The new, the old, and the SRB Meta.

[Pds314]

First off I want to say I love the new SRB motors. I will probably use every single one of them fairly often.

Now, this is about balance. By which I mean every part is useful relative to every other, given the right circumstances to shine. All of them would be missed if they were gone. Let's start with a chart showing the performance of different motors.

SRB Performance specifications (With LFB included for comparison)

Name	Empty mass	Total mass	ISP (vac)	ISP (SL)	Thrust (vac)	Thrust (SL)	Mass ratio	Delta-V (vac)	Delta-V (ASL)	TWR (full, vac)	TWR (full, SL)	TWR (empty, vac)	TWR (empty, SL)	Total impulse (vac)	Total impulse (SL)	IWR (Vac)	IWR (SL)
Separatron	12.5	72.5	154	115	18.0	13.4	5.80	2655	1982	25.3	18.9	146.8	109.7	91	68	1.25	0.93
Mite	75	375	210	185	12.5	11.0	5.00	3314	2920	3.4	3.0	17.0	15.0	618	544	1.65	1.45
Shrimp	150	825	215	190	30.0	26.5	5.50	3594	3176	3.7	3.3	20.4	18.0	1423	1258	1.73	1.52
Flea	450	1500	165	140	192.0	162.9	3.33	1948	1653	13.1	11.1	43.5	36.9	1699	1442	1.13	0.96
Hammer	750	3562.5	195	170	227.0	197.9	4.75	2980	2598	6.5	5.7	30.9	26.9	5378	4689	1.51	1.32
Thumper	1500	7650	210	175	300.0	250.0	5.10	3355	2796	4.0	3.3	20.4	17.0	12665	10554	1.66	1.38
Kickback	4500	24000	220	195	670.0	593.9	5.33	3612	3201	2.8	2.5	15.2	13.5	42070	37290	1.75	1.55
Pollux	8000	51500	225	200	1300.0	1155.6	6.44	4109	3652	2.6	2.3	16.6	14.7	95982	85317	1.86	1.66
Thoroughbred	10000	70000	230	205	1700.0	1515.2	7.00	4389	3912	2.5	2.2	17.3	15.5	135331	120621	1.93	1.72
Clydesdale	21000	144000	235	210	3300.0	2948.9	6.86	4437	3965	2.3	2.1	16.0	14.3	283460	253304	1.97	1.76
Twin Boar	10500	42500	300	280	2000.0	1866.7	4.05	4113	3839	4.8	4.5	19.4	18.1	94143	87867	2.22	2.07
TB + Orange	14500	78500	300	280	2000	1866.7	5.41	4969	4638	2.6	2.4	14.1	13.1	188287	175734	2.40	2.24

Now, notice a few of the motors have colored text. This represents being the best or worst motor in any particular category. For example, the Separatron has absymal ISP, and the worst Total Impulse to Weight Ratio of any motor at sea level. But it has unrivaled TWR in all circumstances and an acceptable mass ratio and Delta-V despite all this. Meanwhile, the Clydesdale is the most efficient motor, with the most delta-V, AND the best total impulse to mass ratio in all situations, but it also has the lowest TWR when fully fueled. Plus, it has a gimbal. For all your gimballing needs.

Now, there's one particular motor I want to take a look at first and foremost. The Flea. It's the best at nothing, but maybe it's a jack of all trades? No. It's got the worst performance, by a significant margin I might add, in mass ratio, delta-V (both in a vacuum and at sealevel), and Total Impulse to Weight Ratio in a vacuum. Meanwhile, it has the second to worst ISP in both a vacuum and at sealevel. And the second to worst Total Impulse to Weight Ratio. Its only redeeming quality is its good thrust, and TWR, but this is significantly hampered by the fact that it still has lower thrust than anything else of the same diameter, and that it has 8.85 seconds of fuel.

The result is that, while all the new motors are at or above power curve, the Flea is so far below it that I can't think of a use case besides as an insane separation motor, or as the only engine you have in the early game.

And about that.. Literally every other part in the game is more or less balanced in sandbox. Yes, unlocking new techs unlocks new parts, but not usually strictly better parts. So the Flea being at the bottom of the tech tree is not a justification for locking it in F tier when normally even low tech parts are still viable (in the right circumstances) even in sandbox.

The hammer and Thumper are also below-curve, but not necessarilly enough to make them irrelevant once you unlock better things.

As to how would I fix this? Well, there is a part upgrade system, right?
Keep the Flea as it is... unless you get a tech that gives you an "advanced flea" part upgrade. What does the part upgrade do? Reduce the empty mass by 150 kg, add 20 extra solidfuel, and increase the ISP by 25 in all situations.

And while we're at it, let's give the Thumper and Hammer upgrades as well. Hammer gets 150 kg weight loss when empty and 20 extra solid fuel too, and the Thumper gets that same bonus, but with 10 more ISP at sealevel.

Proposed Part Upgrades:

Name    ...................

Empty mass

Total mass

ISP (vac)

ISP (SL)

Thrust (vac)

Thrust (SL)

Mass ratio

Delta-V (vac)

Delta-V (ASL)

TWR (full, vac)

TWR (full, SL)

TWR (empty, vac)

TWR (empty, SL)

Total impulse (vac)

Total impulse (SL)

IWR (Vac)

IWR (SL)

Advanced Flea	300	1500	190	165	192	166.7	5.00	2999	2604	13.1	11.3	65.3	56.7	2236	1942	1.49	1.29
Adv. Hammer	600	3562.5	195	170	227	197.9	5.94	3406	2970	6.5	5.7	38.6	33.6	5665	4939	1.59	1.39
Adv. Thumper	1350	7650	210	185	300	264.3	5.67	3572	3147	4.0	3.5	22.7	20.0	12974	11430	1.70	1.49

Nothing too crazy. We want to keep them distinct enough that they retain their identity and character as parts and are not made overpowered. But at the same time, we want them to be on-meta parts. I am thinking these would come into the tech tree around the same time you get the Kickback, but maybe not in the same tech to avoid making it OP.


Anyway, what does everyone else think of the SRB meta?

Edited October 18, 2019 by Pds314

[Kerbart]

To paraphrase John Clark: the Flea has one good property, albeit a truly magnificent one: it is available..

I don’t think there’s anything wrong with early game engines you only pick because there’s nothing else. It’s the very definition of progress. NASA does not use the original engines that were used on the first Atlas rockets either. But if they were that bad then why did we use them in the first place?  We know the answer to that; the same reason we use the Flea; until something better comes along.

[Zedwardson]

I have no problem with the flea as is.

Also all these added boosters make me tempted to make a rocket like the "Scout" used by NASA and the Italian space agency. 

[Pds314]

5 hours ago, Kerbart said:

To paraphrase John Clark: the Flea has one good property, albeit a truly magnificent one: it is available..

I don’t think there’s anything wrong with early game engines you only pick because there’s nothing else. It’s the very definition of progress. NASA does not use the original engines that were used on the first Atlas rockets either. But if they were that bad then why did we use them in the first place?  We know the answer to that; the same reason we use the Flea; until something better comes along.

Yes early on the boosters should be less capable, but that doesn't mean the part should go completely obsolete later. "It's available" is not necessary to justify the existence of anything else in the game.

Basic fin? It's light, small and cheap. I use it all the time in sandbox.
Swivel? Well it's an expensive Gimbal but it's a gimbal nonetheless. I use it sparingly but I still use it.
Thud? You probably can't justify using it even because it's available. But then the solution isn't part upgrades. It just needs a buff.

Edited October 18, 2019 by Pds314

[ExtremeSquared]

You didn't put career game cost in there. You'd want some sort of complex metric like ( thrust² * dV ) / (weight * cost)

I have used the flea for some especially violent short landings in big planes. The burn time and twr are nice for that application.

[Pds314]

5 hours ago, ExtremeSquared said:

You didn't put career game cost in there. You'd want some sort of complex metric like ( thrust² * dV ) / (weight * cost)

I have used the flea for some especially violent short landings in big planes. The burn time and twr are nice for that application.

Yeah that's fair. Although the plane has to be fairly huge for it to justify itself over separatrons for RATOL.

Edited October 19, 2019 by Pds314

[FinalFan]

After reaching higher tech tiers, I use the Flea almost as often as I use the Reliant.  (The Swivel, on the other hand, still occupies a genuine if narrow niche for my vehicles when I want a cheap second stage and the Cheetah isn't strong enough.)

You know, what would really make the Flea more attractive is a built-in decoupler.  I dimly recall that a big reason I stopped using it was that the decoupler cost twice as much as the booster.  It probably just seemed like a stupid waste of credits.  And there's precedent now with those Making History capsules.  On the other hand, that's just an extra complication for people trying to learn the basics of the game, so it's probably best not to have it as standard equipment, but possibly an upgrade as in your idea. 

Edited October 19, 2019 by FinalFan
Added decoupler idea

[fragtzack]

Strongly disagree with the conclusions here but appreciate the effort and chart. 

The Flea is a great career mode engine, especially at the start and even far in when you take a mission that requires a "Splash down item test". Cheapest and easiest engine to launch from the pad into the water.

[Guest]

The RT-5 just needs better Isp. I gave it the same as the RT-10 in my game (175-190). This, I feel, is perfectly reasonable since the descriptions and performance of increasingly larger early game solid motors show that more powerful propellants are being developed alongside them. Presumably it's no major stretch to put the better propellant into a RT-5 casing after it is developed.

 

On 10/19/2019 at 3:57 AM, Pds314 said:

Yeah that's fair. Although the plane has to be fairly huge for it to justify itself over separatrons for RATOL.

RT-5s are a lot simpler to just plug onto the backs of RAPIERs with decouplers, than radially attaching a bunch of sepratrons in places you might not have space for without shifting the craft's centre of thrust. Overkill, maybe, but when it's this easy and affordable, why not.

Edited October 20, 2019 by Guest

[Jimmidii]

I've found the Flea good for getting bigger non-reusable landers off the surface of Duna (EDIT: And Tylo!).

That much thrust from that small a part is useful.

Edited October 21, 2019 by Jimmidii

[PolecatEZ]

Late game, I actually do use the Flea as a separatron for some of the more monstrous launch vehicles.  You can cut the cost and weight a bit by fueling it only partially (you only need about 2-3 seconds of thrust at most), and you can mount it directly to the separating stage without any extra separators.

[Tweeker]

I am so, so, so happy they finally got around to adding proper large SRBs.

My fix for the Flea would have been to delete it, and replace it with the Shrimp.  ---Done. I have long used a flea tweeked to .0625 diameter and stretched 4x, giving it about the same volume, but making it more useful as a strap-on booster. The shrimp is even better, having much less empty weight. I would like to see a higher thrust, so it is demonstrably better than the .0625 LF engines thrustwise,  Maybe 50? 

The Clydesdale is great, I have long held that there should be an SRB scaled to shuttle use, the fuel capacity is almost exactly what I came up with when I was experimenting  with making a properly sized SRB, I think I had 15,000 LF. The thrust should have been al little lower, maybe 2500 to 2750, but it is still a great engine, and long overdue.

[ExtremeSquared]

On 10/18/2019 at 7:57 PM, Pds314 said:

Yeah that's fair. Although the plane has to be fairly huge for it to justify itself over separatrons for RATOL.

Yeah I guess...
 

 

[sturmhauke]

On 10/17/2019 at 5:14 PM, Pds314 said:

The result is that, while all the new motors are at or above power curve, the Flea is so far below it that I can't think of a use case besides as an insane separation motor, or as the only engine you have in the early game.

Ahem.

[fragtzack]

-

Edited October 22, 2019 by fragtzack
Part was from a mod...

[CranialRectosis]

I have been looking for a thread that details why I would use SRBs and this is the closest I can find.

Reading this thread, it seems to me that SRBs are used because they are cost effective. Since I play science or sandbox, I haven't found a use for SRBs where liquid fuel asparagus isn't more efficient.

With almost 4,000 hours in game I can probably count on one hand the number of times I have used SRBs. Now there is a whole update where KSP adds a ton of the things.

I almost posted this question in the general forum and if that would be more appropriate, please let me know. But it looks like from this thread that we have much of the information and expertise here to compare SRBs to liquid fuel rockets and answer my question.

 

With liquid fuel asparagus available, why would you ever use SRBs?

[Jodo42]

@CranialRectosis Other than as sepratrons I can't think of any great uses in a stock/DLCs sandbox game. Often times setting up SRBs is easier than asparagus, but not strictly better. Depends on whether you value ingame efficiency more than your free time. Only takes a few seconds to strap on 8 SRBs and strut them up, while if you needed that much thrust from asparagus it'd take a while to get all the 2x symmetry boosters/fuel lines/struts/sepratrons in place properly. 

More on topic: you get access to excellent engines very early on. I use the Terrier regularly even in late-game vehicles. I see no reason a part can't remain relevant through an entire career game. There's a part upgrade feature in-game for a reason, it makes perfect sense to buff the early SRBs later in the tech tree so they stay balanced. I don't find myself using the Flea particularly often mid-late game, but maybe that's just because it's so bad. It's not unreasonable to say that in a stock game you won't be using probes until well after you've explored much of the system with crewed crafts, and relays around Kerbin, the Mun and Minmus can all be easily established with 1.25m parts, so there's a potential use-case for the Flea, Hammer and Thumper in the mid-late game.

[CranialRectosis]

Sepratrons... Good point and I use them TONS.

Loved the suggestion earlier about using Fleas like Sepratrons on HUGE builds too.

Thanks @Jodo42.

 

[Terwin]

On 10/19/2019 at 11:30 PM, fragtzack said:

Strongly disagree with the conclusions here but appreciate the effort and chart. 

The Flea is a great career mode engine, especially at the start and even far in when you take a mission that requires a "Splash down item test". Cheapest and easiest engine to launch from the pad into the water.

Huh, I have always used rovers for 'splashed down' testing contracts(with a jet engine if I want to recover it at 100%).  A Flea sounds like it may do the job with a lot less player time... I'll need to give that a try.