How come "Thuds" aren't used often? (Looking at Other's Videos and pics)

[JoeSchmuckatelli]

Been checking out other people's work, and I'm surprised I don't see my lander engine of choice used more often.  In fact, what I usually see is something like 4 "Spark" engines on 4 FL-T800 fuel tanks (given 4-way symmetry).  I typically just put two thuds on 2-way symmetry on a single X-200 type fuel tank.

 

Is this a hold-over from the way things were done back in the day - or is there a good reason to not use "Thuds" in the current build?  (B/C... you know... they do work after all.)

 

(FWIW - my Thud's are typically used as the final thrust to SOI change, insertion Pe to gain orbit, land, take off and return to  orbit then to orbit around Kerbin all in the final stage from Minimus / Mun (haven't gotten further afield yet).)

Edited January 26, 2016 by JoeSchmuckatelli

[wibou7]

The "Thud" has a HORRIBLE isp of 305 in vaccum. You 'd better use two Terrier (isp 345) instead, it would have the same thrust for just an additional 0.1 ton of weight. Remember, "isp" is the measure of how efficient an engine is... So that basically mean that for the same amount of fuel, the terrier will give you more delta-v (so it will get you farther).

I sometime use the "thud" for VTOL aircraft as it somewhat fit in a cargo bay... out of those niche use, I don't think you should ever use it.

 

[Kuansenhama]

I think it's the lower ISP that scares most people off. It all depends upon your playstyle and how you build rockets. A part that might seem useless to one person might be essential to someone else's play.

I know that I like everything I build to be in neat, tidy 2.5M stacks whenever possible. I almost always use Clamp-O-Tron sr.s as soon as I have them unlocked, and the engines that I'm always using are the skipper and poodle.

Admittedly I don't use the Thud outside of launcher stages. High gimballing and thrust, and they're pretty cheap, but their ISP is low, and their thrust is higher than what I need for orbital/landing operations, so I don't see a reason to have them on anywhere but launch.

[Empiro]

Since you need to use the Thud in pairs, there's almost no reason to prefer it over the Poodle, which has more thrust than the pair of Thuds at less mass and a far better ISP.

The only time you might want to use the Thud is if you absolutely need a radial attached engine, which is pretty rare.

[Snark]

Poor Isp.  Radially attached = drag.  Lackluster TWR.

So, basically, no reason to use it, unless you have a design where you really can't fit a stack-attached engine.

[mikegarrison]

They are my go-to engine for specialized things like an orbital fuel tank where I want clamp-o-trons on both ends. And occasionally I might use them for a little bit of extra thrust if that's the only way to get it (probably because I haven't unlocked a higher thrust engine I would really rather be using).

[GoSlash27]

The Thud is underappreciated as an early career booster, but not very good as a vacuum engine.

 KSP makes or breaks an engine by the tech level where it becomes available. The LV-T30 maximizes your payload to orbit within an 18t pad limit, but has no thrust vectoring. The LV-T45 limits your payload, but simplifies control. The Thud makes a nice compromise between the two, but becomes eclipsed by the Skipper once you upgrade the pad.

 For vacuum engines, the Terrier is unlocked very early and the Thud can't compete with it in that role.

Best,
-Slashy

[JoeSchmuckatelli]

Hmmm.  Okay - I can see how a single thruster underneath the fuel tank will have less drag - and I understand (I think) about the efficiency.

 

But that doesn't answer why people use 4 symmetrical fuel tanks with small inline thrusters (which had to have been launched with higher drag than two Thuds...)

[Snark]

7 minutes ago, JoeSchmuckatelli said:

But that doesn't answer why people use 4 symmetrical fuel tanks with small inline thrusters (which had to have been launched with higher drag than two Thuds...)

Because the fuel tanks have fuel in them. 

It's always best to have a single central stack, if you can manage it... but sometimes you need enough fuel that the necessary stack would be too tall and wobbly.  So radial boosters may be in order.

For any given fuel tank configuration (whether it's one central stack, or with radial tanks attached), it's always lowest-drag to put the engines underneath.

[fourfa]

Just in case someone wonders - it is possible to use only a single Thud.  Attach centered on the bottom of a rocket, rotate until the nozzle points down, then if you like you can clip the housing up into the rocket until just the nozzle sticks out.  The editor tools in VAB are powerful and most manipulations don't break anything in flight...

Its ISP still blows, clipping is cheating etc etc, but there are applications for a single small high-gimbal engine.

Edited January 26, 2016 by fourfa

[Reactordrone]

I use them on early 2.5m rockets to increase thrust when only the skipper is available. Two of them plus a skipper brings thrust into the 1000kN bracket for about 5t engine weight.

[JoeSchmuckatelli]

Well, I found this Thread where Frostiken explained the answer to my dilemma in This Post (above) (I assumed a radial 1.25 would be dramatically more draggy than the Thud - given the visual models):

Quote

... I decided to do a little test. The drag they produce isn't nearly as much as a radially-attached uncovered 'flat top' 1.25m tank, but it IS more draggy than a covered nosecone'd 1.25m tank. Considering its much smaller size and seemingly more aerodynamic profile (not to mention hugging the side of the rocket, which I have no reason to assume helps, but I assume it does), it should be LESS draggy.

Without doing any math - I've always assumed a single 2.5m tank (which holds as much fuel as the four 1.25s) with two radial engines would be a better solution than cobbling together four tanks and four engines - turns out its not... and I'm going to have to play with a new lander design...

 

[GDJ]

I use them on my interplanetary landers. I set them up so they can lift off on Kerbin Gravity, and then sandbag them to match whichever planet I'm visiting. They work great, and the high gimbal rate is handier then heck.

Stick 16 of them on a booster section and they can lift off a medium sized rocket and have lots of control.

 

Edited January 26, 2016 by GDJ

[mikegarrison]

2 hours ago, GoSlash27 said:

The Thud is underappreciated as an early career booster, but not very good as a vacuum engine.

 KSP makes or breaks an engine by the tech level where it becomes available. The LV-T30 maximizes your payload to orbit within an 18t pad limit, but has no thrust vectoring. The LV-T45 limits your payload, but simplifies control. The Thud makes a nice compromise between the two, but becomes eclipsed by the Skipper once you upgrade the pad.

 For vacuum engines, the Terrier is unlocked very early and the Thud can't compete with it in that role.

Best,
-Slashy

I'm playing a new career in hard mode, and it's dang tough to get enough money to open up the higher tiers. I just launched a rocket that maxed out the second weight limit. I don't have the Mainsail unlocked yet, and the Skipper just wasn't cutting it, as soon as my SRBs burned out. Tacked on four Thuds and that was the ticket.

[Plusck]

Thuds are the poor man's engine of choice for getting just that little bit more out of your lifter stages. More thrust, more control, more options (like adding more engine without adding more stacks). It also helps in mid career by letting you add a stage to a lander without adding a significant vertical height to it, etc.

I'm a bit disappointed to read that it adds so much drag, though. Wouldn't have expected that.

Since its Isp is low, it is theoretically a "better" choice when you are going slowly and propellant is cheap (i.e. lifting off from Kerbin). Its thrust doesn't quite make up for it though, so it only just squeezes into a niche between SRBs and main engines.

[KSK]

2 hours ago, JoeSchmuckatelli said:

Hmmm.  Okay - I can see how a single thruster underneath the fuel tank will have less drag - and I understand (I think) about the efficiency.

 

But that doesn't answer why people use 4 symmetrical fuel tanks with small inline thrusters (which had to have been launched with higher drag than two Thuds...)

Stability. Four tanks with a landing leg on each gives you a nice wide base for your lander, making it easier to land on slopes. That was always my reason anyway. 

[Wemb]

7 hours ago, GoSlash27 said:

 

 For vacuum engines, the Terrier is unlocked very early and the Thud can't compete with it in that role.

Best,
-Slashy

Indeed. I think I mainly use it in first stages if I need to provide a bit of additional thrust to enable me to use asparagus staging/saddle tanks but where the T30 or T45 would be overkill. I can't think of a good reason to take it into orbit most of the time.

Wemb

[RizzoTheRat]

I sometimes use the thud as a strap on extra engine if I'm testing a design that doesn't have enough TWR to take off on Kerbin.

For early game landers I go for radial tanks to keep it low and wide for landing on slopes, but a centrally mounted Terrier so I can drop the radial tanks when empty and fly home on a small central tank.  Other than that I like to go nuclear as soon as I can as my landers are generally designed to be reusable.

[WhiteKnuckle]

8 hours ago, JoeSchmuckatelli said:

 - and I understand (I think) about the efficiency.

You should be really, really clear on understanding ISP. It's one of the most important parts of good rocket design.

 

To put it simply: If you're using Thuds as "lander" engines, you're severely limiting your design because you have to bring a lot more fuel to get to the same place compared to a Terrier or Spark or Aerospike powered lander. Bringing more fuel means you need more engine mass to lift that fuel, means more dry weight, means the tyranny of the rocket equation comes and kicks you in the shins.

 

ISP is the first thing I consider when choosing an engine (aside from booster stages), I'll happily compromise where I can on things like TWR and partcount to try to use the highest ISP engine possible, because I know that using an efficient engine means every other part of my rocket can be smaller, lighter, faster, easier to control, and more awesomer.  

[AeroGav]

The Thud is really an alternative to a solid booster.  Early game, you might have a stack of type 1 fuel tanks with a Swivel at the bottom for the first stage.   I like to boost to 240 m/s as fast as I possibly can off the pad, to minimise gravity losses, then wait around at this speed till i'm above 8km before trying to go through the sound barrier.     Often this first stage will have poor TWR on launch, so I can achieve rapid boost to 240 in one of two ways.

The first is to strap some thuds to the side of your bottom fuel tank, via radial decouplers.   Punch them off when we hit 240.

The other is to put a stack separator below your swivel, a Flea below that, then radially attach four more fleas around this central Flea.    Blast off the pad with truly eye-watering levels of acceleration, before dumping them and relying on the Swivel sustainer.

The Thuds are better than solid rockets in terms of ISP and drag,  so you can achieve a lighter pad weight for same performance.   However it's still cheaper to do it with Fleas,   so I'm not sure it makes sense to always use the highest tech solution available.  Even if you are pad weight limited, you're probably going to upgrade the pad at some point regardless, so might as well bite the bullet and start saving money with cheaper, dumber rockets.

[JoeSchmuckatelli]

Grumble grumble grumble...

I have a knuckle dragger solution that works.  Now you guys are telling me my baby ain't as pretty as I thought she was!  (Grin)

Okay - back to the drawing board.

 

I guess I'll play with some SRBs at lower stages as well - I've been burning liquid fuel + Ox at the cyclic rate.

 

Thanks for the input folks - every time I post on here... I learn something new.

[MagicFireCaster]

Their radial attachment points made me forget about them since is a pain to deal with that extra drag, also they consume fuel pretty fast, they surely need to be buffed, around 335 Isp would make them better to use, since you got structural advantage using them.

[GDJ]

I agree that the Thuds do have parasitic drag that might be unwanted during liftoff, but when landing on a planet with an atmosphere (but not enough for a parachute or balloon to work), drag is kinda your friend here. More drag = slower descents = less fuel burnt.

Everything has it's place.

[Vegetal]

You guys are missing a point: It has huge gimbal range. It's like a tiny Vector, just not OP. I use it for small shuttle-like spacecraft, it's well suited to them.

I don't use it for anything else though, it's terrible in vacuum. Using it as a lander engine is a waste.

[MagicFireCaster]

They're definitively only useful on small shuttle things, but pretty bad for anithing else, that's why i said they shoul be buffed a bit, so you can use them as what they are, a radial engine