Kerbal Space Program - Duna Rocket Launch

collinsmark
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... Continued from part 3.

The Sands of Duna

Part 4: We have ignition

The author of this post (and story), collinsmark, is not affiliated with Squad nor Kerbal Space Program.

We've waited long enough. So let's get this ship out the launch pad.

Kerbals chosen for this mission are
  • Melmon Kerman
  • Jontop Kerman
  • Caller Kerman
Bill, Jeb and Bob, although fully recovered from previous injuries, are taking a well deserved vacation.

We've been through much of this launch business before, but launches are always exciting in KSP. So here's a bit of it.

KSP_326.jpg

[Figure 17: Prepare for launch]

KSP_328.jpg

[Figure 18: We have liftoff (1/3)]

KSP_329.jpg

[Figure 19: We have liftoff (2/3)]

KSP_330.jpg

[Figure 20: We have liftoff (3/3)]

Don't forget to throttle back to keep the surface velocity somewhere around terminal velocity (as a function of elevation. See earlier post near the beginning of this thread). Keep an eye on the surface velocity and altitude; adjust throttle accordingly.

KSP_331.jpg

[Figure 21: First, asparagus stage separation]

KSP_332.jpg

[Figure 22: Shooting through the clouds (clouds are from a visual add-on pack)]

KSP_333.jpg

[Figure 23: SRB separation]

Recall that the SRBs are configured to separate using action groups rather than normal staging, since it's difficult to determine when the SRBs will burn out relative to asparagus stages.

KSP_334.jpg

[Figure 24: Started pitchover (and I remembered to disable SAS and RCS this time!)]

Here we've started the pitchover, which leads into to the gravity turn. With stabilization systems (SAS and RCS) disabled, the rocket naturally pitches over because the center of mass (thus center of gravity) is higher than the center of thrust. Gravity does most of the work bringing the rocket to a more horizontal pointing orientation.

Keep in mind that later we need to counteract this with re-enabling SAS and/or RCS, or by manual steering adjustments. Hypothetically speaking, if we were to allow the rocket to reach a horizontal orientation, the torque caused by gravity would drop to zero, but the rocket still would have a bit of angular momentum that would necessitate steering. Also, realistically speaking, we don't necessarily want to jump straight to a horizontal position. So we'll turn the SAS back on (and maybe the RCS too) once we angle around 45 degrees or so.

KSP_335.jpg

[Figure 25: Continuing pitchover/gravity turn. Notice SAS and RCS are still disabled.]

KSP_336.jpg

[Figure 26: Second, asparagus stage separation]

KSP_337.jpg

[Figure 27: Raising altitude and apoapsis]

KSP_338.jpg

[Figure 28: Third, asparagus stage separation]

It might look like something has gone horribly awry in Figure 28, but no, that's normal. The fireworks are from the sepratrons firing, together with the radial decouplers ejecting, where atmospheric drag is minimal. Those sepratrons are cool. "Never leave home without 'em!"

KSP_341.jpg

[Figure 29: Continuing our initial, orbital insertion]

I'll skip the rest of the initial, orbital insertion for the sake of brevity. We've been through that before.

Suffice it to say the ship got into a low, roughly circular orbit with an altitude just above 70,000 meters (somewhere between 70,000 and 80,000 m). There's a little bit of fuel left in the center stack tank (the one holding the fuel for the high-thrust engine cluster), but not much.

  • Question: Should we raise our orbit to a higher, circular orbit as an intermediate step before heading off to Duna?
  • Answer: No, absolutely not.

  • Question: Why not?
  • Answer: So we can leverage the Oberth effect and save fuel.

  • Question: But our center-stack tank is nearly out of fuel. Once that's gone we're going to be left with a single, LV-N Atomic Rocket Motor that has about as much thrust as sick hamster. We're going to have a hard time keeping our Kerbin escape/Duna injection burn near periapsis, since we're moving so much faster in the low orbit. Oberth effect or not, we want to keep our prograde burn as close to periapsis as reasonably possible, and that will be difficult since burns will take longer with such a low thrust engine. Are you sure you want to keep such a low orbit?
  • Answer: Yes. We have another trick up our sleeve.
To be continued ...

Full Series
Part 1: Introduction and Basics
Part 2: Efficient Launch Into Orbit
Part 3: Rocket Design Basics
Part 4: Rocket Design and Orbital Mechanics
Part 5: Getting to the Mun
Part 6: Getting to the Mun P2
Part 7: Preparing to Land on the Mun
Part 8: Mun Touchdown
Part 9: Staying on the Mun
Part 10: Preparing to Leave the Lander
Part 11: Walking on the Mun
Part 12: Leaving the Mun
Part 13: Rendezvous and Docking
Part 14: Retrograde and Approach
Part 15: Preparing for Docking
Part 16: Docking and Lander Reunion
Part 17: Heading Home
Part 18: Entering Earth's Orbit
Part 19: Landing Back on Earth
Part 20: Launch for Duna
Part 21: Duna Probe Launch
Part 22: Duna Rocket Launch
Part 23: Duna Burn and Periapsis
Part 24: Duna Oberth Effect
Part 25: Duna Mid-course Corrections
Part 26: Using Gravity to Orbit Duna
Part 27: Landing on Duna
Part 28: Exploring Duna
Part 29: Launching Off Duna
Part 30: Fast Return Path From Duna
Part 31: Advanced Duna Flight Math
Part 32: Burn 2 Escape Duna
Part 33: Returning to Earth From Duna
 
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Answers and Replies

pbuk
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I remember the old Battlestar Galactica series (I'm speaking of the original series, back in the '70s. I haven't watched the new ones), spaceships bank when they turn. Why are they banking? Airplanes bank because they get a normal force from the atmosphere, but there is no atmosphere in space! Why bank, I ask. Why?
So the crew don't fall over.

Like when Picard commands "Full stop!" Full stop? In space!? All right, maybe if Q hurls something in their general direction, "Full stop" might just be short for "decrease the ship's relative velocity with respect to that thingy that's approaching." Or if they are in warp, perhaps "full stop" means to drop out of warp. I'll buy that much.

But one time, if I remember correctly, the Enterprise was trapped in some sort of endless, empty void of nothing. (It might have been "Where Silence Has Lease," season 2, episode 2; written by Jack B. Sowards and directed by Winrich Kolbe). In this void there is nothing. No edge, no boundary, no stars, no galaxies, nothing (at least it starts out that way). Eventually Picard commands "Full stop!"

Full stop? Full stop!? Full stop relative to what!? Gah! You - are - in - an - empty - void - of - nothing. It doesn't make any sense!
Relative to the CMB.
 
collinsmark
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So the crew don't fall over.
Hmm. I still don't buy it. It would be more logical to change the ship's heading to the desired orientation first, and then apply thrust only after the desired heading is achieved. This whole banking maneuver just doesn't make sense.

Relative to the CMB.
They were in a void of nothing (at the time). The implication was that there was no cosmic microwave background. There was only nothing.

'Been re-watching all the Voyager episodes recently. Janeway does the same thing except she says "All Stop." Delta quadrant or not, the ship is still well within the Milky Way galaxy, which is moving relative to other galaxies in the local group, and not to mention rotating. "Stopping" relative to the CMB might cause them to shoot through the galaxy quite quickly. "Full Stop" or "All stop" really doesn't have much of a meaning.
 
pbuk
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Hmm. I still don't buy it. It would be more logical to change the ship's heading to the desired orientation first, and then apply thrust only after the desired heading is achieved. This whole banking maneuver just doesn't make sense.
If you change the ship's heading with yaw everyone is going to fall over. To avoid this you need to bank so that the heading can be changed with pitch alone.

Janeway does the same thing except she says "All Stop." Delta quadrant or not, the ship is still well within the Milky Way galaxy, which is moving relative to other galaxies in the local group, and not to mention rotating. "Stopping" relative to the CMB might cause them to shoot through the galaxy quite quickly. "Full Stop" or "All stop" really doesn't have much of a meaning.
2 things, firstly "All Stop" has a quite specific meaning; it means "stop all means of propulsion" which is very different from what is properly called "Dead Stop" but Picard does seem to like calling "Full Stop", which does mean "bring your velocity relative to [some other thing] to zero".

Secondly I didn't say that "Full" or "Dead Stop" always means in relation to the CMB; in the vicinity of a gravitationally dominant body it would make more sense for it to mean "match velocity with the gravitationally dominant body". These conventions are learned at Starfleet Command School.
 

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