Mk-55 "Thud" Liquid Fuel Engine

[More Boosters]

Hello, fans of this engine (there are fans for everything so there must be some fans for this) and others (including me), I would like to ask what this engine is good for if anything at all.

It is radially mounted, but it has some problems, namely:

1. It comes in at the same time as the Terrier and the Poodle is just one node away; making it a rather unconventional choice for a vacuum engine.

2. Since it's radially mounted you must use symmetry of some sort; as such you'll always have more than one Thud and that's not good for Delta-V.

What do you have to say about the Thud?

[yorshee]

I use it to deliver heavier payloads via the skycrane method, it's pretty good at doing that.

[FancyMouse]

Thud is great for skycranes that stick at the top of another (usually big) payload to land somewhere where you need thrust pointing down.

And no, stacking multiple engines doesn't necessarily hurt dV. I believe 3 LV-909 will have the same mass as LV-T45, but 909 has better vac Isp, meaning they will actually perform better than a single LV-T45 in terms of dV given the same amount of fuel regardless of payload mass - of course, atm Isp and TWR will be worse. My point is, it's not always true that multiple engines are bad, so don't feel to bad if you need to stick multiple engines because you have need for TWR or other design considerations.

[juanml82]

It can be useful for landers or medium sized rovers. Also, while the Poodle is one node away, you also need the LT-2 landing strut, as the LT-1s are shorter than the Poodle, so the Thud remains a good choice for landers for a short while

It's a bit of a niche engine, as it trades specs for radial mount. Eventually, if I need a lander which doesn't have an engine at the bottom, I tend to sacrifice gimbal and place a pair of aerospikes beneath radially attached fuel tanks.

[More Boosters]

Thud is great for skycranes that stick at the top of another (usually big) payload to land somewhere where you need thrust pointing down.

And no, stacking multiple engines doesn't necessarily hurt dV. I believe 3 LV-909 will have the same mass as LV-T45, but 909 has better vac Isp, meaning they will actually perform better than a single LV-T45 in terms of dV given the same amount of fuel regardless of payload mass - of course, atm Isp and TWR will be worse. My point is, it's not always true that multiple engines are bad, so don't feel to bad if you need to stick multiple engines because you have need for TWR or other design considerations.

I never said it was always true but thanks for the explanation anyway. ^^

[Bev7787]

I use it to launch payloads without using SAS as it has more gimballing range.

[CalculusWarrior]

If you think it's useless now, you should have seen it before 0.90. It was one of the worst engines in ISP, and so few used it. Fortunately it's been given a few buffs, but I agree with FancyMouse; its best usage is for skycranes or vehicles upon which you cannot fit standard engines.

[Laie]

I use it to launch payloads without using SAS as it has more gimballing range.

This. The Thud (and the other radials) are supposed to act as Vernier Engines: their main purpose isn't thrust, but control. Especially roll authority on a rocket that only has a single engine.

It's main problem is that it's too big, even on biggest single-engine lifter a pair of 24-77s will do the job nicely, thankyouverymuch. So it's actual use is mostly for skycranes and sci-fi type landers, where it's ISP makes you pay out of your nose for the looks.

[Edax]

It's a handy engine to test for a contract cause you can slap it on anywhere on any rocket you've already designed.

[AbacusWizard]

I've been using it on some of my Spacebus designs--rockets intended to launch crew into orbit, rendezvous with a space station or interplanetary craft, transfer crew, wait around for crew to return, and take them back down to the surface. Radially attached engines means the bottom of the stack can be a heatshield.

(Yes, I know bottom-of-the-stack engines make decent heatshields on their own; I didn't know that when I first designed it. I keep using it because I like the way it looks and it works just fine for its intended purpose.)

Side-mounted engines also work well when the bottom of the stack needs something else on it, such as a docking port or a harpoon gun or whatever..

[Norcalplanner]

I use it to deliver heavier payloads via the skycrane method, it's pretty good at doing that.

This. I've also used it on occasion to increase TWR on the first stage on an iterative design.

[cantab]

I feel it's clearly been rebalanced to aim it as a Shuttle OMS engine, especially with its big gimbal range. I will admit I rarely use it. It could be used to give extra thrust to a launcher stage, but I prefer to use SRBs for that.

[bos]

Extra 1st stage thrust. Reliant + 2 thuds can bridge the gap to a skipper.

As an extension of that, why have your second-stage engine be dead weight at launch when you can do this shenaniganry:

[mikegarrison]

In v0.90 I used this engine a lot for orbital tankers. I could take a big tank (3.75m or 5m with mods) and stick a big docking port (2.5m or bigger with mods) on both ends of the tank. Then the tanks could stack on each other as needed and still have docking ports on either side for other ships to attach to and refuel with. When they were almost empty I would de-orbit them. Using a combination of radial chutes and retro thrust right at the end, I could soft-land the tankers and get some money back.

Anyway, the point is that they are pretty decent engines if you need a big radial engine, and there are times when you do.

[Xannari Ferrows]

It is not useful for how I play, which has become small, compact probes over a while. It's kinda fun to have something so small do more than what most people would make something huge for.

[Nothalogh]

The only thing I've used it for is a 1.5 stage booster engine

[problemecium]

TBH I concur with OP. I've never once used the Thud for anything ever.

And sadly I don't actually have any ideas for a solution. If the Thud is even the tiniest bit underpowered, it'll be pointless, and if it's even the tiniest bit superior to its peers, people will just find clever ways of center-mounting it, making it silly to have it radially mount in the first place. TBH I'm thinking the whole radial / stack engine dichotomy ought to be dispensed with and perhaps a few radial engine adapters should be added as a replacement.

[Findthepin1]

I used six of them on a 16-kerbal Mun lander once. However, it had multiple bugs with the probe core and had insufficient fuel to land from and return to low Mun orbit without cutting things really close.

[Kilmeister]

As OP said, the usefulness of a lot of the parts in this game are a matter of style. I generally use the Mk-55 in place of where I would otherwise be using a poodle or skipper, but am trying to reduce the height of the rocket to reduce wobble. Also incredibly useful if you want to insert an intermediate piece of a station such as a large fuel tank and you want to strap docking ports on both ends, or when you have a tightly packed series of vessels that you are launching. In the past I have used them to provide supplemental thrust to lower stages where you need a tiny bit more thrust and don't want to entirely redesign your lift vehicle as well.

[jwenting]

This. I've also used it on occasion to increase TWR on the first stage on an iterative design.

yes, that's the main use I have for it. First stage needs a tad more TWR than my biggest engine can give without going to a wider stack, slap on 2 or 4 Thuds, derate them a bit as needed, and away it goes into the wild blue yonder.

Or ditto for other stages

It's also nice for heavy landers that don't have a lot of ground clearance. And putting the engines way out to the sides of a lander or space tug can make for better control authority as well.

[mythbusters844]

The only time I ever used the Thud was for this thing I did back when I first started playing KSP:

It's an ugly overengineered as heck skycrane for a rover that was so flip happy I never used it

Edited July 31, 2015 by mythbusters844

[SmallFatFetus]

They make nice vernier thrusters for 3.75m rockets with their high gimbal range.

[Captain Vlad]

Mount them to an LF tank on an jet-powered aircraft in the early game to reach altitudes that are difficult or impossible to attain with the basic jet engine. It's relatively light and works well for this, ever since I saw Wanderfound do it and totally stole the idea.

Used four on a two-stage-to-orbit rescue ship that used Rockomax tanks but MK 1 inline cockpits. Boosters got her high enough to make orbit with the Thuds, do the rescue, de-orbit and come down in one piece.

Landing engines, as others have suggested, especially for big ol' long duration semi-bases.

[RocketBlam]

They're pretty specialized, but I've found uses for them. One is for my Eve lander. Most of the engines are aerospikes, which have no gimbaling, so I stick a couple on there just so I have some directional control before the fins start to work.

[Dispatcher]

Yes, I find the 55 especially useful for landers and in tandem with other engines. For performance as a rocket trio or plane pair, see my engine test graph (not the old chart) in my sig line. You may find that they fill the gap between the Skipper and the LVT series in terms of mass lifting and altitude reach. The 55 can be seen to be to be capable for early stage in early career or intermediate stage later.

Edited July 31, 2015 by Dispatcher