Newb question about Solid Rocket Boosters

[Nich]

Ok I just want to set some things straight.

1. Reducing thrust to prevent going faster then terminal velocity.

This is just wrong.  Yes this can happen if your atmospheric TWR is greater then some ridiculously high value at launch.  Some one did some research on it but I cant find the post.  The fix is not throttling down but adding more fuel or using a smaller engine to get TWR down.  Check MJ I believe it will tell you how much dv you have used fighting gravity, friction, and cosine losses.  low TWR = Gravity losses, High speed = friction, bad gravity turn and REALLY bad TWR cause cosine losses.  Gravity losses are the dominant cause of loss.  The most efficient launches are basically fireballs until engines cut out and they coast to AP.

2. Second stage TWR of 2 or 2.5.  This is very wrong if you are doing a gravity turn.  If you are going straight up until 70km and then circularizing it would make sense.  In reality this is very complicated but you can launch a craft with a .6 second stage TWR what is really important is what speed are you going when staged, what altitude, how much dv and time to burn.  Almost all of my 20t+ rockets follow the same pattern.  I put in a .6-.8 twr high altitude engine.  I put in a low altitude stage and add fuel until the atmospheric TWR is .8.  Then I add SRBs until atmospheric TWR is 1.5.  Yes I know it is not optimal but it is easy, cheap, quick and it still does very well.  In fact they generally out perform highly tuned lifters.  For example if I have a 10t lifter that can lift the payload for 900 kredits/t and I use it to lift a 7.3t payload my funds per ton go up to 12,300 funds per ton.  If I use my slap together method it will generally come out to 10,000-11,000 funds/t

3. Pushing MJ and KER asap.  And this is all personal so to each his own.  Learn the math do 2-3 launches maybe 1 mission on paper and then get the mods and never look back.

dv=ISP*g*log(m0/m1)  (I hope I got that right it would be really embarrassing if I flipped m0 and m1)

g = 9.81 m/s^2

m0=starting mass

m1=final mass

4.  dv to kerbin orbit is not 3400-3600, 2450 is possible (but not practical and requires a ton of TWR and no payload)  Ignoring friction and with infinite TWR 2289 I believe is the theoretical minimum.  An interesting note that of the 2289 something like 2024 is just getting to orbital speed and 124 is the transfer from 0m to 70x70km.  (don't quote me that is the best I remember)

5. Skipper is a Great engine and fills a HUGE gap between the reliant/swivel and mainsail.  5 mainsails would be overkill on TWR and a loss of dv

6. I think @GoSlash27 is wrong about SRBs there not cheap their VERY CHEAP generally SRBs will make up 1/3-1/2 the mass of my ships on the pad but only provide 1/5-1/3 the dv.

7. Burning main engines and boosters vs boosters then mains.  Depends on a ship by ship basis.  Yes burning booster first will make more dv because they have a bad ISP so the LFO engine is wasting energy accelerating it, BUT BUT BUT at liftoff you experience most of the gravity losses and higher TWR will generally make up for the loss of dv.  I would argue a middle ground of mains full throttle until 250 m/s then 1/4 throttle until SRBs burn out.

Edited March 13, 2016 by Nich

[KocLobster]

On 3/1/2016 at 4:15 PM, GoSlash27 said:

We have excellent tutorials on the forum for how to do this. I recommend starting here:

+1 to this

When I was just starting out (very recently), this thread helped me out a ton. It'll give you a very good understand of the 'why' behind what works and what doesn't, so that you can start building rockets and know what to expect when you use X part. There are lots of other incredibly well made tutorials too. When you get a little more basic understand of the game down, I highly recommend:

 

[wumpus]

17 hours ago, Nich said:

Ok I just want to set some things straight.

1. Reducing thrust to prevent going faster then terminal velocity.

This is just wrong.  Yes this can happen if your atmospheric TWR is greater then some ridiculously high value at launch.  

This is a leftover from the broken aero model of <1.0.  To leave the souposphere, you would throttle your engines down to terminal velocity for maximum efficiency.

The current aero model is much closer to reality (although I assume that FAR is still somewhat better).  You best bet is to simply lift off with TWR between 1.3 and 2.0 (preferably closer to 1.7) and let the rocket gain TWR as it burns fuel.  Note that if you take off with a high TWR, you will probably need another high TWR second (insertion) stage as the time between the end of the first burn and apogee won't be all that long (high TWR lends itself to steeper trajectories).

* Note that for subsonic speeds, there really isn't such thing as "too high a TWR".  It's just that it suddenly becomes inefficient in a few seconds (and thus a bad idea to bring an oversized engine on the rest of your flight).  If you really need to do such things, it is possible to recover "0-stage" boosters in stock KSP that are ejected under a few km.  Expect to spend a great deal of time tweaking parachute parameters, tuning aerofins (expect to have tiny SRBs stopping the booster in flight and pushing it back to the launchpad).  I can't image it being worth your time, but I did manage to do it once.

[Warzouz]

On 3/3/2016 at 1:10 AM, Giygus said:

...

Does the Dv values shown have some leeway?  For example, it says you need 3200 to leave Kerbol, could you ever design one that needed less? (I would assume an inefficient design would need more?)

3200m/s is a successful launch. If you burnt 3500, even you have a very big and unstreamlined payload or you messed you your ascent profile.

But, on the other hand, trying to reduce dV by design is not efficient. Once you have a design, you can optimized it by choosing a nice ascent profile, thus reducing the fuel you need and save a little. But if you do the other way around, you'lle end with a low dV but very expensive and low payload ratio.

 

Let me give you an example : I tested a high TWR rocket for fun. I built a quick rocket with TWR=5 (that quite absurd). By launching the rocket inclined, I achieve orbit with 2850m/s (if I remember correctly). Was it useful ? To have fun maybe, but no practical value as the rocket had 0 payload.

I build rocket with 3400m/s but I need to deorbit them and recover them. My usually disposable rocket has only 3200m/s.

 

When you get a working rocket, I recommend you to test various ascent flight plan. A good indicator is the altitude where your prograde vector crosses cross 45°. Each time check your remaining fuel while circularized around 75 or 80km. THEN you can tweak your fuel mass to get less dV

[Spaceboy720]

Yeah, what you have to do is if you aren't in early career/science, only use them for lower stages. (unless you use only solid fuel to laythe and back, thats cool too)