As of 1.0.2, do you think the engines are balanced?

[Orbital Vagabond]

Rocket sizes in KSP are quite arbitrary. It's better to compare the relative performance of the engines and the roles they're used in.

The Mainsail is the most powerful single engine in the game at sea level. At ~1.4 MN, it's supposed to be the KSP equivalent of the F-1 engine. Medium-sized SRBs used in real rockets such as the Ariane 5 are about as powerful as a single F-1 engine, while the large ones used in the Space Shuttle and the SLS are about 2x more powerful. Hence a medium-sized SRB should produce 1-1.5 MN of thrust in the game, while large ones should be rated for 2.5-3 MN.

Well, first, if KSP rocket sizes are arbitrary, then "we need bigger bigger SRBs because there are IRL bigger SRBs" is an invalid argument. How do we know the Mainsail is the F-1 equivalent? That's a ton of extrapolation based on one flimsy interpretation.

Separate from physical size, which you've already stated is arbitrary (at least in your view), all your arguments are based on thrust. If the thrust needs to be increased, then increase the thrust. We don't need to double the number of SRBs in the game as the post I was responded to was claiming; You just need to increase thrust on the existing SRBs

Overall, though, all the additional points I made still stand, separate from nitpicking about what rocket sizes.

[NikkyD]

What we need is quite simple, scaleable SRBs for every diameter. If i want 2.5m boosters, give me one, if i want it to have 100t of fuel, it should as a consequence have to be 50m tall etc. Then let me select a burn profile, like 100% thrust for the first 10 sec burntime, then linear down to 50% for 10 sec burntime etc.

And ALL you really need are 3 SRBs, or SRB hulls and everyone can build the booster they want/need/require for their mission.

[Jouni]

Well, first, if KSP rocket sizes are arbitrary, then "we need bigger bigger SRBs because there are IRL bigger SRBs" is an invalid argument. How do we know the Mainsail is the F-1 equivalent? That's a ton of extrapolation based on one flimsy interpretation.

Separate from physical size, which you've already stated is arbitrary (at least in your view), all your arguments are based on thrust. If the thrust needs to be increased, then increase the thrust. We don't need to double the number of SRBs in the game as the post I was responded to was claiming; You just need to increase thrust on the existing SRBs

Please try to see the intent behind the words.

Many large real-world rockets, such as the Ariane 5, the Space Shuttle, and the SLS, use SRBs as their main form of propulsion. Their boosters combine very high thrust with long burn times, while their so-called main engines are weak. We can't build anything even remotely similar in stock KSP, because we can only choose between small, tiny, and puny SRBs.

If part proliferation is a problem, we can always replace the useless Flea and Hammer with new 2.5 m boosters. The Thumper becomes a small 1.25 m SRB and the Kickback becomes a large 1.25 m SRB, while the two free slots can be used for small and large 2.5 m SRBs.

[Orbital Vagabond]

Please try to see the intent behind the words.

Well, then please try to see the intent behind my words in my intial response (to someone else): SRBs are as capable in KSP as they are IRL. Again, nitpicking about how you and I see the interpretation of rocket sizes differently is practically immaterial

Many large real-world rockets, such as the Ariane 5, the Space Shuttle, and the SLS, use SRBs as their main form of propulsion. Their boosters combine very high thrust with long burn times, while their so-called main engines are weak.

!?

This is straight up wrong, and you need to check your facts.

The shuttle's boosters burn for 2 minutes, while the SSMEs burn for almost 9 (source). The Ariane's Boosters provide 90% of the thrust at launch but only fire for 130s, while the Vulcain burns for 10 minutes (source). The boosters are not the "main form of propulsion" for either launch system, as you claim. It's not even close. Strap-on boosters are supposed to provide high thrust for a short time to improve the efficiency what is actually the primary launch systems (the SSMEs and Vulcain, respectively, in the above examples). If you look at the work (F x d) done by various stages you've brought into this conversation, you'll see the contribution of the booster vs main stages isn't even close

Beyond your misleading claims, strap-on boosters like you've mentioned here are NOT what was mentioned in my initial response. That was discussing SRB-Only rockets, e.g. the Minotaur, Vega, and Athena. Please don't change the topic.

tl;dr: There's a reason the SSMEs are referred as "Main Engines".

If part proliferation is a problem, we can always replace the useless Flea and Hammer with new 2.5 m boosters. The Thumper becomes a small 1.25 m SRB and the Kickback becomes a large 1.25 m SRB, while the two free slots can be used for small and large 2.5 m SRBs.

Frankly don't care about part proliferation.

What we need is quite simple, scaleable SRBs for every diameter. If i want 2.5m boosters, give me one, if i want it to have 100t of fuel, it should as a consequence have to be 50m tall etc. Then let me select a burn profile, like 100% thrust for the first 10 sec burntime, then linear down to 50% for 10 sec burntime etc.

And ALL you really need are 3 SRBs, or SRB hulls and everyone can build the booster they want/need/require for their mission.

At least this argument is a more direct and doesn't hide behind false claims of realism about why the engines are "needed." I still disagree, though, that more SRBs are required. This post skates dangerously near the whole 'procedural tanks' issue.

- - - Updated - - -

The LV-909 is indeed excellent.

The Reliant used to be quite versatile, much like a scaled-down Skipper. I used it in many situations where it was by no means perfect, but good enough. These days, it has abysmal ISP. It looks as if both the LV-T30 and 45 are designed to be inferior to everything that comes later in the tech tree. Which is alright from a tech progression point of view, but there's no progression in the 1.25m department. When you're looking for a powerful 1.25m engine late in the game, you're out of luck.

Just saw this. I still don't understand the problem with the 30 and 45. To build a rocket so large that the 30's and 45's aren't sufficient to get it into space, it's a long wobbly mess that's better off as a 2.5 m stack.

While I understand it's not exactly what you said, I don't think a perfect engine needs to exist for every application. Having situations where the best engine is "by no means perfect but good enough" really is good enough.

And despite looking, I can't find any mention of any SRB engine being gimballed. I'm not saying they don't exist, but I just can't find evidence. I'd like to see it, if it exists.

Well, I found it myself. According to this image, the shuttle's SRB's had a thrust vector control system in the skirt attached to their nozzles.

Edited June 1, 2015 by TestPilotTheta

[Ravenchant]

And despite looking, I can't find any mention of any SRB engine being gimballed. I'm not saying they don't exist, but I just can't find evidence. I'd like to see it, if it exists.

Very well.

Vega has pitch and yaw actuation of 6.5° on its first stage. 7° on its second stage, 6° on its third stage. Don't know about roll. Source

The aerojet boosters used on Atlas V can be deflected by 3° on one axis. Source

Both kinds of boosters used on H-II rockets gimbal, though I couldn't find info on how much. This includes the Epsilon 1st stage. Source

The Shuttle boosters had actuators for both roll&pitch and yaw (!) and could gimbal for 8°. Source

India's PSLV uses a liquid injection system for yaw and pitch control of its SRB. Roll control is achieved by small verniers. Source

AFAIK the Titan boosters used a similar system. You can see the fluid holding tank here, it's the red thingy.

Both solid motors of the IUS had thrust vector control; Orbus 21 gimballed for 4°, Orbus 6 for 7°. Source

Really, it's harder to find solid motors that don't have some kind of thrust vectoring.

Edit: a payload capacity of 2 tons is also not that bad if you use solar electric propulsion.

Edit #2: one more link

Castor 120, the motor used as first stage on the Athena and Antares' upper stage, has 5.5° nozzle actuation. Minotaur isn't much different. Source

On the other hand, early Japanese Mu launch vehicles up to M-V didn't have any control that I know of

Holy edits, Batman!:

Just saw this. I still don't understand the problem with the 30 and 45. To build a rocket so large that the 30's and 45's aren't sufficient to get it into space, it's a long wobbly mess that's better off as a 2.5 m stack.

Now on that we agree. Hell, 3.75m rockets can be powered by 1.25m engines, as long as you use enough of them and don't care about the cost that much.

Edited June 1, 2015 by Ravenchant

[Jouni]

Well, then please try to see the intent behind my words in my intial response (to someone else): SRBs are as capable in KSP as they are IRL. Again, nitpicking about how you and I see the interpretation of rocket sizes differently is practically immaterial

I still don't see your point. In what sense SRBs are as capable in KSP as in the real world, if it's not size, burn time, thrust, or specific impulse, either in absolute terms or when compared to liquid-fueled launch stages, or the purposes the boosters can be reasonably used for?

The shuttle's boosters burn for 2 minutes, while the SSMEs burn for almost 9 (source). The Ariane's Boosters provide 90% of the thrust at launch but only fire for 130s, while the Vulain stage burns for 10 minutes(source).

In both cases, SRBs form the majority of the launch mass of the rocket. If most of the rocket is solid rocket boosters, then it feels right to say that the SRBs are the primary form of propulsion.

If you look at the work (F x d) done by various stages you've brought into this conversation, you'll see the contribution of the booster vs main stages isn't even close

The product of thrust and burn time is more relevant, as it doesn't depend on the work done by the other stages.

Beyond your misleading claims, strap-on boosters like you've mentioned here are NOT what was mentioned in my initial response. That was discussing SRB-Only rockets, e.g. the Minotaur, Vega, and Athena. Please don't change the topic.

Then I misunderstood your post. The message you were replying to was talking about the uses of SRBs in a more general sense, and only mentioned the SRB-only rockets as an example.

[Laie]

Just saw this. I still don't understand the problem with the 30 and 45. To build a rocket so large that the 30's and 45's aren't sufficient to get it into space, it's a long wobbly mess that's better off as a 2.5 m stack.

While I understand it's not exactly what you said, I don't think a perfect engine needs to exist for every application. Having situations where the best engine is "by no means perfect but good enough" really is good enough.

Try to do anything with Mk2 parts that isn't a hyprid spaceplane. Like, a straightforward rocketplane. As soon as it becomes big enough to warrant a second engine, you can dramatically increase performance by attaching a 2.5m adapter and a skipper (giving you 3-4 times the TWR and a better ISP to boot).

My current project has four-sided symmetry and could do with a little more thrust. Adding four skippers would be way over the top; adding only two would still be too much, and also a lot of hassle in the construction department. However, using 1.25m engines is totally out of the question as it would actually decrease performance: I'd get barely any TWR increase but a big hit dV. That's how bad they are.

Now you may say that in reality, the Skipper is OP... but in that case, what of the Mainsail, Rhino or Mammoth?

[Orbital Vagabond]

I still don't see your point. In what sense SRBs are as capable in KSP as in the real world, if it's not size, burn time, thrust, or specific impulse, either in absolute terms or when compared to liquid-fueled launch stages, or the purposes the boosters can be reasonably used for?

You clearly can't compare KSP SRBs to IRL SRBs on a unit-to-unit basis any more than you can for any other engine or parameter, with the exception of ISP. And yeah, they do kinda suck from that point of view compared to IRL. However, everything got beat with the Nerf bat, so I really don't feel that bad. But, again, that doesn't justify the need for multiple new SRBs, only modifications of current parameters, but I prefer the increased difficulty. So who's "right" based on their own preferences?

In a more abstract sense IRL and KSP SRBs are comparable on the following two points:

• the SRBs in KSP are useful for strap-on boosters to increase thrust off the pad
  
• SRB-only launch systems frankly suck, and are only useful for small payloads.
  

The rockets under discussion (and that you conveniently chose to ignore to make irrelevant points) were portrayed as being "fully capable" launch systems compared to LFO systems. They aren't.

In both cases, SRBs form the majority of the launch mass of the rocket. If most of the rocket is solid rocket boosters, then it feels right to say that the SRBs are the primary form of propulsion.

It feels right to you, but I don't think many people will agree with you that the shuttle's SRBs were the primary system to get the shuttle into orbit.

The product of thrust and burn time is more relevant, as it doesn't depend on the work done by the other stages.

You only see it as more relevant because it supports your stance. In reality, the boosters are a method to exploit the Oberth effect, which is dependent on work.

Also, I can't see how you can claim 2 minute burn times are "long burning" in any kind of sense. Wikipedia's "booster" entry describes them as "shorter-burning" and used to "augment" thrust, not as primary orbital systems.

Then I misunderstood your post. The message you were replying to was talking about the uses of SRBs in a more general sense, and only mentioned the SRB-only rockets as an example.

Clearly. The post to which you refer never denied or argued that the SRBs worked for strap-on boosters as I stated in my original post. As that point was never under contention, it required no discussion.

But it's all you seem to want to talk about.

And I don't anymore.

So I'm done.

- - - Updated - - -

Very well.

[tons of awesome examples]

Thank you. I was looking for that information in the wrong places.

- - - Updated - - -

Try to do anything with Mk2 parts that isn't a hyprid spaceplane. Like, a straightforward rocketplane. As soon as it becomes big enough to warrant a second engine, you can dramatically increase performance by attaching a 2.5m adapter and a skipper (giving you 3-4 times the TWR and a better ISP to boot).

My current project has four-sided symmetry and could do with a little more thrust. Adding four skippers would be way over the top; adding only two would still be too much, and also a lot of hassle in the construction department. However, using 1.25m engines is totally out of the question as it would actually decrease performance: I'd get barely any TWR increase but a big hit dV. That's how bad they are.

Now you may say that in reality, the Skipper is OP... but in that case, what of the Mainsail, Rhino or Mammoth?

Listen, it's pretty clear my points of view aren't welcome here, so I'm not even going to try to discuss it anymore. Best of luck with your problems.

[rtxoff]

Why was the poll closed? I want to give a vote.

[Jouni]

You clearly can't compare KSP SRBs to IRL SRBs on a unit-to-unit basis any more than you can for any other engine or parameter, with the exception of ISP.

That's why I also mentioned the relative performance comparisons: (real-life SRBs vs. real-life liquid-fueled stages) vs. (KSP SBRs vs. KSP liquid-fueled stages).

Let's look at a few real-life examples:

[table]

[tr]

[td]Stage[/td]

[td]Thrust[/td]

[td]Isp[/td]

[td]Burn time[/td]

[/tr]

[tr]

[td]Falcon 9 v1.1 first stage[/td]

[td]5885 kN[/td]

[td]282 s[/td]

[td]180 s[/td]

[/tr]

[tr]

[td]EAP P241 (Ariane 5 SRB)[/td]

[td]7080 kN[/td]

[td]274.5 s[/td]

[td]140 s[/td]

[/tr]

[tr]

[td]S-IC (Saturn V)[/td]

[td]33400 kN[/td]

[td]263 s[/td]

[td]150 s[/td]

[/tr]

[tr]

[td]Space Shuttle SRBs (2)[/td]

[td]25000 kN[/td]

[td]269 s[/td]

[td]124 s[/td]

[/tr]

[tr]

[td]Edit: Common Booster Core (Delta IV)[/td]

[td]3140 kN[/td]

[td]360 s[/td]

[td]245 s[/td]

[/tr]

[/table]

Except for about 20% shorter burn times, large SRBs seem to be more or less equivalent to kerolox-powered first stages. Edit: I also added the only major hydrolox-powered launch stage I'm aware of. Unlike other hydrolox-powered first stages, the CBC can actually lift off on its own. The specific impulse is about 33% higher and the burn time about 2x higher than for SRBs.

Now let's look at the KSP options:

[table]

[tr]

[td]Stage[/td]

[td]Thrust[/td]

[td]Isp[/td]

[td]Burn time[/td]

[/tr]

[tr]

[td]BACC "Thumper" SRB[/td]

[td]275 kN[/td]

[td]165 s[/td]

[td]26 s[/td]

[/tr]

[tr]

[td]BACC "Thumper" SRB (70% thrust)[/td]

[td]192.5 kN[/td]

[td]165 s[/td]

[td]37 s[/td]

[/tr]

[tr]

[td]LV-T30 "Reliant" + 2x FL-T800[/td]

[td]200.7 kN[/td]

[td]280 s[/td]

[td]109 s[/td]

[/tr]

[tr]

[td]"Kickback" SRB[/td]

[td]599.5 kN[/td]

[td]170 s[/td]

[td]52 s[/td]

[/tr]

[tr]

[td]Skipper + orange tank[/td]

[td]568.8 kN[/td]

[td]280 s[/td]

[td]154 s[/td]

[/tr]

[/table]

In KSP, LFO-powered launch stages have 65% higher specific impulse and 3x higher burn times than SRBs of similar power. These numbers are high, even if we compare them to real-world hydrolox vs. SRB figures.

And yeah, they do kinda suck from that point of view compared to IRL. However, everything got beat with the Nerf bat, so I really don't feel that bad. But, again, that doesn't justify the need for multiple new SRBs, only modifications of current parameters, but I prefer the increased difficulty. So who's "right" based on their own preferences?

I wasn't arguing about making things easier or harder, but about increasing the variety of options. In real life, we mostly see SRBs in three different roles: 1) to increase the payload capacity of a rocket (like in some Delta IV configurations); 2) in fully or mostly SRB-based rockets; and 3) as the main source of thrust, working alongside with an extended "second stage" (like in the Space Shuttle, the Ariane 5, and the SLS). Roles 1) and 2) are possible in KSP, while role 3) isn't, except for small rockets.

The rockets under discussion (and that you conveniently chose to ignore to make irrelevant points) were portrayed as being "fully capable" launch systems compared to LFO systems. They aren't.

Your original post was about using SRBs primarily for providing additional thust for lifoff. Someone replied, saying that it's a narrow view (primary message), and provided some examples, where SRBs were used for other purposes (secondary message). You then replied to the secondary message, while I thought that you were replying to the primary message.

Edited June 1, 2015 by Jouni

[wumpus]

I'd say the engines are balanced. If anything needs the nerf-bat, it is the airbrakes (try using them during re-entry).Has anybody had success sticking fins on the LV-N in 1.0.2? That is supposed to be the answer, but I couldn't get them to stick to my last LV-N powered rocket.I hope SRBs haven't been nerfed too hard since .90. As long as the rules are: "go [practically] into orbit or don't get recovered", I tend to use a lot of SRBs for early stages.

[Laie]

Why was the poll closed? I want to give a vote.

Because when you make a poll, it asks you how long it should remain open. I thought seven days would be generous, expecting the thread to be dead and buried by day three.

[NathanKell]

Jouni, you're comparing the vacuum Isp of solids to the sea level Isp of liquids. Of course you're going to get screwy numbers. Try comparing vacuum all around. 311s for Merlin 1D on F9 v1.1, 304s for S-IC, 412s for CBC. 452s for the SSME, which you didn't list.

It's true that solids in KSP have proportionally poorer Isp than liquids even with that change; but what you're missing is that solids in KSP have maybe 1.5x to 2x (tops) the dry mass fraction of real life solids, whereas KSP liquid engines have 4x to 8x (!) the mass real engines do for their thrusts. So they're nerfed differently.

[Jouni]

Jouni, you're comparing the vacuum Isp of solids to the sea level Isp of liquids. Of course you're going to get screwy numbers. Try comparing vacuum all around. 311s for Merlin 1D on F9 v1.1, 304s for S-IC, 412s for CBC. 452s for the SSME, which you didn't list.

That was some poor googling on my part. I just assumed that the Isp figures listed everywhere for SRBs were sea level figures, as vacuum Isp doesn't make too much sense with them.

I didn't list the SSME, because it's not used in rocket stages that are able to lift themselves at sea level. It's a different kind of engine for a different purpose.

It's true that solids in KSP have proportionally poorer Isp than liquids even with that change; but what you're missing is that solids in KSP have maybe 1.5x to 2x (tops) the dry mass fraction of real life solids, whereas KSP liquid engines have 4x to 8x (!) the mass real engines do for their thrusts. So they're nerfed differently.

My main point was that the SRBs we have in the game have short burn times and low thrust compared to liquids, relative to what SRBs vs. liquids are in the real world.

[tarkhil]

LV-N became useless. Let's see, a cupola+Mk2 Lander Can+MPL-LG-2, like a basic science orbital station, and two Rockomax Jumbo-64 fuel tanks.

Three LV-N's (taking oxidixer out of tanks) gives TWR 0.34, DeltaV 5994

One RE-10L: WR: 0.31, DeltaV 5248

Doesn't see much difference. LN-Ns became too heavy to be useful.

[Superfluous J]

LN-Ns became too heavy to be useful.

I see it as they stopped being the only useful engine.

[tarkhil]

I see it as they stopped being the only useful engine.

It was the engine of choice for interplanetary travel; not it's just a funny looking thing with very limited usage. It seems to have useful TWR below 0.2

[Jouni]

LV-N became useless. Let's see, a cupola+Mk2 Lander Can+MPL-LG-2, like a basic science orbital station, and two Rockomax Jumbo-64 fuel tanks.

Three LV-N's (taking oxidixer out of tanks) gives TWR 0.34, DeltaV 5994

One RE-10L: WR: 0.31, DeltaV 5248

Doesn't see much difference. LN-Ns became too heavy to be useful.

The engines aren't too heavy, just the fuel tanks you're using. Instead of using LFO tanks without oxidizer, most of the fuel should come from LF-only tanks, which have much better mass fractions. The test craft I posted earlier had essentially a 0.90 level of performance:

[tarkhil]

I still do not understand. Cupola+big lander can+MPL+2/4 LV-Ns and fuel up to 40 tonns yields about 3500 deltaV. Replacing LV-Ns with Jumbo-64 and Poodle gives 43 tons (+10% mass) and 4600 deltaV.

With better TWR!

Chemical engines beats LV-N for BOTH deltaV and TWR. I do not understand it at all.

okay, the same payload, two Jumbos, Poodle: twr 0.32, deltaV 5675, 79150 kg

with 85 tons of total weight, 4 LN-Ns finally gives 7600 deltaV, 0.28 twr.

Goodbye, my favorite medium-weight nuclear-powered landers