Theory vs. Practice: The ARM parts.

[UmbralRaptor]

Have a table of SSTO designs:

(download file here)

Notably: using the new tanks knocks ~10% off of your payload (all other things being equal) due to their inferior mass ratios. The new engines pretty dramatically outperform the old size 2 ones. This is especially silly, as the LFB can be used as a drop-in replacement for the Mainsail, and is available at the same tech node. The LFB has slightly better TWR than the quad RS-25, but that gets lost in the chunky nature of KSP's fuel tanks.

More generally, there's the problem that these parts start to make sense "too early" in terms of payloads. Their thrusts are only 1.25 - 2.13x that of the Mainsail, so by the time you're considering using lofting an orange tank, they're the way to go. Compare with the Rockomaxx engines, where the Skipper's thrust is 3.02x that of the LV-T30, and the Mainsail's is 6.78x. Perhaps if they had thrusts of 4000 - 10000 kN (with comparable mass increases), there would still be a role for existing parts.

But really, this is just another general symptom of KSP needing another round of balancing, as half the parts obviate the other half. The Poodle has a clear role as an orbital transfer / lander engine but is generally worse at those than the venerable LV-T30. The LV-909 and 24-77 once had clear roles for lower mass craft (especially landers and probes), but those were eaten away by the 48-7S, and all but eliminated with its thrust buff. The surviving utility of the LV-909 is early in the tech tree. The Skipper has always suffered from being outclassed by arrays of LV-T30s (with perhaps the occasional LV-T45), and the Mk 55 only briefly had a clear role. The tricoupler was rendered superfluous back in 0.13, but nonetheless bicouplers and quadcouplers were added. The Mk1 fuselage is worse at storing fuel than any other aircraft tank, or even rocket tank. The Aerospike has been fixed from a reasonable though poorly executed nerf, but remains a hassle to use outside of (surprisingly efficient) launch stages for small payloads. Nosecones are generally better done without. Ditto adapters. The tech tree is the only reason not to use an okto2 over the other probes, and still leaves the RGUs without a reason to exist. A large number of parts (eg: cupola, Mk2 cockpit, TVR-300L, TVR-400L, docking ports, and perhaps SRBs) have rather WTFy drag values.

Please note that I am not complaining about these parts making the game "to easy." I am complaining that they are not well integrated into the rest. Buff/nerf, whatever. I'm somewhat concerned that the LV-1 and ion changes are solving the wrong problems, but that's a different issue.

[csiler2]

While probably true we have no idea to what degree. For one thing we have no idea what our budget will look like. For another the price listed on the parts is all over the map and make no sense. The Skipper, Mainsail, and KR-2L all have the same price listed. The same price would only get you 3 LV-T30s. The thrust to weight and thrust to cost ratios as they stand are drasticly in favor of the KR-2L over a cluster of T30s.

If the prices are not placeholders then any time you need 4 or more LVT's in a cluster your better off with the KR-2L. If you want to argue that the prices are placeholders(and I'd agree) then we are left with absolutely no way to evaluate relitive costs of various rocket building strategies making this whole argument pointless till squad releases more info on the currency system. Logicly smaller rockets will be cheeper but were playing a game where little green men with 50% of their body mass contained in their heads gleefuly ride unsafe rockets to wherever, logic kinda got left at the door a long time ago.

I want to emphasize a somewhat ironic section from this post "Logicly [sic] smaller rockets will be cheeper [sic] but were [sic] playing a game where little green men with 50% of their body mass contained in their heads gleefuly [sic] ride unsafe rockets to wherever, logic kinda got left at the door a long time ago."

If logic left the game a long time ago when relating to the potential budget system, why are we complaining about unbalanced engine performance?

It's true we don't know what our absolute budget is, but it's still important to compare the baseline costs of the parts involved. The tanking prices are all sorts of strange with Orange tanks holding less but costing more than their ARM counterparts, however just looking at engine prices shows that you're paying a massive premium for ARM engines. The S3 KS-25x4 and LB KR-1x2 both cost 5900 credits, over twice the cost of re-priced Mainsail at 2850 (iirc it used to cost 850), yet neither provides twice the performance of the Mainsail. Is it going to make sense for you to spend the extra money? My guess is no, unless you have a payload that falls into the pricing sweet spot where ARM engines are more cost-effective than Mainsails (eg one S3 would suffice in place of 3 MS CBCs), then you're probably going to want to opt for mainsails, assuming tank prices are rebalanced. The only engine that doesn't obviously fit into the program is the KR-2L which has better performance than the Skipper and Mainsail, but costs the same. I suspect it should probably cost 5900, just like the other ARM main-stage engines, but only time will tell.

[problemecium]

this is kind of silly.

Really all that proves is that a large SSTO can easily do what a smaller non-SSTO can do. I've been building single-stage lifters since well before ARM. It's all a matter of having the proper ratio of fuel tanks to engines, and my hunch is that it was very sub-optimal in both designs; the second especially could easily carry a great deal more fuel with that engine, you just didn't include it.

[Dispatcher]

For more comparisons, see the graphs linked in my sig line; load tests based on flight tests. The "50% payload" means that the other half is fuel used in the flight. NO need to see the chart yet; I'm waiting on the Wiki to be updated before I put up my latest chart, as I still lack TWRs.

Edited April 8, 2014 by Dispatcher

[UmbralRaptor]

Updated, but you can calculate TWR directly from ingame / part.cfg stats.

Also, I distrust an analysis that seems to lock mass ratios at ~2.

[Wahgineer]

The new parts are overpowered, if you use them for satellites. The thing is, every part size has a different purpose: the 3.75 meter parts aren't meant to lift a single crew vehicle, they're meant to launch the sections of interplanetary ships into orbit, to move asteroids into stable orbits, to build a rocket with the same capabilities as before, just with less parts.

[Joonatan1998]

The problem isn't that they can launch interplanetary ships to orbit, the reason they are OP is that they can act as single stage interplanetary ships.

[karolus10]

One of serious problems of engines with better ISP in vacuum is that thrust is not influenced by atmospheric pressure (atmospheric curve in KSP) but fuel consumption - low atmosphere ISP for this engine should be significantly lower but this causing engine to suck fuel at ridiculous rate.

IRL fuel flow is constant for the engine and max thrust is available only at certain pressure range (sea-level, vacuum or something in between for compromise) and thrust is lower in case of under/overexpansion of the exhaust... in short engine should consume same amount of fuel all the time it's turned on and it's thrust should change with altitude (so vacuum engines could be nearly useless on ground in extreme cases).

Also I'm not big fan of single chamber engine of this size I'm rather into clusters (Imagine 3.75 Skipper cluster assembly tweekable between 2,4 or 5 engines) of smaller engines, bigger engines aren't more efficient nor cheaper and harder to throttle or even start them up (not mention engine restarts) It's like kerbal equivalent of M1 engine, monster much bigger than Saturn V F-1 engines and able to replace 5 engines in the Saturn second stage with just one.

EDIT_1: Also I Like new tanks, this diameter tanks where needed in stock from a while.

EDIT_2: Other thing that you should take into account that stock engines TWR is pretty much horrible, in real life rocket engines are pretty lightweight when compared to it's trust (for example Space shuttle main engine it's not that much bigger than skipper/mainsail, weight bit less than skipper and had slightly over 2000 peak thrust).

TWR disproportion between old and new engines are very visible if You cluster "size 1" engines to match thrust of KR-2L at launch.

Here You go, 8x Lv-T30 and 4x 45's with combined thrust of 2520 kilonewtons, only 20 more than KR-2L.

Yet this cluster weight 16 metric tones, nearly 2.5 times more than new engine.

Edited April 8, 2014 by karolus10

[maccollo]

Really all that proves is that a large SSTO can easily do what a smaller non-SSTO can do.

Both rockets have the same mass (197), and both launch the same payload . In fact the single stage KR-2L rocket weighs a about 1 ton less (196). So it got an even better mass fraction.

I've been building single-stage lifters since well before ARM

Of course. The mainsail can single stage about just under 8% or so, which means it would have to be much more massive.

and my hunch is that it was very sub-optimal in both designs; the second especially could easily carry a great deal more fuel with that engine, you just didn't include it.

The KR-2L provides 255 tons worth of thrust, and the rockets weighed around 196 tons, so the TWR was about 1.3.

The first one could obviously be optimized somewhat more with fuel lines and what not, but I think it shows quite clearly how much more efficient the new engines are as lifters when a single stage is roughly equally efficient as a 2.5 stage design.

Edited April 8, 2014 by maccollo

[ScottyDoesKnow]

Both rockets have the same mass (197), and both launch the same payload . In fact the single stage KR-2L rocket weighs a about 1 ton less (196). So it got an even better mass fraction.

Of course. The mainsail can single stage about just under 8% or so, which means it would have to be much more massive.

The KR-2L provides 255 tons worth of thrust, and the rockets weighed around 196 tons, so the TWR was about 1.3.

The first one could obviously be optimized somewhat more with fuel lines and what not, but I think it shows quite clearly how much more efficient the new engines are as lifters when a single stage is roughly equally efficient as a 2.5 stage design.

I think the issue here is that it's not clear what side you're on maccollo. I first saw your video and didn't know what side you were on. Then I read the sentence after your video and assumed you were saying they're not overpowered because they did the same payload. But it sounds now like you're saying they're overpowered because they could single stage the same payload (in which case you're on the same side as parameciumkid).

[maccollo]

I think the issue here is that it's not clear what side you're on maccollo. I first saw your video and didn't know what side you were on. Then I read the sentence after your video and assumed you were saying they're not overpowered because they did the same payload. But it sounds now like you're saying they're overpowered because they could single stage the same payload (in which case you're on the same side as parameciumkid).

They are obviously overpowered in terms of TWR/ISP, and their range of utility overlaps that of the mainsail, essentially making the mainsail completely obsolete. If I had used staging the KR-2L would have beat the mainsail/skipper rocket by a huge amount.

So for anything above 20 or so, use the size 3 parts, and given the extremely high ISP of the KR-L2 that might extend below 20 tons if you stage.

Edited April 8, 2014 by maccollo

[Capt. Hunt]

My whole problem with these arguments about why we don't need 3.75 meter parts, is that nothing stops NASA from building bigger rockets. Why build a Saturn V to go to the moon when they could have strapped a few dozen Redstones together, eh? (that's totally a random example, I have no idea if that would work IRL, but it follows the same logic of these arguments.) I personally like these new parts, it allows me to build heavy rockets without resorting to massive amounts of asparagus staged boosters.

[Rakaydos]

My whole problem with these arguments about why we don't need 3.75 meter parts, is that nothing stops NASA from building bigger rockets. Why build a Saturn V to go to the moon when they could have strapped a few dozen Redstones together, eh? (that's totally a random example, I have no idea if that would work IRL, but it follows the same logic of these arguments.) I personally like these new parts, it allows me to build heavy rockets without resorting to massive amounts of asparagus staged boosters.

What if you could use 3m parts to build TALLER rockets? sure, they'd burn through fuel a lot faster, and wouldnt be able to single stage to jool, but they'd be able to lift even bigger things through the atmo than now?

If so, you should try the Stock Rebalance mod.

[Dispatcher]

Updated, but you can calculate TWR directly from ingame / part.cfg stats.

Also, I distrust an analysis that seems to lock mass ratios at ~2.

Yeah, I see there is beginning to be update activity at the Wiki. I had roughly calculated the TWR of the new engines, but I'll wait and include the published TWR for consistency.

As for flight tests with ships fully maxed in mass, and with half that mass being usable fuel, I think that's a reasonable stress test. That fuel fraction is halfway between launching, and using, most of the mass as fuel (and having no payload to speak of) and either hovering or sitting on the pad and going nowhere.

[m4v]

My whole problem with these arguments about why we don't need 3.75 meter parts

Don't make **** up, nobody is saying that we don't need them or we don't want them.

I personally like these new parts, it allows me to build heavy rockets without resorting to massive amounts of asparagus staged boosters.

Balancing the engines to be in line with the rest won't stop you from doing that.

[Jefzor]

I just did a SSTO with just the big new tank + new "booster". Just a 3 part rocket, zero effort or thought involved. I'd say those stats could be toned down a notch.

If we're going to bring NASA into the argument: if it was that easy to SSTO, then why doesn't NASA do it?

[Xavven]

I just did a SSTO with just the big new tank + new "booster". Just a 3 part rocket, zero effort or thought involved. I'd say those stats could be toned down a notch.

If we're going to bring NASA into the argument: if it was that easy to SSTO, then why doesn't NASA do it?

It takes at least 9400 m/s of dV to get to low Earth orbit in real life, whereas in KSP it takes 4500 m/s to get to low Kerbin orbit.

Edit: I just did some fiddling with my delta-v calculator spreadsheet, and 9400 m/s of dV is impossible to get in a single stage with any non-air-breathing rocket engine in KSP. They seem to max out at 7439 m/s dV vac (6612 atmo). Turns out, it's darn hard to SSTO from Earth without much higher ISPs!

Edited April 16, 2014 by Xavven

[Killerdude8]

Sorry, what is the point of the currency system? Can you give me a link to prove your statement? Honestly, you have no idea how it will work or how it will limit launches so saying that putting a satellite into orbit with a grossly over-powered craft is somehow a bad idea remains to be seen.

Older tech: Cheap and easy to launch and mass produce

New Tech: Expensive to launch.

Nothing to prove, Just use common sense and basic logic.

[csiler2]

I just did a SSTO with just the big new tank + new "booster". Just a 3 part rocket, zero effort or thought involved. I'd say those stats could be toned down a notch.

If we're going to bring NASA into the argument: if it was that easy to SSTO, then why doesn't NASA do it?

You can also make a 3-part SSTO with a Skipper. What's your point?

[masternerdguy]

I just did a SSTO with just the big new tank + new "booster". Just a 3 part rocket, zero effort or thought involved. I'd say those stats could be toned down a notch.

If we're going to bring NASA into the argument: if it was that easy to SSTO, then why doesn't NASA do it?

I've been reading these forums for a while, but the recent ARM debate has gotten me to finally register

You can make SSTOs really easily, but that isn't the point. The point is to put something into space that does something. The new parts are excellent, before these parts I was strapping orange fuel tanks and skippers / mainsails together in onion staging which was slow, tedious, and kind of unrealistic.

Now, I can take that pack of orange fuel tanks and replace it with one large booster and engine, which means the first stage of my rocket is far simpler and pleasant to look at. The new parts aren't OP, they are just replacements for the old way of strapping together a bunch of orange fuel tanks and main sails to make the first stage of your heavy lifter.

You don't even need the new parts for a lot of stuff, they are really just there to make heavy lifters. It's the thing you are lifting that matters.

As for balance, these new parts are expensive. Once career mode rolls into full swing I'm willing to bet these are going to be a little less attractive.

[Jefzor]

You can also make a 3-part SSTO with a Skipper. What's your point?

Oh, I hadn't tried it before, so I was surprised a simple biggest pod + biggest tank + booster thing worked on first try. For some reason I had the impression that SSTO took effort and fiddling with jet engines and whatnot.

It takes at least 9400 m/s of dV to get to low Earth orbit in real life, whereas in KSP it takes 4500 m/s to get to low Kerbin orbit.

Edit: I just did some fiddling with my delta-v calculator spreadsheet, and 9400 m/s of dV is impossible to get in a single stage with any non-air-breathing rocket engine in KSP. They seem to max out at 7439 m/s dV vac (6612 atmo). Turns out, it's darn hard to SSTO from Earth without much higher ISPs!

So the engines may be somewhat realistic, but Kerbin itself is much easier to lift of from.

[csiler2]

Oh, I hadn't tried it before, so I was surprised a simple biggest pod + biggest tank + booster thing worked on first try. For some reason I had the impression that SSTO took effort and fiddling with jet engines and whatnot.

Most people think of spaceplanes as the only SSTO option, and yes, they are difficult to build (I still haven't built a successful SSTO), but SSTO rockets are certainly possible. They're easier because the only requirement you need to worry about is including enough fuel to get into space and enough to return to the planet.

Scott Manley has made a few rocket-based SSTOs that use jet engines as their first stage to save weight, though most of the ones I have seen him use are not stock and rely on B9 parts to work.

I agree the engines (and tanks) are more realistic in their performance. I believe SQUAD made Kerbin smaller than Earth to ensure that it would be easier to achieve orbit, certainly to make sure it takes a bit less time.

Personally, I don't see what the fuss is all about. The ARM parts ARE better than the Rockomax parts, but it looks like they will probably be balanced through budgeting rather than pure performance and it makes tackling high ∆v missions a lot easier for those of us without beefy computers and exceptional rocket building skills. ARM parts lifting power is all but a requirement to lift enough ∆v into orbit for capturing asteroids in my opinion.

[m4v]

You can make SSTOs really easily, but that isn't the point. The point is to put something into space that does something. The new parts are excellent, before these parts I was strapping orange fuel tanks and skippers / mainsails together in onion staging which was slow, tedious, and kind of unrealistic.

Now, I can take that pack of orange fuel tanks and replace it with one large booster and engine, which means the first stage of my rocket is far simpler and pleasant to look at. The new parts aren't OP, they are just replacements for the old way of strapping together a bunch of orange fuel tanks and main sails to make the first stage of your heavy lifter.

And balancing ARM parts won't prevent you from doing just that, just like the people using the balanced mod have shown. The notion that having balanced parts means that you will invariably have to resort to asparagus/onion staging for any launch is wrong.

[csiler2]

And balancing ARM parts won't prevent you from doing just that, just like the people using the balanced mod have shown. The notion that having balanced parts means that you will invariably have to resort to asparagus/onion staging for any launch is wrong.

How do we decide whether or not the parts are balanced? Is it going to be some mathematical mixup of Thrust/ISP/TWR? Just a couple of those factors? Just one? As SQUAD considers ARM an 'official mod' should we also require other modders to adhere to an arbitrary balance? Should we consider cost?

As ARM was in parallel development with .24, I think we should hold off on serious discussion over 'balance' until we see what happens when the budget system comes in. I know, there are counter arguments that we don't even know a budget system will be put in place, but I'm feeling pretty confident about it based on the developer tuesday notes. As we all like to look to the real world for inspiration about how things in KSP should be done, please consider the following:

Like ARM parts, the Saturn V has a huge advantage over other current launchers. It could carry almost five times more payload into LEO than the Delta IV Heavy, the current heaviest lifter. My goodness, you say, that's just unfair that the Saturn V's performance is so much better than Delta, Falcon 9, Atlas, and other boosters! Well, we stopped making them and launching them because they were too darned expensive at $1.2bil per launch (vs appx $170mil for a DIV-H launch).

Now look at the cost of ARM parts compared to older KSP parts. Based on raw performance specs you are not nearly getting the same bang for your buck unless you want to launch really heavy things AND you have the budget for it. That sounds a lot like the Saturn V! Lots of lift capability, but very expensive. The Space Shuttle is another example of an item that does its job pretty darned well, it could lift just a bit more payload than a Delta IV Heavy, but it cost significantly more to launch at appx $450mil.

Yes, we don't know exactly how the finance system is going to work, but as we know we're going to have a budget of some sort, players are probably going to want to spend their budget as efficiently as possible. That's going to limit their ability to use ARM and maintain a sustainable space program as they look to keep their budget balanced. You'll probably end up having a lot of small, cost-effective launches with the occasional big-budget spender.

Besides, in the end, it's just a game. The 'balance' of rocket engine performance isn't as important as balancing weapons in an FPS because you're not competing with anyone. Yes, the ARM parts perform better, but it's not like you can throw an oscar tank on top of an ARM engine for a quick trip around Jool and back.

[Jouni]

Now look at the cost of ARM parts compared to older KSP parts. Based on raw performance specs you are not nearly getting the same bang for your buck unless you want to launch really heavy things AND you have the budget for it. That sounds a lot like the Saturn V! Lots of lift capability, but very expensive. The Space Shuttle is another example of an item that does its job pretty darned well, it could lift just a bit more payload than a Delta IV Heavy, but it cost significantly more to launch at appx $450mil.

There are two metrics on which the Space Shuttle was quite efficient: mass fraction to orbit and the cost of launching that mass to orbit. The Space Shuttle was able to put roughly the same mass into orbit as the Saturn V, for about 2/3 of the launch mass. Most of that mass was not payload, but a more traditional rocket based on the same technology (such as the SLS) would easily beat the Saturn V.