectory is adjusted with tiny burns until one of these lunar flybys kicks the rocket back out to the edge of Earth's SOI.


Earth Return:
Once again, the rocket is in a high, elliptical Earth orbit. Luckily, the process that first got the rocket to this state can also be reversed. the apoapsis is once again poked outside Earth's SOI, such that it re-encounters Earth with a different periapsis. After some adjustment, a flyby that passes directly into the atmosphere is found. On this final pass, the periapsis is adjusted to to 40km in order to aerobrake. You may have noticed that this upper stage is very strange with the two Atlas side covers. Their purpose in this mission was dual: First, they were the lightest way to give the other stage's engines the space to run without burning anything. They are connected to the final stage for their second purpose as air brakes. Reentry heating in realistic mode is set low enough that a heat shield is not actually needed if the rocket is draggy enough. With their length and light weight, they were the perfect part for this. Instead of a 0.5 ton heat shield, the final stage has 0.2 tons of air brakes. The aerobraking is split up to reduce the heating to survivable levels.


Landing:
After all the horizontal velocity is lost to drag, the rocket falls vertically, maintaining its horizontal position to maximize drag. At 130 meters altitude, the rocket begins its flip and burn landing, starship style. At only 35 meters altitude, the high thrust is able to arrest the fall and bring the rocket to a soft touchdown, carefully balancing on the awkward Atlas side cover.


Mission Log and My Findings:
Doing this mission taught me a lot about realistic mode. First off, I learned how thin the upper atmosphere was, which meant most of my ascent happened at below 70km altitude to reduce gravity loss, with almost no drag penalty. Second, I also figured out how gentle the reentry heat was. I also have some surprising insights to the usefulness of mods: ANAIS turned out to surprisingly be of little help. My route to and from the moon is so wacky that not even ANAIS considers it. I look forward to how this mod can help me, but this mission didn't need to use it. Aero trajectory was a nice help in figuring out the reentry, and I likely would have missed the fact that I could skip the heat shield without it. Most useful was Smart SAS: it revolutionized ascent consistency, allowed me to multitask even more, and helped for most of the maneuvers. Now here's the four pages of mission log from all the shenanigans (this many encounters was never actually necessary, but the first working trajectories I found all happened to be many passes into the future).

 

[Jordy]

Pt. 2/2
Lunar Capture and Descent:
Because of this extreme and convoluted route, the delta V needed to capture into elliptical lunar orbit is reduced from around 130 m/s to almost none, with no cost except that to time and sanity. Upon nearing the lunar periapsis, the rocket faces retrograde using Smart SAS and begins the capture and descent burn. Soon, the trajectory is suborbital, and the 6th stage runs out, leaving the final stage to complete the mission. to avoid hitting the surface, the rocket pitches up a few degrees while the last of the velocity is reduced. Once the rocket is falling the final few meters, it is hand flown to touchdown on a crater rim. Due to the upscaled terrain, this is actually above 5km, making the camera act quite strange for the descent.
View attachment 141730 View attachment 141731 View attachment 141732

Lunar Escape:
Like the descent, the ascent from the Moon is performed as horizontally as possible to reduce gravity loss, making it frighteningly close to the terrain. This is however much easier to do precisely on ascent, so low lunar orbit is safely achieved. Then, the spacecraft waits for the right time to escape the moon to reverse the journey. With 1.6% fuel remaining, the trajectory slightly pokes out of the Moon's SOI such that it re-encounters the moon, similar to the Earth escapes earlier in the mission. This trajectory is adjusted with tiny burns until one of these lunar flybys kicks the rocket back out to the edge of Earth's SOI.
View attachment 141733 View attachment 141734 View attachment 141735

Earth Return:
Once again, the rocket is in a high, elliptical Earth orbit. Luckily, the process that first got the rocket to this state can also be reversed. the apoapsis is once again poked outside Earth's SOI, such that it re-encounters Earth with a different periapsis. After some adjustment, a flyby that passes directly into the atmosphere is found. On this final pass, the periapsis is adjusted to to 40km in order to aerobrake. You may have noticed that this upper stage is very strange with the two Atlas side covers. Their purpose in this mission was dual: First, they were the lightest way to give the other stage's engines the space to run without burning anything. They are connected to the final stage for their second purpose as air brakes. Reentry heating in realistic mode is set low enough that a heat shield is not actually needed if the rocket is draggy enough. With their length and light weight, they were the perfect part for this. Instead of a 0.5 ton heat shield, the final stage has 0.2 tons of air brakes. The aerobraking is split up to reduce the heating to survivable levels.
View attachment 141737 View attachment 141738 View attachment 141739 View attachment 141740

Landing:
After all the horizontal velocity is lost to drag, the rocket falls vertically, maintaining its horizontal position to maximize drag. At 130 meters altitude, the rocket begins its flip and burn landing, starship style. At only 35 meters altitude, the high thrust is able to arrest the fall and bring the rocket to a soft touchdown, carefully balancing on the awkward Atlas side cover.
View attachment 141741 View attachment 141742

Mission Log and My Findings:
Doing this mission taught me a lot about realistic mode. First off, I learned how thin the upper atmosphere was, which meant most of my ascent happened at below 70km altitude to reduce gravity loss, with almost no drag penalty. Second, I also figured out how gentle the reentry heat was. I also have some surprising insights to the usefulness of mods: ANAIS turned out to surprisingly be of little help. My route to and from the moon is so wacky that not even ANAIS considers it. I look forward to how this mod can help me, but this mission didn't need to use it. Aero trajectory was a nice help in figuring out the reentry, and I likely would have missed the fact that I could skip the heat shield without it. Most useful was Smart SAS: it revolutionized ascent consistency, allowed me to multitask even more, and helped for most of the maneuvers. Now here's the four pages of mission log from all the shenanigans (this many encounters was never actually necessary, but the first working trajectories I found all happened to be many passes into the future).
View attachment 141743 View attachment 141744 View attachment 141745 View attachment 141746

wow nice landing, this is a impressive mission, if the moon badge still existed this would definitely get a 5/5

 

[QMSP]

Incredible!

if the moon badge still existed this would definitely get a 5/5

Actually, back when the Moon badge existed, the grading system didn’t work like that; instead of a rating from 1 to 5, it was just approved or not approved (but I get your point)

 

[Jordy]

Incredible!

Actually, back when the Moon badge existed, the grading system didn’t work like that; instead of a rating from 1 to 5, it was just approved or not approved (but I get your point)

yeah I knew that I just was saying that it would deserve a 5/5.

I could never do this how long did it take to optimize?

 

[Astro826]

yeah I knew that I just was saying that it would deserve a 5/5.

I could never do this how long did it take to optimize?

Since I've just downloaded the game on steam, I can provide a pretty good number from my play time. Based on the portion of time I've spent working on this mission specifically, I think about 10 hours for everything.

 

[Jordy]

Since I've just downloaded the game on steam, I can provide a pretty good number from my play time. Based on the portion of time I've spent working on this mission specifically, I think about 10 hours for everything.

wow very impressive, I think the longest I've ever spent on a rocket was my shuttle submission that wasn't eligible because of using noheatforcreativeuse on the engines

 

[FPE Fan]

Impressive

 

[yonkee]

what a crazy mission that's insane

 

[TheMacTester]

resulting in this eldritch horror of a trajectory.

tf u mean eldritch horror, that shit is ungodly

ok, so a few questions: why is it more efficient to transfer fuel to upper stages?

Before takeoff, the Smart SAS mod is used to set the heading to 1 degree from vertical. As soon as the rocket reaches 23 m/s, SAS is set to prograde.

how did you figure out that this was the most efficent gravity turn with the least amount of effort

Why is there a stage above the "upper" stage?

But by spending this small moment in solar orbit, the periapsis of the Earth flyby can change

why would it do that?

Now in this strange orbit, a miniscule correction is performed to achieve a glancing encounter with the Moon's sphere of influence in a few orbits. Because of this glancing encounter angle, the moon actually comes close to capturing the rocket without a burn, reducing the relative velocity to the moon even more. Unfortunately, this cannot be achieved while also having a periapsis close to the moon. Instead, this flyby is adjusted such that I get more, closer passes to the moon which also have very low relative velocity, resulting in this eldritch horror of a trajectory.

I dont even know what the fuck happened, did you use a chain of gravity assists to slow down?

Their purpose in this mission was dual: First, they were the lightest way to give the other stage's engines the space to run without burning anything. They are connected to the final stage for their second purpose as air brakes.

very smart, I must say

First off, I learned how thin the upper atmosphere was, which meant most of my ascent happened at below 70km altitude to reduce gravity loss, with almost no drag penalty

so is it better to make a rocket with high thrust in RM?

overall, very nice, good job
mac tested

 

[Astro826]

ok, so a few questions: why is it more efficient to transfer fuel to upper stages?

This is an asparagus type(there are so many names) staging. If I didn't have all the engines going at once, I would need an extra three frontiers and another valiant to get the similar enough thrust for each stage, that's 20 tons . If I didn't transfer fuel back, then all the stages wouldn't be sequential, they'd be in parallel like boosters so it would hardly be a multistage rocket. This method is like having the cake(extra thrust from boosters) and eating it(delta V of multiple stages).

how did you figure out that this was the most efficent gravity turn with the least amount of effort

Trial and error to find the speed to switch at. It is kind of an assumption by me, but prograde the whole time is likely best. The 1 degree step is just to set up the turn right. I went for 60-70km apoapsis before I had to begin pitching up to maintain that altitude, I think that achieved a pretty good balance of drag and gravity losses (a big part of this in realistic is time - if the lower thrust upper stages don't have enough time before the rocket will start falling, they have to pitch up a lot more).

Why is there a stage above the "upper" stage?

For the airbrakes idea to work, the upper stage needed altas side covers on the top and bottom. Reusing these to connect the stages is obviously good, so at least one stage had to be on top.

why would it do that?

Since it's a separate flyby, it can theoretically be whatever distance. I could have gone into a 1:1 resonant orbit to do something similar, but that would be way more tedious. I know SFS isn't an N-body simulation but this video formed the basis for these ideas, it is surprisingly possible to transfer them over to a sphere of influence gravity system.

I dont even know what the fuck happened, did you use a chain of gravity assists to slow down?

Kind of? This ones hard to explain. It's based on lining up these low velocity, grazing encounters with the moon SOI (really it's just trial to find the crazy trajectory though). Here's a diagram of a simpler trajectory doing it:

I've numbered the sections of orbits. What matters most is the second encounter, which I've circled. It is lined up such that the velocity of the craft (C) and the velocity of the moon (M) are identical(direction and magnitude). This means that if they encounter there(which they do), they have almost zero relative velocity, which you can see by the fourth section almost falling straight down to the moon. The first assist is to line this up. In the real mission, this just happened optimally after a bunch of encounters and I didn't want to spend more fuel to find a cleaner way of making it happen. In the absolute ideal perfect case, the craft is going like 1 m/s faster than the moon and the encounter is as close to the edge of the SOI as possible, which would leave the craft in an almost entirely captured, elliptical orbit (like my earth escape orbits).

so is it better to make a rocket with high thrust in RM?

I think it's about the same as normal? I'd need to test more. One thing to note is that realistic mode has lower dry mass, so TWR at the end of the burn is going to get higher than normal mode. The much longer time it takes to reach orbit may also factor into this.

 

[Altaïr]

Wow, I don't know how I missed this, this is dark magic
Well done, it's very impressive. I wouldn't have imagined that a Moon return mission was possible with 341 tons.

ANAIS turned out to surprisingly be of little help. My route to and from the moon is so wacky that not even ANAIS considers it.

For my defense I didn't have that much imagination when I made that tool
Didn't the approach line help you to chain encounters though?

Honestly if I had to make it consider such trajectories I don't even know how I would proceed. First in terms of interface (how should the player specify that he wants that kind of trajectories?), and then the computation time is a constraint: I made the choice of making a calculation tool in real time, which excludes strategies based on brute force.

 

[Astro826]

For my defense I didn't have that much imagination when I made that tool

Yeah it's just maybe a teeny bit too far out there for a general purpose tool to include.

Didn't the approach line help you to chain encounters though?

Perhaps it could have helped the correction of the high elliptical orbit to encounter the moon, but I didn't think of that since I'm not used to the mods.

 

[Altaïr]

Perhaps it could have helped the correction of the high elliptical orbit to encounter the moon, but I didn't think of that since I'm not used to the mods.

That's understandable. The mod interface is not the most intuitive for anyone that doesn't know it. The approach lines should show up automatically though, provided you target the Moon, and your trajectory crosses its orbit (or at least gets close enough). It also works if you crosses several SOIs before trying to encounter the Moon, all future trajectories are considered.

Basically, the approach line is a basic tool that works in many situations, while the transfer prediction is a very powerful one but that doesn't work in as many use cases. In practice, the approach line will be used when the transfer prediction doesn't work.

 

[Astro826]

The approach lines should show up automatically though, provided you target the Moon,

Yeah, I had it turned off around that step of the process because it was (as it's supposed to) showing a maneuver + trajectory to normally encounter the moon, which wasn't what I needed.