Boosters: RT-10 beats BACC in every way?

[Riph]

I've been trying to really fine-tune my first ascent stage, and when I broke down the numbers I saw something kind of odd. Consider this very basic comparison of two RT-10 SRBs vs one BACC. Remember that the BACC is the higher tech option and requires science to unlock.

[TABLE=width: 500, align: left]

[TR]

[TD]RT-10s[/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[TR]

[TD]Cost[/TD]

[TD]325[/TD]

[TD]x2[/TD]

[TD]650[/TD]

[/TR]

[TR]

[TD]Thrust[/TD]

[TD]250[/TD]

[TD]x2[/TD]

[TD]500[/TD]

[/TR]

[TR]

[TD]Fuel (tons)[/TD]

[TD]3.25[/TD]

[TD]x2[/TD]

[TD]6.5[/TD]

[/TR]

[TR]

[TD]Engine (dead weight, tons)[/TD]

[TD]0.4975[/TD]

[TD]x2[/TD]

[TD]0.995[/TD]

[/TR]

[TR]

[TD]isp[/TD]

[TD][/TD]

[TD][/TD]

[TD]225[/TD]

[/TR]

[TR]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[TR]

[TD]BACC[/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[TR]

[TD]Cost[/TD]

[TD][/TD]

[TD][/TD]

[TD]700[/TD]

[/TR]

[TR]

[TD]Thrust[/TD]

[TD][/TD]

[TD][/TD]

[TD]315[/TD]

[/TR]

[TR]

[TD]Fuel[/TD]

[TD][/TD]

[TD][/TD]

[TD]6.37[/TD]

[/TR]

[TR]

[TD]Engine[/TD]

[TD][/TD]

[TD][/TD]

[TD]1.505[/TD]

[/TR]

[TR]

[TD]isp[/TD]

[TD][/TD]

[TD][/TD]

[TD]230[/TD]

[/TR]

[TR]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[TR]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[/TABLE]

So unless I'm missing something huge, the more advanced unit is getting me 5 isp in exchange for being heavier, having less fuel, less thrust, more dead weight, and being more expensive. Five points of isp can't possibly make up for a half ton of additional dead weight, can it?

Since you can stack RT-10s radially outward on to infinity, and tune the thrust limiter exactly how you like, is there any scenario in which the BACC is desired?

[thereaverofdarkness]

It's nice to have a longer booster because you can't stack RT-10s and only use the one engine for both fuel tanks. I like to use it because it is easy to slap onto the bottom of a rocket.

I hadn't noticed that they have less fuel than 2 RT-10s. I thought they actually had more. Perhaps a balance tweak is in order. After all, the BACC is more than twice the length of the RT-10, it should therefore have more than twice the fuel. Its dry mass is three times the RT-10.

[Riph]

Speaking of easy to slap on, I think from now on every rocket I send up will be riding on some variant of this:

Need more thrust? Stack more radially outward.

Need more burn time? Stack more radially outward and then tune their thrust limiter down.

[numerobis]

Since you can stack RT-10s radially outward on to infinity, and tune the thrust limiter exactly how you like, is there any scenario in which the BACC is desired?

When the BACC was introduced, there was no thrust limiter. Even then, the BACC was only good for reducing part count.

[UmbralRaptor]

The RT-10 and BACC have gone back and forth in utility. Currently the BACC is somewhat lacking, though.

[LethalDose]

Huh, interesting question, Riph. I especially like your quincunx of RT10s as a first stage.

I usually use SRBs as strap-on boosters. If you account for the cost of the radial decouplers, the cost of the BACCs comes down. For example, if you use 2 BACCs instead of 4 RT10s

2 BACCs + 2 TT-38Ks costs 2 x 700 + 2 x 600 = 2,600 funds

4 RT-10s + 4 TT-38Ks costs 4 x 325 + 4 x 600 = 3,700 funds

I think it's kinda goofy that radial decouplers cost more than small SRBs, but... it's what we got to work with...

[bdito]

Well, it all depends on what you're design is. You might want one BACC instead of two RT-10's if you want a lower TWR. It also depends if you're using FAR or NEAR. With FAR you don't generally want to build outwards too much and having a large TWR can cause flight stability issues.

Edited December 17, 2014 by bdito

[Empiro]

Yes, I've definitely noticed that the BACC could use the boost (heh). I found myself naturally almost never using them in my hard-mode career games where cost is a big issue. I either use RT-10s for small launches or S1s for big ones. The BACC just sat around collecting dust.

[zerotwo]

Two things I will add; consider cost and consider part count.

It takes half the parts to employ BACC boosters in place of RT-10 boosters and with the 30 limit on the basic hangar this can be a realistic concern early on.

LethalDose's post above highlights the only other thing I would consider important to consider: cost.

As a matter of utmost efficiency in 0.9 I have spent the beginning 500~600 science or so launching everything with an RT-10 as midstage and a BACC as 1st stage.

The price of 2 RT-10s (decouplers) as the basic stage is far too much to accept.

If you have unlimited part count and unlimited funds, then yes, huge volumes of RT-10s in hilariously complex asparagus staging formations is more efficient than BACC boosters in a similar setup-however if you're interested in maximum efficiency with unlimited funds and part count, why are you using SRBs at all?

The point of SRBs is low-cost and low-part count, imo.

Hope that helps.

[Riph]

Two things I will add; consider cost and consider part count.

Point conceded on part count. I'm playing a new game right now and have yet to hit the 30 piece limit, but I am coming close.

However, cost was actually covered in the OP. The RT-10s are cheaper. And if you're putting a decoupler on every booster, then you're doing it wrong. See the pic I posted, an unlimited number of boosters can ride on one decoupler.

[arkie87]

I've been trying to really fine-tune my first ascent stage, and when I broke down the numbers I saw something kind of odd. Consider this very basic comparison of two RT-10 SRBs vs one BACC. Remember that the BACC is the higher tech option and requires science to unlock.

Since you can stack RT-10s radially outward on to infinity, and tune the thrust limiter exactly how you like, is there any scenario in which the BACC is desired?

Even in terms of burn time, RT-10 is better:

Two RT-10's have more thrust, but reduced burn time. Even if you reduce thrust such that thrust is equal to that of one BACC, burn time will still be faster, which means you can drop the mass earlier...

[LethalDose]

And if you're putting a decoupler on every booster, then you're doing it wrong. See the pic I posted, an unlimited number of boosters can ride on one decoupler.

I saw your pic, and actually I complimented it.

I was talking about radial decouplers, but I see your point. IMO, though, mounting multiple boosters off a single radial decoupler is cheaty as ****.

What I described isn't doing it "wrong", I'm doing it different.

[zerotwo]

an unlimited number of boosters can ride on one decoupler.

I guess given that drag is likely irrelevant this is a good point! I did see and read your original post.

Depending on setup you could even apply the decoupler ring to the bottom of a stack, then attach the SRBs to that ring.

Interesting idea.

Regarding part count: It is a helluva thing to try and get to the mun, do a lot of science, and return in under 30 parts! Those big KD25ks are making it possible (theoretically) for me to haul all the basic (under 100 tech) science with me in one go!

Edited December 17, 2014 by zerotwo

[arkie87]

I saw your pic, and actually I complimented it.

I was talking about radial decouplers, but I see your point. IMO, though, mounting multiple boosters off a single radial decoupler is cheaty as ****.

What I described isn't doing it "wrong", I'm doing it different.

Why is it cheating? If you want, he could "attach" it with a strut or two... no need for a full fledged decoupler though, since it will be decoupled with the stack, not individually.

[bdito]

IMO there's no such thing as cheating in a single player sand-box game. Everyone makes their own set of rules and follows them accordingly.

[LethalDose]

IMO there's no such thing as cheating in a single player sand-box game. Everyone makes their own set of rules and follows them accordingly.

Right, so I can't see how my choice not to do it qualifies as "doing it wrong".

Edited December 17, 2014 by LethalDose

[Jouni]

The only real difference is the higher dry mass of the BACC. If you reduced it to 1.0 tonnes, the two SRBs would be quite balanced. The RT-10 would be a better option when you want a lot of thrust at launch, while the BACC would work better with longer burn times.

The BACC has a 2.2% Isp advantage over the RT-10 at sea level, and a 4.3% advantage in vacuum. At full thrust, the average advantage is probably around 2.5%, while a tweaked down BACC could be up to 3% better. The difference in fuel mass is smaller, with 2xRT-10 having an 1.9% advantage over the BACC.

[Laie]

Page two and noone brought up overheating?

If I want to run RT-10s attached (in close proximity) to each other, I need to throttle them to 80-85%. That assembly as pictured would otherwise go BOOM! within ten seconds. BACCs are much more resilient. I habitually used a bundle of seven under every mainsail.

(though with the new weight limits for the pad, I find myself using much fewer boosters: their low ISP means you have higher mass for the same oomph.)

[WanderingKid]

Page two and noone brought up overheating?

At maximum 18t crafts, you're basically going to run those down to 60-80% anyway because otherwise you're blowing way past terminal. Overheating is kind of moot unless you're running cinderblocks of them.

[panzer1b]

I agree that BACCs arent all that useful, but i dont exactly consider anything but the really massive SRBs any use when i actually make rockets.

Reason for not using smaller SRBs, is that most of my rockets that are on the small end, are too small to really bother even using any solid boosters, and the larger ones, are just so large, that anything below the nasa engines/super massive solid boosters is pointless. Perhaps for medium sized rockets, the BACC is not half bad, but at least from my experience, both RT-10s and BACCs are more or less rarely used for me, outside of very early career mode. In sandbox, or where funds and part count arent the ultimate hard limit, they are of minimal use. Even for kinetic missiles, RT-10s are better then BACCs, smaller, and almost as lethal.

Edited December 17, 2014 by panzer1b

[numerobis]

One more point: in FAR, two RT10s side-by-side have twice the cross-section area of a BACC, and thus twice the drag.

[Kesa]

It greatly depends on design. You must take account of decoupler cost, and sometimes heat sinker.

I think the uglyness of the craft should also be taken on account. For example, between my skipper and my jumbo 64, I will never use TR 2 V stack decoupler, despite it being the cheapest end lightest decoupler. It depends of whether you want the most effective but ugly craft, or weather you want a good looking craft that cost only few hundreds more. (and it also depends of what is ugly for you)

[King Arthur]

For what it's worth, the BACC has always been less than stellar.

Back before the NASAmission parts came, the BACC was the largest SRB we had and it was essentially useless. If you needed boosters on your launcher you were far better off building and using some liquid boosters (LRBs) because the BACC can't lift anything to save itself. The RT-10 meanwhile has always been that "trash can full o' boom" that has some hilariously effective applications.

Once you start using the S1 SRB-KD25k, the SRB from NASAmission, the BACC is even further overshadowed because the S1 SRB-KD25k actually does what an SRB does extremely well.

So yes, the RT-10 is better than the BACC in almost every way. I would in fact argue that the BACC is the weakest of all the stock engines currently available because both of the other SRBs outclass it and LRBs just knock the BACC out of the water.

[Craeshen]

R 10's are also cheap enough early game they can be stacked without stack decouplers, yep as long as the staging is good no decoupler needed.

[THX1138]

Point conceded on part count. I'm playing a new game right now and have yet to hit the 30 piece limit, but I am coming close.

What is this 30-piece limit? My rockets have hundreds of pieces before launch.