Completely solid-fueled first stages?

[MedwedianPresident]

I tend to use SRB-only first stages (usually multiple 1.25m SRB's or 2.5m SRB's) in my medium and heavy lifters. I think it is a good solution because in the lower atmosphere, pure thrust is more important than controllability and Delta-V (unless you want to save fuel by keeping to the terminal velocity and hand-tailor a rocket for every single payload).

 

The main advantage of this is the generally higher thrust of SRB's (my SRB-only first stages are usually equivalent to 2.5m liquid-fuelled stages, and especially multiple combined SRB's tend to have a higher thrust than the respective liquid engines, which is very useful for breaching the lower atmosphere) and that it looks cooler (Jeb is happy), but the main disadvantage is the inability to control the first part of the ascent completely - you are effectively riding a giant missile until the SRB's burn out.

 

Does anybody else use SRB's for the first stage of their lifters? Have you fulfilled successful missions? Can you show me your lifters and give some ideas that help to safely handle SRB-only first stages?

[polytechnique]

I occasionally do this for career... it certainly helps save costs! But ... practicality and efficiency isn't so good. It also tends to cause a few issues if you're using FAR, 'cause, those SRBs can sure rip you right out of the atmosphere but the atmosphere is plenty content to rip you right back.

I don't have so many lifter designs using solely SRBs as a first stage, but ... I might be able to go find a few that I had?

Edited August 7, 2016 by polytechnique

[dafidge9898]

38 minutes ago, MedwedianPresident said:

Does anybody else use SRB's for the first stage of their lifters? Have you fulfilled successful missions? Can you show me your lifters and give some ideas that help to safely handle SRB-only first stages?

Fins should do just fine with controlling your ship. Because you're only using SRBs in the dense atmosphere, it seems like the best option.

[MedwedianPresident]

Yes. By "lack of control" I meant speed control, not orientation control. If you don't nerf your SRB's before launch (Thrust Limiter) you may overheat because the acceleration may be very high.

[GoSlash27]

I use SRBs for my first stage when cost is a factor but mass isn't. I cluster them into an assembly and adjust the limiters so that thrust diminishes over the time of flight.

http://s52.photobucket.com/user/GoSlash27/slideshow/KSP/CnCRocketFactory

Best,
-Slashy

[foamyesque]

Fuel's cheap. I stretch what would be my 2nd stage and co-fire it.

[Snark]

4 hours ago, MedwedianPresident said:

Does anybody else use SRB's for the first stage of their lifters? Have you fulfilled successful missions? Can you show me your lifters and give some ideas that help to safely handle SRB-only first stages?

I use SRB-only first stages practically exclusively.  I just like 'em. 

One thing that makes them easier to handle:  I stick religiously to a consistent TWR on the pad.  (I like to use 1.5.)  That means that every launch has a pretty similar shape to the gravity turn, so it gets me lots of repetitive practice, to the point where I can judge that initial eastward nudge pretty well. The fact that I can't control the throttle is irrelevant to me-- if I were launching on liquid fuel, I'd have the throttle maxed out for that part of the climb anyway.

It's true that SRBs can make for a rough ride.  The TWR gets a lot higher near the end, not just because your mass has gone way way down, but also because their vacuum Isp is 20% bigger than on the pad.  That TWR spike at the end can be a bit... enthusiastic.

For cases where I have an even-number-greater-than-two of radial SRBs on my rockets, I like to smooth out the ride a bit with a technique I've come to think of as "poor man's asparagus".  The idea is:  run two identical symmetry groups of SRBs, both staged together at launch, but one of them has a higher thrust limiter setting and its decouplers are staged first.

For example:  Let's say I've got 8 radial Thumpers on my lift stage.  I look at my ship mass, do the math, and work out that in order to get a launchpad TWR of 1.5, I need to set their thrust limiters to, say, 85%.

Well, instead of putting all the boosters in one 8-way symmetry group at 85%, I instead put them in two 4-way symmetry groups, rotated by 45 degrees from each other.  Call them groups A and B.  I set group A to 100% and group B to 70%.  All 8 boosters fire upon launch.

The rocket takes off with the same 1.5 TWR on the launchpad as it would have if I had set all of them to 85%.  But group A burns out first and is immediately jettisoned, and group B carries the ship along for a good while further before I need to ditch them and light the liquid engine.  Group B does just fine without assistance until it burns out, even though there are now only half the boosters and they're running at a lower thrust setting than the others.  Why?  Several reasons:  1. rocket mass is a lot lower; group A gone, most of the mass of group B gone.  2. by now the engines are getting close to their vacuum Isp and have 20% more thrust apiece than at launch.  3. the rocket is tipped over mostly horizontally and needs less TWR than at liftoff anyway.

Pretty quick to set up, works nicely.

[Xavven]

After a couple accidents involving lack of control authority from fins alone, I've sworn off all-SRB stages. If I use SRBs at all, I make sure to always have a central liquid+oxidizer rocket engine that has thrust vectoring.

In career, once part count is no longer a limiting factor (by upgrading my VAB), I switch to all liquid+oxidizer rockets due to role-play reasons. SRBs are a bit more polluting IRL.

[Guest]

The Aries I launch vehicle relied on a Shuttle SRB as its first stage. So there's a real-life example. Personally, I use SRBs on a very limited basis in crewed flight. Currently, I'm employing a Titan IIb configuration (never flown in reality), which is a Titan II with two short SRBs. I use it for launching the Advanced Gemini craft. But it doesn't qualify as a pure solid-fuel first stage. I can't quite bring myself to take that step.

Edited August 7, 2016 by Jack Wolfe

[Curveball Anders]

I usually go with a main core assisted by two (or more) SRB's.

SRB's burning out during gravity turn with meco and jettison just a shade before orbit (automagic de-kessler).

Mainly because it's my vision of a launch and it looks cool.

[Adelaar]

Well, I have the Alfa C and Alfa D (of which the Alfa D is displayed here). It can carry a maximum payload of 6000 kg into the Kerbin SOI. For smaller or larger rockets I've found a SRB powered first stage to be too unreliable, due to a complete lack of thrust control and thrust vectoring.

[Joco223]

I actually use pure SRB first stage rockets a lot in my 6.4x save. Since i am using procedural parts there, i make the first SRB little smaller in diameter than the second stage which is powered by LiquidFuel+Oxidizer or sometimes if the payload is lighter Hydrogen+Oxidizer. First stage SRB push trough the thicker part of the atmosphere more-less quickly and you have enough control authority from fins there since the gravity turn is less agressive in stock

[magnemoe]

Even then nerfed the srb tend to produce too high TWR close to burnout as they provide constant trust while mass decreases 
Real world SRB or solid fuel stages burns slower and decreases trust as they empty. Has launched some small satellites using SRB to to get so high I can discard fairing and continue on internal fuel. 

[moogoob]

I use them from time to time. This particular one uses an odd-ish launch profile - you activate the first stage, then ride it up. When it goes out you don't activate the second stage (or Mechjeb's ascent guidance) until you're past the stratosphere - not only does the Terrier get full thrust up higher, but aero tends to flip it  otherwise if you're too low!  You'll also notice that I'm trying the Scott Manley "tilted" launch profile, without control surfaces of any kind on the first stage. Keeps it simple!\

 

I call it the Fool-Justice HALP - High Altitude Launch Pad. (Posted as images because imgur albums are borked right now)

 

 

 

[X-SR71]

I also often use SRBs only for first stage. When doing so, I don't bother with turning - too risky, and simply launch straight up until the boosters are dropped.

[GoSlash27]

In the "tips" department:

Assuming you have adjusted your thrust limiters as described above, I design my first stage to provide 1,800 m/sec DV at 1/2 ATM. This is enough to get you roughly 700 m/sec at 27 km altitude on a proper gravity turn. 27km is the point where you can safely jettison the first stage and ditch the fins without fear of tumbling.

In this scheme, the first stage carries all the fins.

I make 1.2G at liftoff, 1.8G at 60° pitch, 1.5G at 45°, and 1G at 30°. If you have too much thrust, it tends to not want to gravity turn.

1 hour ago, moogoob said:

You'll also notice that I'm trying the Scott Manley "tilted" launch profile, without control surfaces of any kind on the first stage. Keeps it simple!\

*Grumble*... Scott Manley gets credit for a lot of things he didn't invent. We've been doing that around here since ditching the souposphere and the FAR folks have been doing it for years.

Best,
-Slashy

 

[Majorjim!]

Just now, GoSlash27 said:

Scott Manley gets credit for a lot of things he didn't invent.

Yup, he is indeed massively over rated. 

[OhioBob]

1 hour ago, moogoob said:

You'll also notice that I'm trying the Scott Manley "tilted" launch profile, without control surfaces of any kind on the first stage.

It's been done since the 1950s.

[GoSlash27]

16 minutes ago, Majorjim! said:

Yup, he is indeed massively over rated. 

 Well... I don't mean to turn this thread into a tirade about Scott Manley and I don't mean to disparage him. He's an indispensable member of the community and he does fantastic work with his tutorials. It's just that he's a teacher, not an inventor or super- skilled pilot. His skills and knowledge are on par with a lot of folks around here, and I can think of several players who are far better.

 But again... thank Kraken we have him around to teach new players!

-Slashy

Edited August 7, 2016 by GoSlash27

[wumpus]

14 hours ago, MedwedianPresident said:

Yes. By "lack of control" I meant speed control, not orientation control. If you don't nerf your SRB's before launch (Thrust Limiter) you may overheat because the acceleration may be very high.

The various changes between 1.0 and 1.1.3 make it difficult to tell the current options, but any reduction in thrust should be done for control only (unless you have cranked up the re-entry heating and are having heating issues.  1.1.3 base difficulty should overheat the rockets).  Efficiency also seems to go up as the thrust goes high (around TWR>2.0 off the pad, which gets interesting to control).

One low-cost means to control an otherwise rowdy amount of SRBs is to place AV-R8 winglets (the ones that act as control surfaces) on the north and south surfaces of your rocket (the door is facing East).  This lets you control the gravity turn with the fins, and let the capsule or whatever control everything else (which shouldn't require much).  Wrangling a rocket with high TWR can be different from simply setting a course and letting it turn, so expect a few more controls.

Note that the "tilted when launch" does not allow fine tuning of pitch, and tends to reset the angle when touched (to something extreme).  There was a recent forum suggestion where mechjeb could be used to just set the angle when launched (and type in the specific number of degrees), this seems a better choice.

[Majorjim!]

Just now, GoSlash27 said:

But again... thank Kraken we have him around to teach new players!

I agree some have found his tutorials very helpful. I never have as I didn't know of him when I first started playing. I had a fairly good idea of orbits before and the rest was just trial and error.

 I just find some of his comments laughably self reverent.

[GoSlash27]

3 minutes ago, wumpus said:

The various changes between 1.0 and 1.1.3 make it difficult to tell the current options, but any reduction in thrust should be done for control only (unless you have cranked up the re-entry heating and are having heating issues.  1.1.3 base difficulty should overheat the rockets).  Efficiency also seems to go up as the thrust goes high (around TWR>2.0 off the pad, which gets interesting to control).

Weeelll.... I wouldn't be so quick to say that.

 Aside from the control and heating issues, there's also the matter of how you define "efficiency". While it is true that a high t/w is most efficient in terms of DV expenditure, it is not the most efficient approach in terms of payload fraction or cost per tonne to orbit.
 Since we're talking SRBs, the overriding concern is "cheap", else we would just go with LFO (which is much easier to work with and lighter). In this case, lower thrust is preferable.

See the entries in the "cheap & cheerful challenge" here:

 All of the high- ranked entries use SRBs and low thrust off the pad.

Best,
-Slashy

[moogoob]

1 hour ago, GoSlash27 said:

*Grumble*... Scott Manley gets credit for a lot of things he didn't invent. We've been doing that around here since ditching the souposphere and the FAR folks have been doing it for years.

Best,
-Slashy

 

Didn't mean to start something! I would never have tried it if I hadn't watched his "serious business" series, so that's why I credit the guy, it's how I learned it.

You can get some degree of fine control by trimming the thrust limiter, too, especially if your launch TWR is high to begin with. Hasn't high launch TWR become fashionable?

Edited August 7, 2016 by moogoob
space

[wumpus]

1 hour ago, GoSlash27 said:

Weeelll.... I wouldn't be so quick to say that.

 Aside from the control and heating issues, there's also the matter of how you define "efficiency". While it is true that a high t/w is most efficient in terms of DV expenditure, it is not the most efficient approach in terms of payload fraction or cost per tonne to orbit.
 Since we're talking SRBs, the overriding concern is "cheap", else we would just go with LFO (which is much easier to work with and lighter). In this case, lower thrust is preferable.

See the entries in the "cheap & cheerful challenge" here:

 All of the high- ranked entries use SRBs and low thrust off the pad.

Best,
-Slashy

But did they "dial down" the power off the pad?  Getting low thrust by limiting the number of SRBs (compared to the mass of the rocket) in the first stage is completely different than nerfing the thrust of the SRB.  SRBs provide two things: cheap delta-v (from the pad) and cheap thrust.  Typically nerfing the thrust hurts the delta-v as well.

On the other hand, it certainly isn't an absolute thing.  But in almost all cases, if you lowered the thrust of 10 boosters to 90%, you would be better off with 9 boosters firing full thrust, and then moving the remaining (ignoring the issue of balance) on the second stage.  Using SRBs as a second stage is rare, but sometimes makes sense (I'm pretty sure I've made interplanetary probes that worked that way).

[GoSlash27]

3 hours ago, moogoob said:

Didn't mean to start something! I would never have tried it if I hadn't watched his "serious business" series, so that's why I credit the guy, it's how I learned it.

You can get some degree of fine control by trimming the thrust limiter, too, especially if your launch TWR is high to begin with. Hasn't high launch TWR become fashionable?

moogoob,

 Oh, by all means there's no apology necessary on your part. That was all me. And I do hope you didn't take anything I said as a gripe about you.

 IRT your question, yes high t/w has become somewhat fashionable in certain circles, but it depends on what you're trying to accomplish.
 I've noticed a pattern in the schools of thought about the "best" way to get payloads to orbit that tends to mirror the progression of the individual player. First comes "moar boosters* because that's a running joke around here. Next is most DV you can stuff into a lifter, followed by least DV expended to orbit, followed by highest payload ratio, and eventually ending at "cheapest and most reliable".
 There may be another tier beyond that, but if so I haven't seen it yet...

 If you wish to get your payload to orbit cheaply and reliably while keeping it easy to handle, you're not going to wind up putting a lot of t/w in the booster. It'll take more m/sec DV to accomplish the job, but the price tag is reduced and it's easier to fly.

 

Apologies,
-Slashy

2 hours ago, wumpus said:

But did they "dial down" the power off the pad?  Getting low thrust by limiting the number of SRBs (compared to the mass of the rocket) in the first stage is completely different than nerfing the thrust of the SRB.  SRBs provide two things: cheap delta-v (from the pad) and cheap thrust.  Typically nerfing the thrust hurts the delta-v as well.

We did nerf the thrust off the pad, as it is necessary in order to tailor the thrust curves over the duration of the gravity turn. Having achieved that balance, the question then becomes one of the economy of adding boosters in order to boost thrust. While adding boosters will improve the DV efficiency (DV required to get through the boost phase), it actually hurts in terms of cost per tonne of upper stage. Even though less boosters requires more DV to do the same job, it still works out cheaper, which is the point of SRBs.
On top of this is the balance issue; lower thrust SRBs require less active control to keep them on course. Sometimes no active control whatsoever. This adds to the economy of SRB first stage designs, and makes them much easier to pilot correctly... which reduces launch failures.
 Finally, there's the issue of using SRBs for a job they're no good at. An ideal first stage is geared strictly towards cheap, so it's heavy for what it does. A transstage, OTOH, needs to be light. SRB propellant is too heavy and the boosters too inefficient for the job. So it's better, in terms of cost, to transition to LF&O as soon as practicable. That means that the DV savings of high thrust are simply employed in a job they're no good at. In theory, this cutoff would be up around 8km @Mach 1.2, but controllability issues usually push this up to 25-30km @Mach 2.5, since it's convenient to dump your fins at this point.

TL/DR;

 So while *theoretically* high initial thrust is good for efficiency, cost- effective and practical designs will wind up cutting that way back. The super- cheap designs generally work around 1.1-1.3 t/w off the pad.

HTHs,
-Slashy
 

Edited August 7, 2016 by GoSlash27