MedwedianPresident

Completely solid-fueled first stages?

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8 hours ago, Alshain said:

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.

I've tried both this and tilting the rocket at launch, and I've found tilt + small amounts of thrust limiting to trim to be more controllable and reliable. YMMV, of course!

6 hours ago, numerobis said:

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.

I've not melted one of these since... 1.0.3? If you're melting them, you're probably going too fast.

No, the main issue with these fins is that they offer very little drag. Which in some designs is just enough, but with an all sold first stage I'd use like 8 of them.

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Fast is the goal, so I feel like I'm going just fast enough!

The first-stage I've started using for those probe-in-particular-orbit-with-thermometer missions is a BACC. Four fins on that, a fairing for the payload, and I'm flying quite well. I use the same trick as others of using launch clamps to initialize the gravity turn -- just a degree or two. I dial back the thrust to 60-70% or so (I forget, I'd have to look) or else the fairing melts; even then the fins melt after I've gotten through the thickest point of the atmosphere.

The payload is a probe core, ant, two oscars of fuel, panels, battery, antenna, thermometer.

Launch cost is about $5k, and 400 of that is launch clamps.

Flight plan is: SAS *off*, and go! The first stage flames out shortly before 30km. Around 40km (10s later) I discard the fairing and first stage and light up the Ant and turn SAS on. Point at the horizon, full throttle, until you reach space. Ap should be about 300km. Figure out roughly what your horizontal speed is, probably about 500 m/s. You need to reach 1830 m/s horizontal at 300km in order to keep from dipping into the atmosphere, so probably about 1330 m/s. Divide the difference by your acceleration (a bit over 3 m/s^2) to see how long your burn will be, probably about 400s (6 minutes and a half). Start burning horizontally when your time to apoapsis is about a third of that -- a smidge over 2 minutes. Why a third? Because as you burn, apparent gravity falls off, so your TTA increases. If/when you reach apoapsis, pitch up a bit to keep from falling too much.

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So, if I understand you correctly, you're compensating for the inefficiency of an "up then over" ascent, by using MOAR BOOSTERS?

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4 hours ago, moogoob said:

I've tried both this and tilting the rocket at launch, and I've found tilt + small amounts of thrust limiting to trim to be more controllable and reliable. YMMV, of course!

I've not melted one of these since... 1.0.3? If you're melting them, you're probably going too fast.

No, the main issue with these fins is that they offer very little drag. Which in some designs is just enough, but with an all sold first stage I'd use like 8 of them.

It depends on your initial TWR. With a TWR from 1.4-.1.6 you will find, starting with a tilt will turn much too quickly.  You will want to build speed before beginning your turn.  

OTOH, If you have a TWR around 2.0, starting at a tilt will not turn as quickly so it will work just fine.

However, starting with a TWR of 1.5 will get you further, as the SRB's will not burn out as fast and the acceleration difference is of minimal impact.

Edited by Alshain

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1 hour ago, suicidejunkie said:

So, if I understand you correctly, you're compensating for the inefficiency of an "up then over" ascent, by using MOAR BOOSTERS?

No, I'm compensating for the high cost of vectoring by using big dumb boosters what are cheap, though they are inefficient.

 

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I rarely use only SRB's alone. Ability to alter cavity patterns would change that though! 

This really needs to become part of the game. Either in vanilla or in mod.
PropellantGrains.gif

Here's STS SRB. Note how propellant is profiled to give a hard kick first, then to keep it constant and in the end to gently die down.
SRB_SRM_Diagram_1977.gif

Edited by ImmaStegosaurus!

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I only use SRB only stages in early career/science mode, it looks and costs better than stacking LFO tanks. I rarely do it in sandbox mode.

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I used a BACC lower stage to get my first kerbal into orbit. Slap a Terrier with a T400 on it as the upper stage, and it will make orbit on fumes (only a 30 m/s margin of error). But hey, it was dirt cheap :P Final cost (not including recovery) was a nice 4,500 Roots.

Edited by TotallyNotHuman_

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I've had a few super light (as in 5 tons or less) 1.25m lifters that had nothing but a kickback SRB for stage 1, but I'm not using anything like that in my current save (I've started using SSTO spaceplanes for payloads that lightweight). 

Sometimes early on in career if I have a fairly light payload, I can get away with 2-4 Thumpers attached radially to a 1.25m liquid fuel core, and stage it so that the liquid engine doesn't ignite until after SRB seperation. 

In both of those designs, I've had ascent profile issues with how fast the solids get you going by the end of their burn, making gravity turns difficult due to drag. 

Most of the time I use SRBs it's for adding a bit of extra DV and TWR to a liquid 2.5m or 3.75m stage, when the launcher i'm using just isn't quite beefy enough. 

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At the risk of being a NecroBones fanboy, I have to put in a plug for the SRBs in SpaceY. Most of them have gimbaling nozzles, just like the real STS SRBs. You can avoid fins and still have great attitude control with a low cost solid first stage.

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On 8/6/2016 at 9:03 PM, MedwedianPresident said:

(unless you want to save fuel by keeping to the terminal velocity and hand-tailor a rocket for every single payload)

A bit late to this thread, but this is not really a thing anymore in the 1.0+ atmospheric model. Terminal velocity can be safely ignored, most fuel efficient is to get as fast as you can as low as you can without burning up (assuming the craft is not stupidly unstreamlined).

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28 minutes ago, Norcalplanner said:

At the risk of being a NecroBones fanboy, I have to put in a plug for the SRBs in SpaceY. Most of them have gimbaling nozzles, just like the real STS SRBs. You can avoid fins and still have great attitude control with a low cost solid first stage.

I've added fins to the stock SRBs with some pretty great results. Depending on the payload, it can work quite well.

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I do it all of the time- They're cheap, They're powerful, and SpaceY is a mod that exists. Nuff' said.

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3 hours ago, Red Iron Crown said:

A bit late to this thread, but this is not really a thing anymore in the 1.0+ atmospheric model. Terminal velocity can be safely ignored, most fuel efficient is to get as fast as you can as low as you can without burning up (assuming the craft is not stupidly unstreamlined).

 

Pretty much. I do frequently thrust limit my SRBs, but that's generally because they're co-firing with a sustainer stage and I want it to have a better TWR post-booster jettison.

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On 8/6/2016 at 4:33 PM, MedwedianPresident said:

hand-tailor a rocket for every single payload

You don't have to do it for each payload though, just have a 5t lifter, 10t lifter, 20t, 30t, 50t, etc.

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On 18.8.2016 at 4:50 AM, the_Demongod said:

You don't have to do it for each payload though, just have a 5t lifter, 10t lifter, 20t, 30t, 50t, etc.

I actually like making new boosters, I stuck to a 15- and 30-ton booster (15-ton looks like Delta IV medium, 30-ton like Delta IV heavy) for quite a long time, but especially with new problems for me, as getting 100-ton eve landers in orbit, I've found out that creating tailored boosters, all with ~3.5k delta V for the payload they're supposed to lift. But if you rather fly a craft than designing it, this is a very good technique to get your stuff to LKO and beyond.

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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?

Yes, I use SRB-only 1st stages all the time.  I really don't care if this is efficient---I do it because it makes the whole rocket simple.  No matter how huge my payload, I get it off the ground with about 1700m/s of SRB followed by about 2000m/s of LFO in a single stage.  As my payloads increase in mass, I just get 5m and 7.5m lifter parts from @NecroBones.  Thus, my late-game lifters have the same basic design as my early-game lifters, just bigger:  Mission payload on transfer stage on LFO lifter core on SRB lifter stage.

I use MJ's dV report instead of KER to design my lifters.  I pick the tanks and engine for the LFO core stage to have the necessary ~2000m/s and a TWR of 1.75-2.0 without thrust limiting.  Then I add however many SRBs are needed to supply the rest of the dV needed to reach LKO.  I then limit the thrust on the SRBs so they have an initial TWR of 1.5.  I do a long, slow, early-starting gravity turn at full throttle all the way and usually only get a hint of flames as I pass through about 30=35km.  I make extensive use of Sepratrons (both stock and mod) to fling the SRBs away when the rocket isn't pointed straight up.  Everything flies nice and stable thanks to a fair amount of available torque and the use of tail fins.  But because the LFO core does the circularization burn and is necessary long (so huge moment of inertia), I put 4 Vernors on the bottom of it to help orient to circularize.

Like I said, I just do this because it's simple.  There's like zero chance of making a staging error, and there are also far fewer parts and joints.  Plus, for a given payload, usually it's cheaper to use a single stack of your biggest LFO parts plus SRBs than it is to make an asparagus lifter out of the next smaller size of LFO stacks.

Here are some examples of my lifter philosophy:

From my current game, a heavy base module with a 3.75m transfer stage on a 5m LFO lifter core with 3.75m SRBs.  https://flic.kr/p/KRBvKz

A similar thing in terms of size from my previous game:  https://flic.kr/p/D2btA5

2.5m cores with 1.25m SRBs from early in an earlier game:  https://flic.kr/p/Bx2WKd

1.25m core and 1.25m SRBs from early in my current game:  https://flic.kr/p/JdSPpw

 

 

 

Edited by Geschosskopf

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4 minutes ago, Red Iron Crown said:

@Geschosskopf Are you igniting the core LFO stage at liftoff or do you wait for the SRBs to burn out first?

SRBs and only SRBs to start with.  The LFO only goes when the SRBs are done.

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14 hours ago, Artheus said:

I actually like making new boosters, I stuck to a 15- and 30-ton booster (15-ton looks like Delta IV medium, 30-ton like Delta IV heavy) for quite a long time, but especially with new problems for me, as getting 100-ton eve landers in orbit, I've found out that creating tailored boosters, all with ~3.5k delta V for the payload they're supposed to lift. But if you rather fly a craft than designing it, this is a very good technique to get your stuff to LKO and beyond.

I only play RP-0/RSS where getting to orbit is quite a bit harder and thus takes more time to properly design and tune a good launch vehicle, so while I do like creating boosters too, it's just not time efficient to spend 3 hours designing, testing, tweaking, and tuning a launch vehicle when my payload only took 30 min to design.

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On 8/6/2016 at 7:33 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?

you can limit their throttle in the VAB with a tweakable so they don't rip themselves apart in the dense lower atmosphere

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On 20.8.2016 at 6:52 PM, peachoftree said:

you can limit their throttle in the VAB with a tweakable so they don't rip themselves apart in the dense lower atmosphere

True, set it to around 1.5 at launch.
I have used them for small satellites and probes. 
An thumper works nice for kicking an 909 terrier up so high its efficient and can take over, you can add an extra stage with an spark if you need range. 

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On 07/08/2016 at 0:33 AM, MedwedianPresident said:

pure thrust is more important than controllability

famous last words...

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On 10.08.2016 at 3:25 PM, moogoob said:

 you're probably going too fast.

Heretic! Burn him! Strap him to an SRB and burn him with the air!

(the only allowable form is "your payload mass is too low.")

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I tried to rebuild a Vega rocket and used this concept in career once. But it never had too much relevance to be used on a regular basis. Maybe when the satellite contracts come up more frequently but up until now it's mostly a little toys. And now, costs are not so much a factor now.

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