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Author Topic: What is the acceleration of the DGIV?  (Read 4385 times)

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Offline GXE3

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11 March 2008, 20:19:34
Just as the topic asks, what is the acceleration of the DGIV?


-GXE3
A 15 year old.
Long Live Java


Offline Pirx

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Reply #1 - 12 March 2008, 00:04:21
There are at least 3 accelerations in DGIV :)

1. ACC displayed on Surface MFD. It shows the rate of change of ship's velocity. This would have been closest to the
definition of acceleration, except that Surface MFD considers only the length (modulus) of the velocity vector. In
physics, though, change of the speed direction without change of its amount is still acceleration.

2. Acceleration on the D3 display. It shows the acceleration experienced by the crew. This is the acceleration of the
ship due to all forces, except gravity. Unless I am missing something, in Orbiter this leaves us with engine thrust and
aerodynamic forces (lift + drag).

3. ACC (bottom line) on Engine Display. This is the acceleration of the ship due to engine thrust not considering any
other forces

ACC [ m/s^2 ] = Thrust [ N ] / Ship mass [ kg ]

In case of a circular orbit and pro- or retro-burn this happens to be the actual acceleration of the ship.

So if you want to calculate at what distance from a Moon base to start braking, the initial approximation could be
(typical numbers for DGIV in "Moon" configuration, launch from KSC):

ACC = 260 kN (= 260,000 N) / 16500 kg = 15.76 m/s^2

If the orbital velocity is V = 1640 m/s, the "stopping distance" of DGIV will be

V^2 / (2 * ACC) = 85.3 km

But as I said this is just the initial approximation. What we are ignoring is:

A. As the orbital velocity decreases, part of the thrust should be directed "up" to prevent the ship from impacting the
surface short of the target.

B. As the ship burns fuel, its mass decreases, and, given constant thrust, its acceleration increases.

Both affect the stopping distance in opposite ways: A increases it and B decreases it.

Unfortunately, it is not possible to derive a simple formula for the stopping distance that takes A and B into account.
However, as a worst case estimate, you can consider that a portion of the engine acceleration counters the gravity
during the entire braking. Then the "useful" (for reducing the horizontal velocity) acceleration would be

Sqrt( ACC^2 - g^2)

where g is the acceleration at the surface. 9.81 m/s^2 for the Earth, 1.62 m/s^2 for the Moon.

V^2 / (2 * Sqrt( ACC^2 - g^2)) = 85.8 km

Luckily, due to the high thrust / mass ratio of DGIV and low Moon gravity, the optimistic and pessimistic estimates are
very close.


Offline Pagir

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Reply #2 - 12 March 2008, 02:16:39
Since there are many kind of engines with the DGIV, I suppose the acceleration varies accordingly :wonder:

Pagir


Pagir

Offline GXE3

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Reply #3 - 12 March 2008, 03:30:32
Thanks Pirx.....again. I can always count on you to thoroughly answer my question. First it was the "IMFD Help" topic (where
you literally wrote an entire book on the IMFD), then it was the "Orbiter Formula" topic, an now this. Thanks a bunch.

Quote
Since there are many kind of engines with the DGIV, I suppose the acceleration varies accordingly :wonder:

Yes, the acceleration does vary. I couldn't figure out why I kept miscalculating until I realized that always use the 320kN
engine. :). The formula works now.


-GXE3
A 15 year old.
Long Live Java


Offline Pirx

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Reply #4 - 12 March 2008, 20:41:49
Everybody who asks a question I have asked myself while learning Orbiter can count on me. As a matter of fact the sole reason I registered here was to answer one such question (by Tachyon).

Too bad most of the posts on Dan's English forums are not dealing with space navigation or more creative uses of
DGIV than smashing it into the ground :turning:



Post Edited ( 03-12-08 21:05 )


Offline James.Denholm

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Reply #5 - 13 March 2008, 09:46:32
And how is smashing a DGIV into the ground not creative? It is an art form! Or, at least, it could be considered art when you
look at some of the "contemporary" junk you see these days... some of it is good, most of it is cra... b... bish


-------------------------------------
The etiquette of a cigarette, vinaigrette mixed with anisette, the silhouette of a clarinet, is but a stockinet in a landaulette.
http://en.wiktionary.org/wiki/Wiktionary:Rhymes

Offline Tachyon

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Reply #6 - 14 March 2008, 16:41:24
err .. ah ..what ... did I hear my name ??? :zzz:

.... I always enjoy reading your posts to anyone's questions as they are a treasure trove of
information. :applause: :wor:


My god - it's full of stars !

Offline GXE3

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Reply #7 - 15 March 2008, 23:37:17
Pirx, what are some of the most important formulas that you personally use when playing Orbiter?


-GXE3
A 15 year old.
Long Live Java


Offline Pirx

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Reply #8 - 16 March 2008, 13:22:14
Hmm... I think I only use braking distance on bodies without an atmosphere and launch azimuth calculation (as
documented in DGIV manual). The rest of the computations are performed by MFDs.

Do you have a particular problem in mind?


Offline GXE3

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Reply #9 - 16 March 2008, 21:03:23
nope. Just wondering if I could dig any more information out of that valuable head of yours.
:D

Thanks


-GXE3
A 15 year old.
Long Live Java


Offline James.Denholm

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Reply #10 - 16 March 2008, 23:40:22
Do as the Freeman does: crowbar.


-------------------------------------
The etiquette of a cigarette, vinaigrette mixed with anisette, the silhouette of a clarinet, is but a stockinet in a landaulette.
http://en.wiktionary.org/wiki/Wiktionary:Rhymes

Offline Pirx

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Reply #11 - 17 March 2008, 07:54:10
Well, my first association about brain digging was the dessert in the dinner scene from "Indiana Jones and the
Temple of Doom". But young people of today, they have probably not even heard of this movie :badsmile:


Offline James.Denholm

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Reply #12 - 17 March 2008, 08:15:25
I've seen the first one! On VHS!

Wait... I probably shouldn't have admitted to that...


-------------------------------------
The etiquette of a cigarette, vinaigrette mixed with anisette, the silhouette of a clarinet, is but a stockinet in a landaulette.
http://en.wiktionary.org/wiki/Wiktionary:Rhymes

Offline Kadet

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Reply #13 - 18 March 2008, 00:51:44
Ion engines for 2045, vote now!

---------------
"Tobi is a good boy!"Why let HIM join Akatsuki!?


Offline FetDaniel

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Reply #14 - 24 March 2008, 15:34:22
Great information Pirx, I am sure alot of people can find value in such an intelligent answer. =)
I remember that they do have a soup in "Temple of doom" where they have eyes I think.

I guess young is relative, I still feel young but I also feel old in certain technological aspects that many are very used
to. =) But I still am very fresh.. ^^

« Last Edit: 24 March 2008, 15:34:22 by FetDaniel »