Question about Rocket Stats (MechJeb info window) Newb

[Giygus]

First, still learning at this, and I have been asking some questions around here, hope I am not getting annoying.

So, I am trying to use MechJeb information to help build a better rocket ship, but am finding it opaque.  Here are three pictures of a launch rocket with different types of stages.  Tell me if my understanding of what I am reading is correct.  Note that I am mostly interested in the lifting stage designs, and will be talking about "in the atmosphere" stages exclusively.   Pictures behind spoilers because they are large.

Spoiler

OK, here is a simple rocket. 

Weight: 80 tons, down to 48 tons when lift stage is empty.

Sea Level Thrust starts at 1.75, which is good enough for leaving Kerbin.

Atmosphere DV is 1423, which is less then half of what I need.

If this stage were in a vacuum, the DV would have been 1548, which is only a bit better.

Spoiler

So needing more DV, I add 3 side tanks, made to be ditched during the accent.  This changes the numbers.

Weight Question:  Why does stage 7 have an ending mass of 106.8, but stage 6 has a start of 80.1?  Do the three tanks/engines/crap weigh 26 tons, and it goes down when we decouple them?

SLT:  Our thrust at launch is down to 1.41, which is the low end of OK.

DV:  We added over 1800 DV, which is nice, but 1800 + 1400 is still not enough to go all the way up.

Spoiler

Still needing more power to get out of the atmosphere, I add 6 med. sized SRB set to full thrust.

Weight: We are up to a total of 252 tons.

Weight Question:  Why does ending mass of stage 8 say 175 tons and not 202 tons?  Is it because we burned about 25 tons of fuel while the SRBs were also firing?

SLT:  We are up to 1.74 thrust from 1.41, a good number.

SLT Question:  SLT of Stage 7 now says 1.77 as opposed to 1.41, which it was before the SRBs.  Is this because of the liquid fuel we burnt while Stage 8 was running?

DV:  We now have 860 in stage 8.  When stage 7 starts, we now have 1201.  I assume that its 1201 and not ~1800 because of fuel spent during stage 8.  860+1201= 2061, so adding the 6 SRB gave us about +200DV

Overall, 2061 + 1423 = 3484, which is the bare minimum to get into orbit.  Since I am not very good at this yet, I will need more.

OK, that's it for pictures.  Does anyone see something I am missing, or understanding incorrectly?

 

Last question I asked , I received an answer that, after thinking about it, is confusing me more and more.

The comment was: 

Quote

GoSlash27

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Posted Wednesday at 6:00 PM · Report post

Giygus,

 In both examples, you already had a t/w of 1.6 and enough DV to get the job done. All adding SRBs was going to do for you is increase thrust, which isn't going to help you. Now... if you had added fuel so the SRBs had something to lift, you would've noticed a dramatic increase in DV. Or if you had left the first stage turned off and used the SRBs exclusively as the first stage.

 As you can see, just tacking on "Moar Boosters®" as an afterthought does very little to help, and can easily cause problems that render a design useless.

The central problem is that you're following an incorrect process for designing a lifter.
1) You're taking a working design, adding payload to it, and then trying to get it to work again with additional boosters.
2) You're neglecting aerodynamics, which is actually important in 1.0 and beyond.

Foxster's tutorial addresses both of these problems and walks you through how to design a lifter the right way.

Best,
-Slashy

Specifically, I do not understand this:  "...SRBs was going to do for you is increase thrust, which isn't going to help you. Now... if you had added fuel so the SRBs had something to lift, you would've noticed a dramatic increase in DV."

I don't comprehend how you can increase thrust without increasing DV.  If Dv is a measure of the total amount of acceleration your ship can output, how can increasing thrust not really increase DV?

Thanks for time spent on this stuff, still trying to wrap my head around it.

[Norcalplanner]

There's a staging problem where the mainsail isn't firing at the same time as the vectors, which means you're not really doing onion staging. What slash was saying is that adding SRBs without changing the rest of the rocket is going to have a very limited effect. I'd recommend ditching the radial stages, add 1 or 1.5 more orange tanks above the mainsail, then adding SRBs firing at the same time as the mainsail to bring up the initial TWR to 1.4 - 1.5. Thrust limit them as needed in the VAB so that the TWR after you stage the SRBs is at least 1.1.

[FancyMouse]

> hope I am not getting annoying.

First of all, don't feel bad about asking question - it's always good to ask. We all learn from questions, regardless of who asked.

> Note that I am mostly interested in the lifting stage designs, and will be talking about "in the atmosphere" stages exclusively

You may not realize that atmosphere gets thin enough even at just 10km altitude - you can right click your engine to figure out its thrust/Isp when running. For a typical rocket launched on Kerbin, most of the time from the second stage it's already almost vacuum.

> Weight Question:  Why does stage 7 have an ending mass of 106.8, but stage 6 has a start of 80.1?  Do the three tanks/engines/crap weigh 26 tons, and it goes down when we decouple them?

Yes. I didn't check the exact number but the magnitude sounds right. And that's the exact advantage staging gives.

> Weight Question:  Why does ending mass of stage 8 say 175 tons and not 202 tons?  Is it because we burned about 25 tons of fuel while the SRBs were also firing?

> SLT Question:  SLT of Stage 7 now says 1.77 as opposed to 1.41, which it was before the SRBs.  Is this because of the liquid fuel we burnt while Stage 8 was running?

Yes. Since you have both SRB and LFO engine activating at stage 8, most info mods should be smart enough to calculate both fuel burnt and calculate mass/deltaV from there. Furthermore, most mods should assume you run full throttle for LFO engines. When you actually execute the launch, you may choose to limit the throttle, in which case these number in reality could be different (start mass higher, delta V higher, TWR lower) for the next stage where LFO engines are still running.

> If Dv is a measure of the total amount of acceleration your ship can output, how can increasing thrust not really increase DV?

A simple sentence is: you misunderstood. GoSlash said "help", this does not equal to "increase dV". More SRB tends to increase 1st stage TWR which leads you to the trap of going too-fast-too-low. It's not helping in that sense.

 

Really, atmo 3.4km/s dV is more than enough to go to orbit. I suggest you can start optimizing/practicing launching the rocket now.

[Giygus]

1 hour ago, FancyMouse said:

...

> If Dv is a measure of the total amount of acceleration your ship can output, how can increasing thrust not really increase DV?

A simple sentence is: you misunderstood. GoSlash said "help", this does not equal to "increase dV". More SRB tends to increase 1st stage TWR which leads you to the trap of going too-fast-too-low. It's not helping in that sense.

...

OK, that may be what I am missing.  Would you agree that SRBs are mainly used to increase THRUST and not DV?

For example, lets say I need more DV, so I add a few fuel tanks.  That lowers my TWR, so I add a few boosters to help increase thrust at the bottom of the atmosphere.

[FancyMouse]

Just now, Giygus said:

OK, that may be what I am missing.  Would you agree that SRBs are mainly used to increase THRUST and not DV?

For example, lets say I need more DV, so I add a few fuel tanks.  That lowers my TWR, so I add a few boosters to help increase thrust at the bottom of the atmosphere.

Both will be increased, but the purpose is only for thrust most of the time. You don't add SRB for the purpose of increasing dV because it's inefficient.

And no - if you need more dV, you should really redesign the rocket. Especially when you find you need to do add extra engines to compensate extra fuel - it's really a sign of redesign. Most of the time a redesigned rocket performs a lot better than a patched rocket whose original intention was different.