ESS Guides and Q&A

[septemberWaves]

Eloquent Spacecraft Systems: Guides and Q&A Thread

I've had a couple of people asking me questions recently about how I design spacecraft; things like what my thought process is, how I design a vehicle for a specific task, where I start with that. So I've decided to create this thread for similar sorts of questions, and also for general guides on various things. I have a fairly good amount of experience with most aspects of KSP, and so I feel like I'll be able to provide some good advice. I'll be making general guides about how things can be done (not necessarily the absolute best way, just the way that works best for me), and I'm very much open to requests for certain things.

The guides I make will make use of a couple of mods (MechJeb, Editor Extensions Redux, Fuel Tanks Plus, and a couple of aesthetic mods), but will otherwise be stock (unless the guide is specifically for something modded, such as a Tekto lander).

Existing guides

•  

Current plans

• Comprehensive guide to using MechJeb in construction and flight
• Generic mission guide for an Apollo-style Mun mission (will cover many of the basics of spacecraft design)
• Guide for designing efficient and effective launch vehicles
• General guide for mission profiles & how to choose the most effective vehicle design for a particular location
• Guides for creating landing vehicles for various locations (particular celestial bodies may be done by request, including modded worlds)

Edited November 19, 2016 by eloquentJane

[septemberWaves]

Mun Mission: Apollo-style

This guide will follow the design process for a stock-balanced Apollo-style Mun mission. It will go through many of the basics of planning out a mission and designing vehicles to accomplish that mission. The techniques in this guide can be applied to other missions and are not exclusive to a Munar mission. Delta-v information comes from this map, and is rounded up to allow for some room for error.

Step 1: Plan
My mission plan will replicate an Apollo moon landing. Since I know what the vehicle will look like as it is an approximation of the Saturn V, the first step is to consider the different parts of the vehicle.

Vehicle components (top to bottom)

Command/Service module - main spacecraft. Will contain the crew, some science equipment for orbit, a docking system, an antenna, and the Kerbin return system. Will circularize in Munar orbit whilst docked with the Munar Excursion Module. Consists of 2 components:

    Command module - crew capsule. Needs a docking port, basic RCS, and the Kerbin return system. Also needs a launch escape system.

    Service module - the section beneath the command module for science equipment, an antenna, the RCS, and the propulsion system (including fuel and an engine).

Munar Excursion Module (Mun lander) - the vehicle that will land 2 kerbals on the Mun and return them to Munar orbit. Consists of two stages:

    Ascent stage - crew module. Equipped with docking system and antenna, as well as propulsion.

    Descent stage - contains science equipment. Equipped with landing legs and propulsion.

Launch vehicle - Saturn V. Will launch the command/service module and Munar Excursion Module into a Munar transfer from the surface of Kerbin. Consists of 3 stages:

    Third stage - executes Munar injection burn.

    Second stage - finalizes ascent (on the actual Saturn V it is the third stage that circularizes, but that is not needed in KSP).

    First stage - lifts off from Kerbin surface and begins the ascent.

 

Step 2: Design

The order of design should start with the Munar Excursion Module. The reason for this is because the Command/Service module will circularize in Munar orbit whilst docked to the MEM. The next component to design is the Command/Service module, as both it and the MEM are its payload to Munar injection. Finally, the launch vehicle can be designed.

Munar Excursion Module

Spoiler

The Munar Excursion module has two components. The first to design is the ascent stage, since it is the payload of the descent stage and so the descent stage must be made to carry it.

I started with the crew capsule, a lander can which holds 2 kerbals plus some monopropellant and electric charge. I added solar panels (fixed ones are fine for landers like this one), as well as an adapter and a docking port. The docking port is rotated 45 degrees because the RCS thrusters will be positioned at 45 degree angles in order to avoid getting in the way of the hatch.

The four-way RCS thrusters are positioned on cubic octagonal struts because they look nice, but also because if they are further from the center of mass they generate slightly more torque for rotation. The absolute offset function is used to position the thrusters perfectly on the struts.

Once the four-way ports are positioned properly, the complete RCS can be added.

Four Oscar-B tanks are placed beneath the capsule. The MechJeb core is also added at this point.

The location shown by MechJeb's delta-v reading is very important, as it will show thrust-to-weight information for that location.

Ant engines are chosen because of their low mass and high vacuum I_sp. They have low thrust, but the Mun has low gravity.

Four Ant engines are used in order to achieve a thrust-to-weight of 1.21 on the Mun. The vehicle technically has 101m/s more delta-v than it needs, but the low thrust-to-weight ratio means that much will be lost to gravity. A decoupler is placed in the center, and will activate at the same time as the engines.

A couple of structural components are added to give more separation between the ascent module and the descent module. The descent module will now be constructed.

I start out with a service bay, to hold science equipment and a battery. I also add some structural components to hide the bottom of the ascent module (purely an aesthetic choice).

The service bay is filled with science equipment and a large battery.

A small fuel tank (from the Fuel Tanks Plus mod) is added, plus an adapter and a Spark engine. The spark engine has fairly low thrust, but again it is enough for the Mun. I initially tried using a Terrier, but the additional mass meant that the Spark gave slightly more delta-v despite its lower efficiency.

Landing legs were added, and the Spark engine was clipped slightly up into the adapter to avoid it touching the ground.

Now that the landing gear is in place, ladders can be added and configured so that the kerbals can reach the ground. Ladders aren't strictly necessary in the Mun's low gravity, but it's good to get into the habit of including them.

All science experiments (including the crew report) are assigned to an action group. I usually use the 0 key (action group 10).

Finally, in order to ensure that the Munar Excursion Module can be merged with the rest of the Mun vehicle when necessary, the entire vehicle is rerooted so that the docking port is the root part.

Command/Service Module

Spoiler

The next component is the command/service module. This consists of two components: the command module, and the service module. The first one to design is the command module.

The command module should seat 3 kerbals, so I use the 3 kerbal pod.

Three parachutes are added, as well as monopropellant, a MechJeb core, and a docking port.

Two drogue chutes are added in radial symmetry. This tilts the pod, but that is not an issue for landing.

The next components are small RCS thrusters. They are monodirectional, and the 4-way ports will be attached to the service module.

Finally, a heat shield completes the command module. This entire assembly will have to re-enter and land, so it is important to test it.

After successful landing tests, the command module is mostly complete. There is one last component needed for it though: the launch escape system. In the event of a malfunction during launch, the launch escape system is needed to pull the crew away from the vehicle. However, it is rather massive and so should be jettisoned once in space.

A separator is added to separate the command module and service module before re-entry. For the launch escape system separation, a decoupler (pointing down) is attached on top of the docking port.

The command module is not overly aerodynamic, so an upside down fairing is used to create a shroud for the LES. The staging is turned off for the fairing, and it will be pulled away when the LES separates.

The launch escape system is completed with the tower, a solid rocket motor designed to briefly provide high thrust in the "away from exploding rocket" direction. Action groups are set up so that the tower ignites at the same time that the separator decouples. Now it needs testing.

The LES decoupled successfully.

And in the flight test, it successfully pulled the command module away from the rocket.

The LES is jettisoned, and the command module executes its landing.

Now it's time to design the service module.

The LES can be removed at this point, as the service module engine will not be propelling it. It will be added back again later.

A fuel tank, the 4-way RCS thrusters, and an adapter (purely for aesthetic purposes) are the main parts of this service module. It could be modified to include a service bay for various payloads, but this particular mission has no need of one.

I chose to use a Reliant engine, as it's reasonably large and will provide good thrust. It's not particularly efficient, but that's not really important as there is an excess of fuel anyway.

I also added solar panels. The real Apollo missions used fuel cells, but I don't want to risk leaving them on and consuming all of my fuel.

The first burn of the service module engine will be a Munar insertion burn, and during that it will be attached to the MEM. For this purpose, the MEM can be merged in temporarily and attached to the command module docking port.

A Munar insertion burn takes about 350m/s on average, and the same is required to get back on a trajectory to leave the Mun. The command/service module easily has enough fuel for these purposes. It actually has far too much fuel, but I want it to resemble the Apollo command/service module.

Now the LES can be replaced. Conveniently the fairing returns to the shape it was initially in, though it would not be difficult to approximate the same shape again if necessary.

The command/service module is complete. I renamed the save to the name for the complete Saturn V vehicle ("Apollo-style Mun mission") in order to keep the command/service module as a separate vehicle. I then added a separator in order to mount the MEM beneath the service module when configuring the launch vehicle.

Launch Vehicle

Spoiler

Now that the two payloads are designed, the launch vehicle can be assembled.

The first step is to mount the payloads in the correct configuration for launch.

The MEM is mounted beneath the service module.

A small decoupler and adapter connect to the bottom of the lander for attaching it to the fairing.

The fairing is added and built. The solar panels on the service module are adjusted slightly so that they do not get broken when the fairing is deployed.

The third stage needs about 900m/s of delta-v to take the payloads from low Kerbin orbit to a Munar transfer orbit.

A fairing is added as an interstage. Four engines are removed because the third stage of the real Saturn V only has one engine.

The second stage needs about 1400m/s in vacuum. This is only approximate and the value would change depending on how powerful I want to make the first stage, but I've found from a lot of experience that a 2200m/s first stage with 1400m/s second stage is a good launch configuration.

Separation motors are added, in order to propel the spent second stage away from the third stage once it has served its purpose.

The first stage needs about 2200m/s of delta-v, and a lot of thrust. It also has separation motors.

This is the first design iteration. The engines are too close together though, and it doesn't have enough delta-v and so requires an extra fuel tank.

This is the second iteration. The first stage has less than my target delta-v, but I added an extra tank to the second stage to make up for the difference. This does however have the downside of the second stage having a low thrust-to-weight ratio.

I moved the engines out, and gave them some extra nosecones to make them more aerodynamic.

Small fins are added, not because the vehicle needs them in this case, but because the Saturn V had them.

And with that, and the addition of launch clamps, the Saturn V for the Apollo-style Mun mission is complete.

 

Step 3: Flight

Launch and Mun Transfer

Spoiler

Saturn V on the launch pad.

At this point, it is important to conduct pre-flight checks. Ensure that stages are correct, ensure that the right crew are on the vehicle, check delta-v, etc.

All systems nominal, and the mission is go for launch (my acceleration limiter was wrong so I quickly had to fix that).

Gravity turn begins at around 100m/s.

First stage flies perfectly.

First stage cutoff and separation, second stage ignition. It's worth noting that MechJeb will not autostage if you have used an interstage fairing like I did here, unless it is high enough in the atmosphere (which it is not in this instance).

Intended apoapsis reached, second stage cutoff.

The LES is jetissoned at this point.

The circularization burn is planned, and is executed by the second stage.

Once in circular orbit, the second stage is jettisoned. It should fall back into Kerbin's atmosphere.

A Mun transfer burn is planned. It places the vehicle on a collision course with the Mun, and this is by design. The third stage of the Saturn V will crash into the Mun, whilst the command/service module (whilst docked to the LEM) will make a correction burn to avoid the same fate.

The trans-Munar injection burn is executed successfully.

Now the fairing separates, and the solar panels on the service module are deployed.

The command/service module separates from the LEM, and redocks with its docking port. This allows for use of the service module engine.

Once in Munar SOI, the spacecraft executes a small radial burn to raise the Munar periapsis to 15km.

Finally, the spacecraft circularizes in low Munar orbit in preparation for the landing.

Landing and Return

Spoiler

Once in low Munar orbit, the crew prepare to land on the light side of the Mun.

The landing crew transfer over to the MEM.

The MEM then undocked and executed the descent burn.

MechJeb can show the intended trajectory and predicted landing location. It usually performs a suicide burn to slow the spacecraft, which is very efficient but also rather dangerous.

When approaching landing, an issue was identified with the landing gear: they wouldn't deploy.

The landing gear bug meant the loss of the engine on touchdown, but luckily nothing else was damaged. Strangely, the landing gear did deploy properly once the craft was already landed.

Once on the surface, science and EVAs can be conducted.

The crew return to the MEM and begin the ascent. The low thrust-to-weight ratio makes it a slow and tedious process, and at times it was necessary to use the RCS thrusters for a boost.

The engines didn't have enough fuel to complete circularization, so I had to use the RCS to help out. Luckily the vehicle is light enough and contains enough monopropellant that this was possible.

The MEM is left in orbit whilst the command/service module performs several maneuvers to rendezvous with it.

I also got a rather good angle of the front of the command module. It's quite a pleasing design I think.

The two spacecraft approach each other and zero their relative velocities.

The MEM ascent module had enough monopropellant remaining to execute the docking, but if it hadn't then the command/service module could've done it.

Crew and science are transferred back to the command module, and then the MEM ascent module is de-orbited.

The spacecraft performs a burn to return to Kerbin, with a 20km periapsis for safe aerobraking.

The spacecraft slowly returns to Kerbin.

Before entering Kerbin's atmosphere, the service module and heat shield shroud are jettisoned. Due to the way the staging is set up, the parachutes are also staged at this point, and will deploy autonomously as soon as it is safe. Due to this, the entire descent requires no player input even without the use of MechJeb.

The re-entry and landing were successful, and the crew returned to Kerbin.

 

I made some minor mistakes with this mission, such as the lack of antennas, but there was enough redundancy included for it to work (such as the excess monopropellant system on the lander). That itself highlights how important redundant systems are on spacecraft even in KSP.

Edited November 24, 2016 by eloquentJane

[RA3236]

You might also want to add that the two-stage LM design is actually extremely inefficient in KSP because all the bodies are much smaller and closer.

[septemberWaves]

8 hours ago, RA3236 said:

You might also want to add that the two-stage LM design is actually extremely inefficient in KSP because all the bodies are much smaller and closer.

That guide is incomplete. And anyway, I used the two-stage design because it was supposed to be Apollo-like. Same with the launch vehicle; most payloads don't actually require 3 stages, but the Saturn V is a 3-stage rocket.

[septemberWaves]

Apollo-style Mun mission guide is complete. It covers many of the basics of spacecraft design and mission planning, though most will be elaborated upon in future guides.

[XB-70A]

It is really nice. I would love to get associate it to my Apollo-like module made or the Saturn IB-like launcher.

Spoiler

Alas I had to use three Mk 1 pods to keep it small, cheap and able to be docked to a stock Skylab (so with medium size parts)... even the Clamp-O-Tron and the antenna are too big for it.

 

 

[Clipperride]

Nice job!

[septemberWaves]

@XB-70A Nice launch vehicle, it captures the aesthetic of the Saturn IB quite well. As for the capsule, I suppose there will always have to be sacrifices made to preserve either realism or appearance. I think for improving appearances it's generally best to start with the payload (in your case, the capsule) and design the launch vehicle around it (like I did with the Mun mission).

The three Mk1 pods are cheap and easy to use, but if I was designing such a vehicle I would probably use the larger crew capsule as it is more Apollo-like and also is able to use either size of docking port without looking kind of odd. The service module is then not too difficult to design for an Apollo-like aesthetic. As on my Saturn V, I prioritized appearances over efficiency even though it made the final vehicle more expensive. I could, however, have used a fuel tank half the size of the one I did use, and make up the rest of the length with a service bay.

For a Saturn IB, I think I would go for a 3.75m design because it would suit my Apollo capsule quite well in terms of size - although it would then have the unfortunate effect of being the same diameter as the stock Saturn V. That being said, I usually use SpaceY, so my Saturn V would probably end up using 5m diameter parts.

In any case though I think your Saturn IB is quite an effective design considering the limitations you were working with (though there is one suggestion I would definitely make - clip an 0.625m docking port inside the ring of a 1.25m docking port on a station like Skylab, that way you have the option to use either size).

Edited November 25, 2016 by eloquentJane

[The Flying Kerbal]

Excellent!!!  The finished launch vehicle looks fantastic, I really must try to copy this tutorial and launch an Apollo of my own.

And turning a fairing base upside down, what a simple, brilliant idea!  I'm not having much fun with fairings, I find them to be a devil of a job to get them to close where I want them to.  Having tried this upside down trick though, while I won't say it has fied all the problems I have with them, it certainly helps enormously.

Great job Eloquent Jane, I hope you do more threads like this.