Someone Fancy Checking My Math For Me?

[jakeb1993]

Hello All,

So i have been playing KSP for a while now and until now I have been generally adopting the Jeremy Clarkson approach to rocket science -i.e if it doesn't work it needs more thrusters and engines. However after adding 8 more 'thumper' solid fuel boosters to my rockets and gaining very little out of it, I decided maybe I do need to start doing some math. I wouldn't mind if someone would just glance over it to make sure I haven't missed something.

So for example this is my test rocket. Yes I know its not very good. So i started by trying to work out the thrust to weight ratio. As I understand you work this out by saying:

Quote

TWR = Thrust Of Engines (kN) Divided by the Multiplication of The ships Mass (t) and the Surface Gravity of Kerbin (m/s)

or in other words

Quote

So i sat down and worked out the mass of the ship by hand - Though the mass indicator gave me a different reading:

Ships Mass

Parts	Total Mass (Tonnes)
MK1 Command Pod	0.84
Crew Cabin	1.00
MK16 Parachute	0.01
2x Radial Mount Parachute	0.02
2x Radial Separators	0.050
2x Nose Cones	0.06
2x RT-10 SFB's	7.125
Total Mass	9.105

I worked it out to be 9.105 Tonnes but the came said that it was 9.4 Tonnes. Even when i double checked i couldn't tell where the extra weight was coming from. But i took the KSP reading of 9.4 Tonnes since it was heavier.

I have two RT-10 SFB's that give out 197.897kN of thrust ASL (At sea level?) So between them they give out 335.794kN of thrust.

The final thing to do is get the surface gravity of Kerbin which i think is 9.81m/s.

So:

9.4t X 9.81m/s = 92.214

TWR = 335.794kN / 92.214

The Number i got was long but when rounded it was 3.6415

That is the answer i think.

I spent all this time working it out and i still  cant get a cheap orbit around Kerbin xD

[Whizzkid]

Your calculation for TWR (Thrust to Weight Ratio) looks alright to me.

Not sure about the mass discrepancy, but your problem isn't TWR, as it is greater than 1, which means that your craft is capable of taking off. Your problem is Delta V and your launch profile.

Delta V is the total velocity change that your craft is capable of. You can use the Rocket Equation to determine this. Alternatively, a mod such as Kerbal Engineer Redux (KER)  will calculate this for you. This should be at least 3500 to 4000 m/s to get into Low Kerbin Orbit, depending on the launch profile.

As for your launch profile, see here for a proper launch profile.

Also, i would advise using liquid fuel engines, as these have throttling capability and the ability to be shut off, unlike Solid Fuel Boosters. They are also much more efficient.

Also, use decouplers to stage the craft, to discard empty stages.

More design tips can be found here.

Hope this helps.

Edited February 21, 2016 by Whizzkid
Added detail and corrected grammar

[jakeb1993]

Hi Whizzkid, 

I knew it was going to be more than that. I just thought that it would be a good start. So i went away and adapted it to this:

Now the TWR on that is 2.916 - Does that mean I can turn the throttle on that to around one third - as at that level of thrust it is still sufficient to provide lift, or does it just mean it can fly? xD

[GoSlash27]

I would guess that the discrepancy is due to the RCS propellant stored in the capsule.

[edit]  all the stuff that @Whizzkid already said. ^ What he said. [/edit]

 You're never going to reach orbit on Fleas. They're simply too inefficient and cumbersome.

 I definitely recommend sticking with the math and forgetting about "moar boosters".

Here's how I run the math:

Rather than building a stage and then checking it's t/w and DV, I do it the other way 'round. I decide how much DV and T/W is needed, and then compute how much engine and fuel is required to achieve that.

Good luck!

-Slashy

Edited February 21, 2016 by GoSlash27

[jakeb1993]

Thanks for that Slashy,

I have been reading it trying to get my head around it. I am a computer scientist but getting my head around rocket science is proving to be a challenge!

So i haven't gotten a orbit yet, but after many hours of trial and error and discovering a few things like gravity turns etc. I made this:

Its not great, but it can do a suborbital route, upto to 70,000 and then down to the ground, using the heatshield to slow down. One of my biggest landing errors was that i was coming in nose down, making myself less resistant to air. Just need to start adapting this design to make it orbit worthy xD

[Bill Zarr]

3 hours ago, jakeb1993 said:

i couldn't tell where the extra weight was coming from.

Your parachute masses were incorrect. 0.1 tonnes not 0.01.

[bewing]

The weight discrepancy comes from the mass of the solid fuel for the two Hammers, I think. The mass of the fuel is not added into the base mass of the part.

The easy way I use to figure launches to orbit is: Use enough SRBs in stages to get you sub-orbital. Then, once you are up there, you need about 400 fuel (or a little more) to get to 2300 m/s and into orbit. Your design would get 3 kerbonauts to orbit. That's a lot for just starting out. It's much easier just to get an OKTO into orbit.

A command pod and crew cabin + fuel tanks + engine takes about 540 fuel to close your orbit and then get back down safely, after the SRBs have gotten you above the atmosphere. SRBs may be a little less efficient, but they cost a lot less than liquid fuel stages, at the same time.

 

[neamerjell]

I know some people like to go completely stock, but if you want to go that route, I would recommend that you install just one mod - Kerbal Engineer. It does most of the math for you. You can see right away the differences in delta-v and TWR just by switching the engines and watching the numbers.

It is good that you want to do the math long hand to gain some true understanding of how certain changes affect the result, and you will also understand what Kerbal Engineer is actually doing "under the hood".

[Whizzkid]

48 minutes ago, neamerjell said:

I know some people like to go completely stock, but if you want to go that route, I would recommend that you install just one mod - Kerbal Engineer. It does most of the math for you. You can see right away the differences in delta-v and TWR just by switching the engines and watching the numbers.

It is good that you want to do the math long hand to gain some true understanding of how certain changes affect the result, and you will also understand what Kerbal Engineer is actually doing "under the hood".

For this, i recommend this video by the great Scott Manley.

[Snark]

One other thing-- beware of setting your TWR too high.  It causes various issues (too fast too low = lots of aerodynamic drag = wasted fuel; exacerbates any problems with aerodynamic stability).

Note:  you can set the thrust amount on SRBs at design time in the VAB.  Just right-click on them and choose the thrust percentage you want.

For example, that ship you showed that has four Hammers around a central core with a Reliant:  that's going to be vastly overpowered on the launch pad unless you dial down the power on the SRBs.  Running them at lower thrust may seem counterintuitive, but the important thing to remember is that if they're burning slower, they'll also burn longer.

Here's a handy technique to use when dialing in your SRBs:

1. Decide what launchpad TWR you want on the launchpad.  I typically like to go for 1.5, which is a bit on the high side.
2. Do the math to figure out "how much thrust do I need to get that TWR."
3. Now add up the default thrust you'd get from all your SRBs if they're at 100%.  (The ASL thrust is listed in the info window on the parts tab of the VAB.  For Hammers, it's like 198 kN apiece).
4. Divide the number in #2 by the number in #3.  That's the fraction you want your SRBs to be at.

 

[FancyMouse]

I thought the title is summoning me.

But seriously, as pointed out earlier, parachute is 0.1/0.2t, not 0.01/0.02t.