Require help in designing new method of building my rockets.

[Moonfrog]

My current method is to build a payload, the lander or probe etc. Then have a transfer stage, based mostly on how much Delta V I need. Next stage is for orbital insertion and pushing through last two atmosphere stages, or thereabouts. I build this to around TWR of 1.05, roughly try to get 3000+ Delta V out of it. Then I build a launch stage with TWR of 1.4 1500+ Delta V for takeoff.

I read somewhere while looking for stuff to do on ksp. that solid boosters are sometimes paired with liquid engines to help with thrust. I was trying to think how to incorporate them into my design.

I imagine solid boosters are used in the launch stage, but if I use say the 250 thrust ones, pair them with a 200 thrust ones. I would want to keep my TWR at 1.4 for lift off.

normally I would do this. 215 thrust, + 500 from two side boosters. 715 total.

Then I do, 715 / 9.8 / 1.4 = 52.1 which would roughly be my total mass.

then do 52.1 minus Payload of what i'm building. Then subtract engine mass and any bits that are not fuel. The rest of the mass I use as fuel. Then calculate its Delta V.

My problem is when the boosters finish the TWR always drops below 1, so it falls backwards. I know I have to build whatever is left to have twr with the liquid engine of 1.4 as well or a bit higher since it would have travelled anyway. The mass of the fuel becomes much less, so TWR with the two boosters is no longer 1.4.

Got to the point, where I don't know how I am enhancing the liquid engine by using the boosters as a sort of hybrid thing to increase thrust. I'm sure it has something to do with the time the engine is active, till it runs out of fuel. I don't know how to time the booster to match the time of the liquid engine.

Idea is if booster with more thrust lasts as long as the liquid engine, during that time, the liquid engine could lift higher payloads, even though engine is much smaller.

Just can't figure out a way to do it while i'm building a rocket. I have always wanted to find a genuine use for boosters, I think this may be it f or me. As a thrust booster, not as delta V, since in terms of Delta V the liquid engines are far superior.

[celem]

I dont let my srb's enter the math like that. Rather than allow for them, and then have your TWR dropping off halfway through your main launcher's ascent, you still design the launcher to have the TWR you want. You then run it at greatly reduced throttle while the srb's are active, and ramp it up again once the srb's are dropped. Net result is usually extra fuel reaching orbit in the form of a half-spent orbital burn stage or whatever. Just take it with you as bonus fuel or use the left-overs to de-orbit your insertion stage.

Otherwise as you point out, your craft ends up a little gutless in that window between srb's dropping and ditching whatever jumbo/mainsail style launcher you are using. For me this is often round the 20km point, which is a dangerous place to be strapped for thrust. An alternative is to use several rings of the smallest booster instead of a single larger one, they burn a little over half the duration, so 2 rings equals out to about 1 ring of big boosters, and by carrying that second ring into the trouble-zone with you, you can kick through the low TWR phase by applying that thrust exactly when you need it. (On some of my launchers i carry a ring of the smallest srb's to fire after I drop the last asparagus tank, this is a point where im suddenly on 1 mainsail yet still pushing a pair of orange jumbo tanks from the launcher, along with interplanetary/payload. Even in thin air, thats a lot to shift)

Net result is exactly as you suspect. srb's dont do delta-V, they are all about grunt when your rocket lacks it on an uphill climb

Edit: Incidentally: Top-down is definately the smart way to design, especially since you seem to actually be doing your math. (or using ker for it, either way)

Edited November 18, 2013 by celem

[Seret]

The SRBs are good for providing tons of thrust at launch, but are big heavy lumps so you obviously fire 'em and drop 'em as soon as possible. They can be handy if your TWR without them is a little low and you want to get yourself a few thousand metres of altitude quickly. If your TWR with them is too high you can always launch at partial thrust on your liquid engines, and throttle up when you ditch the solids. I try to keep my G meter in the green, which does mean launching at less than 100% throttle on the liquids for some designs. I do tend to attach SRBs to most designs, they're a simple cheap boost to ÃŽâ€v that makes for a nice frisky performance off the pad.

[TechnicalK3rbal]

Use asparagus staging, or add clusters of SRB's and stage them at different times to increase your twr throughout the flight. Also, you could launch smaller payloads.

Edit:ninja'd

[celem]

Another thought, and something I use from time-to-time on smaller launches.

You can use a solid only first stage. Drape the ship in enough booster to get a twr of something like 1.4 and dont light the liquid at all till the drop.

It's like launching off a mountain-top and often lets you do away with a lot of liquid anyway.

Beware stability and control when running solid only. No thrust vectoring so you better have built it right

[Soda Popinski]

You really need a higher thrust to weight ratio. I kept adding more rockets / fuel, but delta-V wasn't the problem, it was the thrust to weight ratio (and adding asparagus stages with the same ratio didn't help). I usually shoot for 1.7-2.2. Lower and you waste energy on gravity drag, higher, you waste it on atmospheric drag.

So you need to build your rockets to have BOTH enough delta-V to get to orbit and have sufficient TtoW ratio to utilize that delta-V properly.

On this Basic Maneuvers guide, you can see the optimal speed at specific altitudes based on terminal velocity. Try not to go much below this.

Before I unlocked the Mainsails, I was launching 60t payloads using a 12 asparagus staged rocket, with the first 6 stages running skippers with 2x Rockomax 32 tanks, and an inner ring with single Rockomax 32 tanks.

[Nao]

Use asparagus staging, or add clusters of SRB's and stage them at different times to increase your twr throughout the flight. Also, you could launch smaller payloads.

Edit:ninja'd

If you use boosters like that, a simple one stage skipper or mainsail will give better performance. The reason for boosters to have good performance is their "zero" engine mass, their mass efficiency of burning fuel is bad and if we don't use all of them from launch it just means that we tug along an inefficient dead weight.

As for OP's question. The original idea of boosters comes from increasing burn time (fuel amount) of current design filling the thrust gap at launch with them. The problem with KSP is that we don't have much parts to choose from right now. And current boosters have really big amounts of thrust. For example for only two RT-10 boosters it takes rocket with two mainsails to have same launch TWR at launch and at booster separation.

But there is more to them, due to way KSP does drag. It is most efficient to fly just below terminal velocity, and that takes only ~1,8 TWR mid flight to maintain it. It takes much larger TWR thou, to just accelerate the ship from launch pad to desired 100-120m/s early. This is where it's good to have boosters.

While building, just design for around 1,5-1,6 after booster separation, and let any excessive thrust at launch reduce deltaV requirement to reaching orbit. (resonably: up to around ~2,2 TWR at launch with boosters).

edit:

You really need a higher thrust to weight ratio. I kept adding more rockets / fuel, but delta-V wasn't the problem, it was the thrust to weight ratio (and adding asparagus stages with the same ratio didn't help). I usually shoot for 1.7-2.2. Lower and you waste energy on gravity drag, higher, you waste it on atmospheric drag.

TWR of 1,5 is perfectly fine for me, a little on the low side but if the stage is long burning it's plenty. Having more than 1,8 on average will just waste fuel.

Edited November 18, 2013 by Nao

[700NitroXpress]

Don't use the SRB's if you're building lifters that need to get heavy payloads into orbit. Drop tank asparagus is one of the best options there is. If it's a light payload, then you could still use drop tanks, or an air lift to high altitude and then fire an orbital engine. I would recommend building a "one size fits all" lifter solution and drop it into the subassemblies area. That way, you can just build your payload, fix it to the transfer stage, then just grab the same lifter and use it every time. In case you haven't tried it yet, the drop tank asparagus design allows you to maintain full thrust the entire time until you run out of drop tanks and then it switches to regular asparagus staging. By the time you run out of tanks, you should already be in high altitude or close to orbital altitude. This means that you can throttle down as you go while maintaining TWR. You drop dead weight and don't drop tanks with engines on them.

Overall, I find that SRB's are useless outside of campaign mode.

Here's a video for you. Download link is here: https://drive.google.com/file/d/0B-vTRL2n8wvzQkEtUTBCNHpIMW8/edit?usp=sharing

[Vanamonde]

As others have said, SRBs have no steering and burn out relatively quickly, so the only way I use them is for a quick kick to help a ship get off the pad and up to an efficient ascent speed for the liquid engines to sustain.

[SRV Ron]

As others have said, SRBs have no steering and burn out relatively quickly, so the only way I use them is for a quick kick to help a ship get off the pad and up to an efficient ascent speed for the liquid engines to sustain.

Some SRBs have vectoring and some don't. When using a ring of SRB for first stage, brace them in a ring and X pattern. That will eliminate twist and rotation. Properly braced, it should not be necessary to brace to the core stage. The radical decouplers can hold them on so long as there are no twisting forces.

Design your launch vehicle so that it can fly without boosters. Then, adding the booster ring will give you additional fuel for missions once you reach orbit.

This example of a ring of SRBs and asparagus staging works quite well for my interplanetary probes.

Stock career mode

NovaPunch design;

The design drops the liquid engines once in orbit allowing the more efficient LV-N to use the remaining fuel from launch for interplanetary travel. I stage them just prior to reaching orbit so that they will drop out of orbit.

SRBs only are used for the boost phase of launch. The X bracing makes the rockets easily controllable during launch to orbit. The orbital turn starts after staging - usually from 5000 to 8000 meters.

The fuel meter is measuring the fuel in the drop tanks. The core tank is full.

Edited November 18, 2013 by SRV Ron

[Vanamonde]

Some SRBs have vectoring and some don't.

There are only 2 models in the stock game, and neither can vector thrust. Are you saying some modded SRBs do?

[SRV Ron]

There are only 2 models in the stock game, and neither can vector thrust. Are you saying some modded SRBs do?

Some are listed as doing so. Not the stock ones. However, the designs posted are stable enough that the SAS at the probe is sufficient to control the rocket during launch to orbit.

[SRV Ron]

Duplicate posting