Is Swivel almost as efficient as Terrier?

[temetvince]

Obviously the swivel engine has more mass than the terrier, but their vacuum ISP's only differ by 25. Does this mean the more fuel(mass) we add, the closer these two engines become to being about the same in total delta-v?

Background:
Hello, I've come back to ksp after a long time absence. I don't have any mods installed and am trying to play completely "stock". I've been trying to calculate all my delta-v without a mod, but I'm not sure it's worth it yet or not. Is there a hotkey to automatically empty or fill all fuel tanks? This would speed up the process considerably.

Edited January 1, 2016 by temetvince

[Snark]

25 minutes ago, temetvince said:

Obviously the swivel engine has more mass than the terrier, but their vacuum ISP's only differ by 25. Does this mean the more fuel(mass) we add, the closer these two engines become to being about the same in total delta-v?

Background:
Hello, I've come back to ksp after a long time absence. I don't have any mods installed and am trying to play completely "stock". I've been trying to calculate all my delta-v without a mod, but I'm not sure it's worth it yet or not. Is there a hotkey to automatically empty or fill all fuel tanks? This would speed up the process considerably.

The Terrier is more efficient.  It has a higher Isp, which means it uses less fuel, and it has less mass, which means it's less to lug around.  The higher Isp means that it will always give you more dV than a Swivel would, regardless of how much fuel you're carrying:  7.8% more just for the difference in Isp, even without taking its lower mass into account.

So, whether it's "almost" as efficient depends on your definition of "almost".    To me, 7.8% of extra dV is nothing to sneeze at, and the mass savings is a lot.  If I'm lugging enough fuel mass that the mass savings of a Terrier becomes less significant, then I'd say it's time to start thinking about an LV-N.

That said, the Terrier is awful for atmospheric launch (relatively low thrust, and abysmal atmospheric Isp).

So:  for upper stages, Terrier is better, as long as it has enough thrust for your application.

(Note:  "upper" doesn't have to mean "space"-- even at 10 km altitude, the atmospheric pressure is low enough that a Terrier is a reasonable engine choice. Just don't, under any circumstances, try to use it on the launch pad.)

There's no hotkey for filling tanks.  It's the same as it's been since forever:  right click a tank, alt-right-click one or more additional tanks, then choose the "In" or "Out" buttons to transfer fuel.  (I think it transfers faster than it did before 1.0, though.)

Edited January 1, 2016 by Snark

[Geschosskopf]

If  you right-click on a tank in the VAB, a menu will come up showing how much fuel is in the tank.  You can then drag the slider up and down to put however much fuel in it  you want, from 0 to full.

As for the relative values of the engines, it depends on what you're using them for.  Their most obvious difference is their thrust, the T-45 being considerably more powerful than the 909.  So if you need to make big burns with a heavy ship, the T-45 will do it in less time.  But it will burn more fuel doing it.

[GoSlash27]

temetvince,

 There are no circumstances where an LV-T45 will approach the vacuum efficiency of an LV-909. We wouldn't rate them by DV, since that's a design consideration for the stage itself. A better way to compare them would be payload fraction at a desired DV.

 As a case approaching the extreme, let's say we want a 2km/sec DV and .5 thrust to weight. This is about the point where the LV-N becomes preferable to both.

The LV-909 can handle 5.5t of payload and the whole stage would weigh 12.7t (43% payload)

The LV-T45 can handle 17t of payload and the whole stage would weigh 41.7t (41% payload)

Looking at the cost, the LV-909 assembly would cost $2,190 ($398/tonne) , while the LV-T45 assembly would cost $7,500 ($441/tonne)

Happy New Year!,

-Slashy

 

[Wcmille]

3 hours ago, temetvince said:

Obviously the swivel engine has more mass than the terrier, but their vacuum ISP's only differ by 25. Does this mean the more fuel(mass) we add, the closer these two engines become to being about the same in total delta-v?

Background:
Hello, I've come back to ksp after a long time absence. I don't have any mods installed and am trying to play completely "stock". I've been trying to calculate all my delta-v without a mod, but I'm not sure it's worth it yet or not. Is there a hotkey to automatically empty or fill all fuel tanks? This would speed up the process considerably.

When I used to hand-calculate dV, I took the mass (which you can get from the information window in the Map View) and just subtracted the resources to get dry weight. Since I used a spreadsheet, I just typed in the resource values and let the spreadsheet multiply the units of fuel times the weight per unit of fuel. That way, I didn't have to fill and drain my tanks in design either.

Edited January 1, 2016 by Wcmille
Incomplete answer.

[GoSlash27]

4 hours ago, temetvince said:

I've been trying to calculate all my delta-v without a mod, but I'm not sure it's worth it yet or not.

Oh, I forgot to address this...

 I have my spreadsheet set up this way:

It's absolutely indispensable! No mucking about in the VAB trying to guess which combination of fuel and engines will get me my DV and t/w. No guesswork about which engine is best. I just key in the mission specs and it tells me exactly what to build.

 Virtually all of my rockets come from this spreadsheet.

Best,

-Slashy

[temetvince]

Thank you all for the amazing responses. They helped me a lot. 

I always assumed the atmosphere isp was for anything less than 70k. Seeing as its generally around 10k opens up new doors.

And GoSlash27, that spreadsheet is amazing! I'm going to have to come back to it for sure. Thank you. I broke down and installed KER yesterday, and it's been invaluable. Unfortuantely, I just suck at this game even though I've played it on and off for years. Even with ker, the delta-v charts I use... I can't get anywhere close to the numbers they show. The furthest I've ever made it in this game was Duna (with MechJeb), and that was a 1 way trip.

I think I'm going to do a full career-mode with mechjeb, including visiting all planets and such. And eventually, once I start to learn what in the world I'm doing wrong so much, I'll probably start again without any mods at all. It's getting frustrating to be so inefficient at everything, no matter how many flights and how much tutorials/videos/reading!

[Snark]

2 hours ago, temetvince said:

I always assumed the atmosphere isp was for anything less than 70k. Seeing as its generally around 10k opens up new doors.

It's not an either/or thing, there's no sudden switch that happens at 70 km.  It's a smooth curve.  At sea level you get atmospheric Isp, at 70 km you get vacuum Isp, and in between it's a smooth curve.  By the time you're at 10 km, you're only a small percentage of sea-level pressure, so you're practically in a vacuum already as far as the engines are concerned.  By the time you're at 25 km, the pressure is under 1%!

So at 10 km, the engines won't be as good as in a vacuum... but their Isp will be a lot closer to the vacuum value than for the atmospheric value.

2 hours ago, temetvince said:

I think I'm going to do a full career-mode with mechjeb, including visiting all planets and such. And eventually, once I start to learn what in the world I'm doing wrong so much, I'll probably start again without any mods at all. It's getting frustrating to be so inefficient at everything, no matter how many flights and how much tutorials/videos/reading!

If I might make a suggestion-- try it the other way around?  Do it without MechJeb first, so you can learn to do it the hard way.    Once you've done that, then if you like, switch to MJ to reduce tedium (if you find it tedious).

Have never used MechJeb myself, I like doing stuff by hand.  But from what I've seen in the forums, I get the impression that if you start out with MechJeb, it can be harder to wean yourself off it.

Any MechJeb users in here who'd like to chime in?

[GoSlash27]

I'm with Snark on this one.

A lot of players aren't interested in the nuts and bolts. They just want to build rockets with little fuss and have fun. Certainly nothing wrong with that, *BUT*...

If you're interested in understanding the math and improving your engineering skills, it's best to not let yourself become dependent on aids like KER early on. It'll just make it more difficult for you later.

Best,

-Slashy

Edited January 3, 2016 by GoSlash27

[technion]

That curve can be hard to predict. I had a terrier in my Laythe launcher and, even at 30km alt, it turned out to be hampered by the atmosphere enough that it pulled a TWR of <1 and wouldn't make orbit. I was sure the combination of Laythe's thin atmosphere, and that height would do it. But no, I forget the exact thrust output, but it was nowhere near what it was capable of.

[MagicFireCaster]

Swivel is always a problem because it is in a "limbo" between Atmospheric and Vacuum efficiency but not one of those two, Swivel would be a good choice in middle stages when you're high enough, like when your small solid busters run out, also on places like duna with a thin atmosphere, since getting to an altitude were the engine will be optmal will take no time, it's pretty nice for medium landers and spaceplanes on places like that.

If you ask me i've never used it, if i started a new career understanding is advantages i would use the swivel for optimal entrances to orbit. 

[Snark]

4 hours ago, MagicFireCaster said:

Swivel is always a problem because it is in a "limbo" between Atmospheric and Vacuum efficiency but not one of those two, Swivel would be a good choice in middle stages when you're high enough, like when your small solid busters run out, also on places like duna with a thin atmosphere, since getting to an altitude were the engine will be optmal will take no time, it's pretty nice for medium landers and spaceplanes on places like that.

If you ask me i've never used it, if i started a new career understanding is advantages i would use the swivel for optimal entrances to orbit. 

I use Swivel all the time, it's a great workhorse.  I almost always use SRBs to get off the pad, and by the time they burn out, the atmospheric pressure has dropped enough that the Swivel's Isp has climbed above the Reliant's.  It makes a great second-stage engine for 1.25m stacks.  Lots and lots of my 1.25m launches are 1. SRBs, 2. Swivel, 3. Terrier.

[MagicFireCaster]

4 hours ago, Snark said:

I use Swivel all the time, it's a great workhorse.  I almost always use SRBs to get off the pad, and by the time they burn out, the atmospheric pressure has dropped enough that the Swivel's Isp has climbed above the Reliant's.  It makes a great second-stage engine for 1.25m stacks.  Lots and lots of my 1.25m launches are 1. SRBs, 2. Swivel, 3. Terrier.

Maybe i'll start using it more, i made a small munar lander i sended to the mun north pole. Too bad i didn't pack enough fuel to get back, but with some tweaks it will make it next time

[Snark]

11 minutes ago, MagicFireCaster said:

Maybe i'll start using it more, i made a small munar lander i sended to the mun north pole. Too bad i didn't pack enough fuel to get back, but with some tweaks it will make it next time

Sounds like a plan. 

Regarding the Swivel-- I almost never use it beyond LKO.  In fact, I almost never send it all the way to orbit-- it's a great 2nd stage engine and I use it all the time, but once you're boosted to the point where TWR doesn't matter much, the Terrier is a far superior engine (much lighter, and has the second-best LFO vacuum Isp in the game).  So I'd never send a Swivel to to the Mun-- that's what the Terrier is for.  (Or the Poodle, if I've got something big.  Or the Spark, if it's something really tiny.)

[MagicFireCaster]

31 minutes ago, Snark said:

Sounds like a plan. 

Regarding the Swivel-- I almost never use it beyond LKO.  In fact, I almost never send it all the way to orbit-- it's a great 2nd stage engine and I use it all the time, but once you're boosted to the point where TWR doesn't matter much, the Terrier is a far superior engine (much lighter, and has the second-best LFO vacuum Isp in the game).  So I'd never send a Swivel to to the Mun-- that's what the Terrier is for.  (Or the Poodle, if I've got something big.  Or the Spark, if it's something really tiny.)

Yup i made a little mistake and my swivel got in a higher stage than i needed, with a terrier in there the flight could be better.

[cantab]

I don't see the Swivel and Terrier as really in competition, the former is a significantly more massive and more powerful engine.

The fairer comparison is Swivel VS Poodle. In vacuum the Poodle is superior in both TWR and Isp, but the Swivel has a little less absolute mass, so in practical designs they can perform very similarly. The Poodle is usually the better option though I think, except of course on a first stage where it takes an Isp nosedive.

[Snark]

37 minutes ago, cantab said:

I don't see the Swivel and Terrier as really in competition, the former is a significantly more massive and more powerful engine.

The fairer comparison is Swivel VS Poodle. In vacuum the Poodle is superior in both TWR and Isp, but the Swivel has a little less absolute mass, so in practical designs they can perform very similarly. The Poodle is usually the better option though I think, except of course on a first stage where it takes an Isp nosedive.

I don't see Swivel & Terrier as competition either, since I never use them for the same purpose.    Swivel is good for 2nd stage, i.e. from SRB burnout to suborbital trajectory.  Terrier is for use in orbit.

The Terrier and the Poodle occupy the same ecological niche, with the Poodle good for larger applications.  Anything that needs more than 3 Terriers, I switch to a Poodle.

The Swivel's not a terrible engine for use in orbit... I just end up not using it there because a Terrier or Poodle works better in that situation.