Completely solid-fueled first stages?

[Monger]

SRBs are really cheap, so especially during early career mode they are a great choice. If tweaked correctly, they can also serve as second stage, and as very cheap platform for Mini-Satellites.

[wumpus]

4 hours ago, GoSlash27 said:

 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
 

More experimentation for me.

My career mode is mostly halted, and no idea what I'll do with the current mission (which seems to have taken much longer than was budgeted).  No idea when they are getting outside of Kerbin's SOI.  Now I have to face that I still don't understand 1.1's atmospheric model.

I think most of my issues involved use of terrier/poodle upper stages, with those you have to be moving *fast* to be able to move fast enough.  I'll have to wade through the whole thread, I'm guessing at 1.1 you can use kickbacks for a second stage.

[GoSlash27]

20 minutes ago, wumpus said:

More experimentation for me.

My career mode is mostly halted, and no idea what I'll do with the current mission (which seems to have taken much longer than was budgeted).  No idea when they are getting outside of Kerbin's SOI.  Now I have to face that I still don't understand 1.1's atmospheric model.

I think most of my issues involved use of terrier/poodle upper stages, with those you have to be moving *fast* to be able to move fast enough.  I'll have to wade through the whole thread, I'm guessing at 1.1 you can use kickbacks for a second stage.

Wumpus,

 We're probably wandering off into the weeds here, but...

 The Terrier and Poodle are absolutely outstanding transstage engines, but yeah... if you're trying to use them in the boost phase when the rocket is still mostly vertical and going slow, they're not very good. Once you get to the transition point I mentioned earlier, apoapsis is a long time ahead and the rocket is more horizontal than vertical so you don't need much thrust to keep the show rolling. I design these stages to have 1600 m/sec DV and a t/w of 0.7 if I've got another stage or 1700 m/sec budgeted if I'm going to ditch it at the next body.
 That's a personal preference thing; I hate having junk floating around up there, so every stage comes down somewhere.

As for a Kickback second stage, you could do that... but I wouldn't recommend it. It's still cheap for the DV, but it's weight penalty is so high compared to LF&O that it makes your first stage more expensive, which winds up costing more overall. Plus they can't be throttled, gimballed, turned off, or restarted. This makes them just about useless for circularization burns.

Best,
-Slashy

[foamyesque]

@GoSlash27: I was checking through that challenge you linked; correct me if I'm wrong, but didn't the most effective one on 2.0 ruleset leaderboard use a cofiring Rhino?

[GoSlash27]

1 hour ago, foamyesque said:

@GoSlash27: I was checking through that challenge you linked; correct me if I'm wrong, but didn't the most effective one on 2.0 ruleset leaderboard use a cofiring Rhino?

Actually, no. The winner was @Nefrums with a quasi-asparagus design utilizing 2 cofiring twin- boars. He got under $600/ tonne.

All of the cheapest entries utilized quasi- asparagus staging, where the SRBs carried drop tanks for the core which fed it asparagus- style. Hella- cheap and elegant design, but unfortunately it only works with fairly large payloads due to the cost of decouplers and plumbing.

In the case of those parallel designs, we found about 1.6 t/w off the pad to be optimal. 

Best,
-Slashy

 

[WeirdCulture]

19 hours ago, moogoob said:

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)

 

 

 

looks like mine.... exactly like mine 

[numerobis]

I didn't think @Nefrums dialed back the SRBs. At least my reproduction of the craft doesn't, and my autopilot could squeeze out about 50 m/s more and circularize the orbit.

The entire gravity turn profile depends on how your TWR changes. Nefrum's craft starts at 1.5 so a tiny little error in pitch over sends you into the sea. Then it builds to TWR 3, and error sends you too high on an inefficient trajectory (which eventually means into the sea). If you nail it, you can be within a degree of prograde and suffer very little drag.

I don't have a really good way of analyzing that; some numerical experiments helped though. Ignore drag; trace out the gravity turn assuming that at 1m/s (when direction becomes well-defined) you pitch over then thrust perfectly prograde. You'll see how incredibly small the margin is. I use the experiments as guidance of where I should be pointing at various altitudes (1km, 2500m, 5km, 10km).

[Sharpy]

If you use ONLY SRBs for launch, that means the engines you carry are just a dead weight not producing any thrust. Which is plain inefficient.

What I do:

- leave the main LF engine exposed.

- stick some smallish fuel tanks on top of the SRBs.

- crossfeed them to the main fuel tank.

That way the moment I drop the SRBs, my main fuel tank is still full, my main engine kept working, providing valuable thrust, and the only extra cost for that thrust is some cheap pipes and tanks.

[Gordon Fecyk]

On ‎8‎/‎6‎/‎2016 at 6:33 PM, 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?

Have you tried just slapping six Kickbacks on the stock Kerbal-X?

Until I learned proper gravity turns, that gave me enough delta-V to get around the local moons and back. Now that I've learned proper turns and angled launches, the "Kerbal-O" (for "overkill") could take me to multiple biomes on Minmus, or two or more biomes on the Mun, even hauling a 2.5 m service bay full of science kit around.

Definitely works if the boosters alone can get you off the launch pad and 10 km up.

--

Edited August 8, 2016 by Gordon Fecyk
Minor grammar

[VincentThacker]

I've tried before, and everything failed horribly. Firstly, you cannot lower the throttle once started, so that means you either have to have low thrust all the way or high thrust and getting ripped apart. Secondly, SRBs are heavy and expensive. I only use a pair of Kickback clusters for my Space Shuttle (which is accompanied by the engines of the orbiter and fuel tank). In my opinion, it isn't feasible to have a stage made entirely out of SRBs.

Edited August 8, 2016 by 100055

[HafCoJoe]

On ‎8‎/‎7‎/‎2016 at 6:13 AM, moogoob said:

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)

 

 

 

What's the mod that let's you take staging pictures like that? I'd love to take a few myself

[Archgeek]

2 hours ago, Sharpy said:

If you use ONLY SRBs for launch, that means the engines you carry are just a dead weight not producing any thrust. Which is plain inefficient.

What I do:

- leave the main LF engine exposed.

- stick some smallish fuel tanks on top of the SRBs.

- crossfeed them to the main fuel tank.

That way the moment I drop the SRBs, my main fuel tank is still full, my main engine kept working, providing valuable thrust, and the only extra cost for that thrust is some cheap pipes and tanks.

No, they are payload effectively.  You use them later in the launch, when they work better.  Using them earlier is inefficient, as it'll make you bring extra fuel.

I do that too, it works a treat, but the plumbing costs extra, and may not be available depending on carreer progress.  It's also not as cheap as a purely solid stage, though it can be a lot easier to control.

35 minutes ago, 100055 said:

I've tried before, and everything failed horribly. Firstly, you cannot lower the throttle once started, so that means you either have to have low thrust all the way or high thrust and getting ripped apart. Secondly, SRBs are heavy and expensive. I only use a pair of Kickback clusters for my Space Shuttle (which is accompanied by the engines of the orbiter and fuel tank). In my opinion, it isn't feasible to have a stage made entirely out of SRBs.

You won't actually have low thrust all the way, as even SRBs get stronger as they approach their vaccuum Isp.  I think you mean SRBs are cheap, but heavy.  Expensive definitely doesn't describe 'em.  It's definitely feasible, just annoyingly tricky.

[Sharpy]

 

51 minutes ago, 100055 said:

I've tried before, and everything failed horribly. Firstly, you cannot lower the throttle once started, so that means you either have to have low thrust all the way or high thrust and getting ripped apart. Secondly, SRBs are heavy and expensive. I only use a pair of Kickback clusters for my Space Shuttle (which is accompanied by the engines of the orbiter and fuel tank). In my opinion, it isn't feasible to have a stage made entirely out of SRBs.

Oh, that's what staging is for.

One Kickback in the middle.

Some (3?) BACCs around it.

2-3 Hammers on each of these.

Payload on top.

As outer engines burn out, enough fuel have burned in the (smaller number of) inner engines to maintain roughly the same, maybe a little lower thrust.

Also, SRBs are heavy but CHEAP.

[HafCoJoe]

Actually the oldest rocket I have in production (Version I was built in 0.21, I'm on version XIIV now) uses a completely solid first stage  It's the rocket in the final sequence of this video: 

 

I modeled it loosely off my memory of an Ares 1 rocket but didn't intend it to be a remake. It's plainly called the "MkII Orbiter XIIV". It's survived through old aero and now has a fin in the first stage that makes it auto-gravity turn. All I need to do for controlling the first stage is yaw and roll and it automatically pitches itself into a perfect gravity turn. I added that partially for simplicity, but mainly because I was getting very annoyed at the smoke trails not leaving the rocket in the direction of thrust, but rather in the direction of retrograde making them veer upwards of 30 degrees to the side after leaving the engine bell.

[Tyko]

On 8/6/2016 at 9:31 PM, Snark said:

<SNIP>

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.

Really great idea...

[moogoob]

2 hours ago, Avera9eJoe said:

What's the mod that let's you take staging pictures like that? I'd love to take a few myself

I think you meant to quote @Adelaar, the mod he used is KVV: Kronal Vessel Viewer. I have it installed myself - very cool!

[HafCoJoe]

12 minutes ago, moogoob said:

I think you meant to quote @Adelaar, the mod he used is KVV: Kronal Vessel Viewer. I have it installed myself - very cool!

Ach! I think you're right Many thanks

[Adelaar]

2 minutes ago, Avera9eJoe said:

Ach! I think you're right Many thanks

He's right.

[HafCoJoe]

45 minutes ago, Adelaar said:

He's right.

Which means you're right

lol

[Archgeek]

1 hour ago, moogoob said:

I think you meant to quote @Adelaar, the mod he used is KVV: Kronal Vessel Viewer. I have it installed myself - very cool!

Oooh, Kronal still works in 1.1.3?  Very nice, I haven't used it since .90.

[suicidejunkie]

I'm sure it isn't super efficient, but I use an all-solid bag of hammers to create a High Velocity Launch Pad (HVLP); the rockets themselves then carry 1.0 TWR, and cruise at an aerodynamically safe mach 0.9 through the lower atmosphere.

I have not had any need for or use of struts this game.

[AlexisBV]

On 06/08/2016 at 7:33 PM, MedwedianPresident said:

(...) unless you want to save fuel by keeping to the terminal velocity (...)

 

Is this still a thing in 1.1? I remember that from early access, but the atmo model was changed since...

[Archgeek]

5 hours ago, AlexisBV said:

Is this still a thing in 1.1? I remember that from early access, but the atmo model was changed since...

Nope, better efficiencies are to be had by drilling through the sound barrier and going a little bit hypersonic.  Gravity losses tend to trump aero drag unless you're doing something silly.  Fear the temp gauge, but don't fear the flames.

[Alshain]

I've been considering solid/control fin stages recently, though I never bothered to do it in the past.  Now that the thrust limiter actually affects fuel consumption (finally fixed in 1.0) I would expect it to be a lot more worthwhile.  All you really need the fins for is to start the gravity turn after all, then you don't really need control at all.

[numerobis]

The tiny fins are cheap, $25 each. But they melt easily. Any other aero part is expensive, hundreds each, which obviates much of the savings from using an SRB rather than a liquid-fuel engine... unless you put the fins up top on your payload.

As a bonus, if you lock the elevons and deploy them on re-entry you get spin-stabilization. It would kill a human, but kerbals are fine with spinning at incredible rates.