[Community Guide] How to build Linear Motion Airships

Saelem Black

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#1
Introduction
I’ve been meaning to make this guide for a while after questions from several community members concerning how to make airships around linear motion engines. From a historical perspective (as in 1.0 Stable and before) there were only two ways to make a true aerial tech. These were the good old airplane (yes I’m an American, don’t judge me) and then by use of the hoverbug. The hoverbug was essential because, aside from the source of thrust, it fooled the game into thinking the tech was on the ground, allowing techs to continue being controlled like a car no matter the height (ordinarily a tech would “switch” into aerial mode after lift-off which was meant for planes, not helicopters). In the 1.0 era, any airship that wasn’t a plane needed a hoverbug to function.


With the inclusion of control schemes, Better Future, and the new adjustment thrusters, hoverbug is no longer strictly necessary for airship building. In this guide, I will attempt to discuss how to make a well controlled airship using LMEs instead of hoverbugs. I’ll also discuss how to make a ship which handles well in the air.

Control Schemes
Linear Motion Airships are fundamentally constructed according to how you want to control it. I’ll be focusing on two types of control schemes as described below. Make sure you’re familiar with the control scheme interface and terms like yaw, roll, pitch, and the vector thrust indicator.


Dirigible Control Scheme: This control scheme is the simpler of the two. Lift is controlled exclusively by the upward thrust vector and turning is handled exclusively by yaw. Pitch and roll are LOCKED. This control scheme is how blimps and zeppelins function (hence the name).

Starfighter Control Scheme: This control scheme is how you would expect a scifi starfighter to pilot. With this scheme, the weight of the tech is lifted by LMEs, but you control your altitude by pitching up or down while travelling forward. These handle very similarly to planes in that you need to be moving in order to climb or descend. However, if you stop moving, you should simply hover in place. This control scheme is the more advanced of the two and requires a good handle on general principles first.

Basics of the Linear Motion Engine (LME)
The LME is a 1x1x2 Better Future Flight Block with attachment points on all sides. It has two functions; 1.) it provides a vectored thrust in the direction it’s pointing and 2.) it converts all adjustment thrusters and propellers pointing in the same direction to vectored thrust. This is very important. The thrust of one LME at 100% by itself is enough to lift a total mass of 16. The LME itself weighs 2, so you have 14 more to play with (remember that a GSO One block weighs 1).
 
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Saelem Black

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#2
Basics of Airship Building

Before I get into the use of LMEs, let’s discuss basic general airship building.

Rule 1: You have to have more upward lift than total weight. This is a common-sense thing, but there’s a subtlety: If you pitch or roll, any block you’re using for lift is turned slightly. The angle of this turn reduces the lift on your tech (by cosine of the angle, for mathy people). At a 45-degree pitch (or roll), your lifting blocks will only be 70% effective, because much of your LME thrust will be pushing you sideways. If your tech’s weight is close to the hairy edge of its lift capability, pitching or rolling may cause you to drop altitude very quickly.

Rule 2: Your “center-of-lift” must match your “center-of-mass”, and they must match closely. If it’s not clear, to think about center of lift, look exactly top-down on your tech. Each lifting block contributes an upward force in the same way every block contributes a downward force (from gravity). The center of lift is then the average location of all of your upward thrust. More powerful lift blocks, like HE rotors, will pull the center of lift closer to them.

Pitch Forward.png

99.99% of the time, the issue with making an airship is getting this right, particularly in the fore/aft direction. For a tech with left/right symmetry, your center of thrust will be on the centerline of your tech. But if most of your lift is in back, then you’ll pitch forward, and vice versa. If you can’t get your airship off the ground but its design seems otherwise sensible, odds are that this is the issue.

Rule 3: Your “center-of-thrust” must match your “center-of-mass”. In this case, thrust is the forward force on your tech. Perform the same exercise as rule 2, but look at your tech from the back. Are most of your thrust blocks above or below the center-of-mass? This will determine whether you pitch up or down (or drag left/right if your thrusters are not symmetric).

Pitch Forward 2.png

Now obviously, you’re rarely going to be perfectly symmetric like these pictures. But if I put one LME in front and three in back instead, but I also put all my heavy batteries in back, then I just might have a balanced tech. In other words, your center-of-mass doesn’t need to be the physical center of your tech. This is where ballast blocks would be useful (looking at you, Matt and Zanzistar), to allow you to fine tune the location of your center of mass. In absence of a true ballast block, Hawkeye batteries work fairly well as ballast. With a mass of 8 (being a 1x1x2 block), they're incredibly dense and heavy and can be substituted in place of many common body blocks.

No Pitch.png

Things like gyros and adjustment thrusters (with a stabilization computer) give you more room to wiggle. With these parts correctly used, the centers of mass, lift, and thrust need not be so tightly aligned. But you should *always* attempt to get them as close together as possible before relying on other components or you risk giving yourself a huge design headache.
 
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Saelem Black

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#3
Adjustment Thrusters, Basic controls, and Gyroscopes

So far, we’ve discussed the lift and acceleration axes. To understand how to control the remaining axes, we need a good understanding of adjustment thrusters, how they work, and where to place them.

First, I want to address something I’ll call the “thrust reserve”. Thrust reserve is simply the remaining thrust a block can put out before being maxed out. For example, if I have a tech with one single LME which hovers at 70% thrust (which means it weighs 11.2 weight units), then my thrust reserve is the remaining force of 4.8 units. Okay, so why does this matter? Because you’re frequently asking your thrust blocks to do multiple things at once. The thrust reserve represents how effectively you’ll be able to do any secondary tasks. This is especially important for pitch/roll on airships because the any adjustment thrusters aiding in lift are also responsible for pitch and roll. Likewise, the same relationship exists between forward acceleration and yaw – the same blocks are responsible for both motions. If you’ve ever built an airship and wonder why you have troubles turning at full speed, this is why. You have no reserve in your forward thrust. This is also why planes use roll instead of yaw, so turning doesn’t interfere with your (very vital) forward speed.

Interestingly enough, LMEs do not respond to any command besides vectored lift. However, adjustment thrusters do, so make sure you have thrust reserve if you're planning on using pitch or roll.

Rule 4: if you want to use some control axis (yaw, roll, pitch, etc), you need thruster blocks which point that direction. Seems self-evident right? But then look at the tech below. This is the Gloombird, one of my small airships. It cannot perform roll at all. The reason is that it has no thrusters which enable the roll direction. It relies solely on yaw to turn. If I wanted to roll, I would need to put adjustment thrusters on the outside of the wings.

[This is a snapshot, feel free to try it out]
Gloombird.png


Roll and Yaw.png

Rule 5: For pitch, roll, and yaw, the further away you place a thruster from the center-of-mass, the more effective it will be. Yaw thrusters should be on the very nose and very tail of your tech. Roll should be as far out on the right and left sides as possible. Pitch, roll, and yaw are all rotational axes, meaning they work according to the both thruster force itself as well as the distance from the center-of-mass (this property is called torque). You can increase torque by increasing the thruster force or increasing the lever arm (i.e. distance from center).

Gyroscopes
The last thing I want to talk about in this section is gyroscopes. Gyros apply a torque on your tech to try to restore it to being perfectly level. As I understand it, they apply torque about the center-of-mass of your tech, regardless of their position. This means they also affect the pitch, yaw, and roll directions, making them more resistant to imbalance. If your tech is marginally front or back heavy, you can use gyros to correct this. Same with left and right. This effectively allows your center-of-lift and center-of-thrust vectors to be slightly off of your center-of-mass without suffering pitch problems. Furthermore, having at least some gyro force is 100% necessary to keep your LMEs pointed in the right direction. Once you get much past a 45 degree roll or pitch, so much of your lift is pointing in the wrong direction that you'll probably crash. Having even a tiny amount of gyro force prevents this.


However, gyros have drawbacks. Because they actively resist pitch, roll, and yaw, they make maneuvering in those directions much more sluggish. During design, they can also mask center-of-mass problems and make it harder to debug. The number of gyros you need also doesn't scale linearly with tech size, meaning you have to dedicate disproportionately more space in a large tech to get the same gyroscopic behavior versus a small tech. You can read about this more in depth here. When I get into the two control techniques below, I'll discuss how gyros play a different role in each.

Lastly, as an advanced technique, you can use pitch-trim gyros to significantly displace your center-of-lift. They allow you to cluster your LMEs in front or back and still keep level. I recommend using at least one on any airship to fine-tune pitch balance in the end stages of design.
 
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Saelem Black

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#4
One more comment before getting into control styles; your LME airship should be fundamentally designed around its LMEs. The LMEs will form the foundation of your tech and may very well be a significant percentage of its total volume. You should decide on their placement first before building anything else. Chose a symmetric pattern to build around, such as a single column of LMEs fore to aft, or a pair or nacelles left and right of the main hull. Nothing else in your tech matters if you can't get off the ground and control it once its in the air.

Dirigible-Style Airships

Okay, so we’ve pretty much covered all the techniques to build and control an airship. Let’s look at the dirigible style airship. This style airship is excellent for beginner and veteran builders alike, and a nice heavy airship of this style pilots like a Cadillac. Interestingly enough, the default hovercraft control scheme is perfect for this. Just keep in mind your lift direction will now serve to lift you off the ground.

The fundamental objective of this airship style is to stay perfectly level at all times. It should feel like a car which can lift off the ground. As mentioned before, this control scheme uses yaw to turn, and lift for vertical control, so the pitch and roll axes should remain unassigned. Because staying level is such a priority, gyros should be heavily used to really lock in the pitch and roll. It’s not going to turn the best, but there are a few things we can do about that. Use the heaviest gyros that are realistic for your tech, and add a few single-axis gyros for a bit of extra stiffness especially in the roll direction.

To increase stability, I recommend giving this type of build a “wide stance”; locate your LMEs on the wide edges of the tech. This gives you extra stability in the roll direction which you’ll need. You want your stabilization computer to have to do as little work as possible.

A design like this should focus on bottom mounted weaponry with turning turrets or homing projectiles. You will likely not often be low enough to hit with any weapons on the top of the tech, and you will not be able to pitch down to aim.


Starship-Style Airships
These airships are a bit more complicated because the method for controlling height is much more versatile. With this type of airship, you'll be piloting it like a plane, but without wings. You'll use pitch to point up or down, and your acceleration or propellers axis to move. Turning can be accomplished with yaw or with both yaw and roll. With these builds, maneuverability is much more important, which means a couple things. First, you must minimize the number of gyros on this type of tech. You need some, but use as few as possible to keep yourself stable. You need to rely on pitch and yaw to maneuver correctly.


To increase maneuverability, I recommend giving this build a narrow stance for much the inverse of the reasons listed in the previous section, especially if you want to use roll. As mentioned before, LMEs do not react to secondary input, so if their thrust remains constant, you need to make sure they're not preventing the direction you want to go.

Starship-style airships need to include thrusters which properly handle pitch. There should be thrusters focused at the front and back of the tech alongside the LMEs. Normally these thrusters will help with lift, making it easier for the LMEs to function. But be prepared, in order to pitch correctly, you need to make sure you have plenty of thruster reserve to spare or you won't be able to pitch up. I recommend targeting 70-80% lift as your hover point if you want to use pitch. You need that overhead so your adjustment thrusters can lift the nose of your tech properly.

These airships allow you to pitch down to aim at targets with front-mounted weapons. Weapons can be located anywhere, though fixed guns like HE battleship cannons may require some practice to master.

The Downburst (below) is an example of a starship-style airship.

Downburst.png
[I'm getting tired, I may edit this more tomorrow]
 
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Saelem Black

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#5
FAQs and Comments

You didn't mention the Stabilization Computer much. Should I use it for LME airships?
Yes. Always.

I feel like 80% of my tech is just flight blocks. Is that normal?
Yep. LME airships are not conducive to overkill-gun-pyramid type designs. Not that that's needed for campaign anyway.

Why do hawkeye airships require so many more LMEs than other corps?
Hawkeye blocks are notably heavier than other corps. They will need about 50% more LMEs to do the same job.

My airship seems to drop out of the sky when I try to pitch up. What's happening?
Your tech is trying to pitch by dropping the thrust in back and increasing the thrust in front. Depending on how much power you have in front, it can mean the back end just drops and that's it. Move some LMEs to the back of your tech and make sure you have enough overall thrust reserve.

The handling of my tech goes crazy when I'm shooting. What's happening?
The recoil of many weapons in terratech are very very powerful, especially the autocannons. It can be so much that firing an autocannon can meaningfully add or subtract from from your LMEs or thrusters, forcing you up, down, or pitching your tech. Avoid using autocannons in particular unless your tech is very big or you are very good at maneuvering.

[more to follow]

If you made it this far, I want to give you a sincere thank you for reading. Please feel free to ask any questions about LME tech design, and post your own hoverbug free airships!
 
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MrTwister

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#9
Nice guide, but linear motions don't nearly have the utility of the hoverbug the way they are implemented.

Hoverbug provides constant thrust completely independent of control schemes and doesnt interfere with other motion and other vertical thrust engines you may have on the tech.

Linear engines not only occupy an (vertical) axis on the control scheme, they also disable your ability to control that axis through momentary full-power thrust, as they turn the axis into a slider which take a lot of time to change direction either way.

So, once you place LMEs on vertical axis, you lose full power "feather trigger" control on the thrust and any other engines you place on the axis have to stick to their power setting as well and have to be controlled by the same keys. Which sucks big way.

While it may be good for certain zeppelin-like airships and other slow-moving air things with limited vertical mobility, it certainly cannot replace hoverbug for most techs, especially the agile ones.

If there was a block to completely exclude LMEs from the control schemes to be controlled by the control block instead, it could be much better. But without it - it is vastly inferior to hoverbug, not to mention that it has a lot less power per unit of space as well.
 
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Saelem Black

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#10
@MrTwister , while I appreciate your thoughts, I do not want this guide to turn into be a debate thread on the merits of the hoverbug versus LMEs. I've made no assertions about hoverbug for that purpose. I'm simply providing a guide on how to build with LMEs if you want to.

However, I will absolutely defend the utility of LMEs. As such, I wholesale object to this comment.
While it may be good for certain zeppelin-like airships and other slow-moving air things with limited vertical mobility, it certainly cannot replace hoverbug for most techs, especially the agile ones.
A big reason for this guide is precisely because this isn't the case, but people often don't realize it, usually because very few community members are well experienced in building with LMEs. In point of fact, both the tech snapshots included in this guide are clear examples of very agile techs which use LMEs. However, you can't pilot them in the same way you'd pilot a hoverbug ship, which is the point I've been trying to make.

I agree with you that LMEs have their limitations. They do need to be stronger, and vectored lift could use improving overall. But these are in no way prohibitive, you just have to practice thinking in a slightly different way than hoverbug.

Feel free to respond, but let's try to keep this on topic - specifically, how to build with LMEs (not whether to build with LMEs).