A better Mun landing approach

[90VG]

@MPDerksen Tools used for the video:

OBS - Open Broadcaster Studio - Free

Blender - For video editing, Free

SaveFrom.net - for stealing videos from youtube.

Note that within 3-4 minutes after processing, I got a copyright violation for that upload.  I just make those unlisted to start with and it's not a big deal, they don't give a darn other than spam emails.  I could have done better by fading into the landing and still having the music playing.  Next time.

As far as landing, yeah, you just set your intersect with the surface (not PE) just beyond your target, maybe 10-20% from current location.  Then you can burn retro a little, but you gotta wait until you think you can stop all horizontal movement about on time before hitting the surface.  It's like driving, separate the events so you don't do it all at once.   Brake then turn, don't brake and turn.

 

 

Edited March 29, 2018 by 90VG

[GoSlash27]

MPDerksen,

 

1 hour ago, MPDerksen said:

(what is Oxidizer anyway???)  But since dV, as I understand it, is a measure of ability to change the velocity of a mass, we need to know that mass, don't we?  For now, I'll simply post my amount of remaining fuel until I understand it better.

This is correct. I need to know:

1) exactly how many units of fuel and oxidizer are available to the current stage,

2) the ship's current total mass, and

3) the Isp of the engine (s) currently in use.

I can calculate the DV of the current stage from those data points.

Best,
-Slashy

[90VG]

Just approach, circularize, and quick save.  Practice.

[MPDerksen]

You are being VERY patient and helpful.  I really appreciate it.  Here is Slashy's craft, in LMO ~20K over the surface.  Next, I need a big burn to incline to land where my mission person is stranded.  BTW, I was only 1.2 degrees inclined this time.  Way more stable.

How do I determine the mass? (Math?).  The engine in use is the swivel (you know this, since you built it...)  LSP?

[GoSlash27]

Okay, so I'm assuming that both of those FL- T800 tanks have the same quantities of fuel and oxidizer.

23.97 units of fuel ( x2) plus 29.30 units of oxidizer ( x2).  Adding them all together  yields 106.54 units of propellant. dividing this by 200 yields 0.5327 tonnes of propellant.

What's your current ship mass? Check in the map view, under the "i" tab.

[MPDerksen]

Am I looking at the wrong place?

and yes, your assumption about the x2 is correct.

[GoSlash27]

Yeah, you're looking at the wrong place. Your focus needs to be on your ship. Keep hitting the 'tab' button until the focus is on "MPDGus"

[GoSlash27]

Sorry, I'm winding down for the night. I'll check back in the morning to help you with the math. Find your ship with the tab button and let me know the mass. I'll assume the Isp is 320 s ; vacuum Isp for a Swivel.

We're gettin' there...

Best,
-Slashy 

[MPDerksen]

getting there indeed.  I'm learning at every step.  
9.96t and yes, 320 isp for the swivel in vacuum.

I'm about to go off grid for vacation.  But I'm very interested to see your math when I return.

[GoSlash27]

Okay, 0.5327 tonnes of propellant. Ship mass 9.96 tonnes.

Wet mass is 9.96 tonnes
Dry mass is 9.96- 0.5327 = 9.4273 tonnes
Wet/dry ratio is 9.96/9.4273=1.0565
Take the natural log = 0.05497
Multiply by 9.81= .539
Multiply by the Isp 320s*.539= 173 m/s DV remaining.

I did that leg down to 8km orbit and had 325 m/sec left. Not that big a difference, probably accounted for by the launch from Kerbin.

From my mission
Surface: 2,119 m/sec DV remaining
Orbit: 1,470 m/sec
Kerbin interface: 1,131 m/sec

Best,
-Slashy

 

 

[MPDerksen]

That's 90% clear.  Next step (kinda on my own).  I took the lander portion only.

6.74 tonnes wet mass
800 Units of propellant = 4 tonnes
wet/dry ratio 6.74/4 = 1.684 --> ln = 0.5218
x9.81 = 5.119
Terrier ISP in VAC is 345 = 1,766 m/sec

Why 9.81?
Where is the mistake in my math?  We're using the same lander/fuel.  Yet I see a starting dV of 1,766 m/sec, and you got to the surface with >2K?

[GoSlash27]

MPDerksen,
 Your dry mass is your wet mass minus your fuel mass. 6.74/(6.74-4)

It should work out to 3,046 m/sec DV.

9.81m/sec² is standard kerbin surface gravity. In this case, it's used to convert between specific impulse and exhaust velocity.

Best,
-Slashy