Topic Actions

Topic Search

Who is online

Users browsing this forum: Google [Bot] and 53 guests

the Destroyer future - a new take, with fission!

Join us in talking discussing all things Honor, including (but not limited to) tactics, favorite characters, and book discussions.
Re: the Destroyer future - a new take, with fission!
Post by SharkHunter   » Sat Jul 11, 2015 5:04 pm

SharkHunter
Vice Admiral

Posts: 1608
Joined: Fri Nov 14, 2014 3:53 pm
Location: Independence, Missouri

--snipping--
Jonathan_S wrote:6 km/s works out to about 612g; fast but something many RMN ships can now do.
And 610g for 4 hours does give me about 0.288c top speed.
But I came up with the ship covering 621,831,168 km; or 35.5 LM.

Using Bill's 510g accel I got 5.1 hours to 0.306c, covering 842,386,910 km (46.8 LS) -- right in line with his calculation; if you allow for some rounding.
Trying to figure out the math... can y'all give me an assist there? We ended up with the same speed at the same point, so my distance calc must be botched somewhere....
---------------------
All my posts are YMMV, IMHO, and welcoming polite discussion, extension, and rebuttal. This is the HonorVerse, after all
Top
Re: the Destroyer future - a new take, with fission!
Post by Jonathan_S   » Sat Jul 11, 2015 8:28 pm

Jonathan_S
Fleet Admiral

Posts: 8306
Joined: Fri Jun 24, 2011 2:01 pm
Location: Virginia, USA

SharkHunter wrote:--snipping--
Jonathan_S wrote:6 km/s works out to about 612g; fast but something many RMN ships can now do.
And 612g for 4 hours does give me about 0.288c top speed.
But I came up with the ship covering 621,831,168 km; or 35.5 LM.

Using Bill's 510g accel I got 5.1 hours to 0.306c, covering 842,386,910 km (46.8 LS) -- right in line with his calculation; if you allow for some rounding.
Trying to figure out the math... can y'all give me an assist there? We ended up with the same speed at the same point, so my distance calc must be botched somewhere....
[First, I edited the quote because I should have had 612g; not 610]

Ok, you must have gotten accel and time right, since you got terminal velocity correct. But I'll run through both accel and distance formula; just to be thorough.
Since RFC seems to ignore relativity for wedge acceleration, we can just use Newtonian formulas for all this.

And I'm going to work all these in meters and seconds; which will make for some large numbers but keeps the units clean.

612g = 5997.6 m/s^2
4 hours = 14,440 seconds
And I used initial velocity of 0 m/s [Though a warship could carry as much as 0.05c down from the Alpha bands]

First terminal velocity:
<velocity final> = <velocity initial> + <acceleration> * <time>
<velocity final> = 0 m/s^2 + 5997.6 m/s^2 * 14,440 s
<velocity final> = 86,365,440 m/s
86,365,440 m/s ~= 0.288c


Now distance covered
<distance covered> = <velocity initial> * <time> + 1/2 *<acceleration> * <time>^2
<distance covered> = 0 m/s * 14,440 s + 1/2 * 5997.6 m/s^2 * 14,440^2 s^2
<distance covered> = 1/2 * 5997.6 m/s^2 * 207,360,000 s^2
<distance covered> = 1/2 * 1,243,662,336,000 m
<distance covered> = 621,831,168,000 m
621,831,168,000 m ~= 34.57 LM



(Now in actuality I didn't work it out like this until now, I simply plugged the numbers into a spreadsheet I already had for missile accelerations; treating the ship like a very slow, very long endurance, missile)
Top
Cheatsheet for the relativistic rocket (was Re: the Destroye
Post by Bill Woods   » Sun Jul 12, 2015 9:55 am

Bill Woods
Captain of the List

Posts: 571
Joined: Tue Jun 11, 2013 12:39 pm

Jonathan_S wrote: Since RFC seems to ignore relativity for wedge acceleration, we can just use Newtonian formulas for all this.
I didn't, since 0.3c is mildly relativistic. Here's the set of formulas I've got:

----

t, d, v, a are measured by the moving observer.
T, D, v, A are measured by a 'stationary' observer.
From a standing start, so at t_0 = T_0 = 0 s, d_0 = D_0 = 0 m and v_0 = 0 m/s;
a is a constant.

Newtonian rocket*: v(t) /c = at/c, while,
Einsteinian rocket: v(t) /c = tanh(at/c)
http://math.ucr.edu/home/baez/physics/R ... ocket.html

* Though a 'rocket' is an unlikely vehicle to be producing constant acceleration for long periods of time.


With x == at/c
gamma(x) = 1/sqrt{1 - [v(t)/c]^2} = cosh(x)

t(x) = c/a x
d(x) = c^2/a [1 - 1/gamma(x)] = c^2/a [1 - 1/cosh(x)]
v(x) /c = tanh(x)

T(x) = c/a sinh(x)
D(x) = c^2/a [gamma(x) - 1] = c^2/a [cosh(x) - 1]
A(x) = a /gamma(x)^3 = a/cosh^3(x)


With y == aT/c = sinh(x)
gamma(y) = sqrt{ y^2 + 1 }

t(y) = c/a arsinh(y)
d(y) = c^2/a [1 - 1/gamma(y)] = c^2/a [ 1 - 1/sqrt{ y^2 + 1 } ]

T(y) = c/a y
D(y) = c^2/a [gamma(y) - 1] = c^2/a [ sqrt{ y^2 + 1 } - 1 ]
v(y) /c = y /gamma(y) = 1/sqrt{ 1 + 1/y^2 }
A(y) = a /gamma(y)^3 = a { y^2 + 1 }^-3/2

----

Setting a = 5000 m/s2, and plugging in v/c = 0.3, I got
T = 18,900 s = 5.24 hr,
D = 868 e9 m = 48.3 lt-min.

Setting a = 6000 m/s2,
T = 15,700 s = 4.36 hr,
D = 723 e9 m = 40.2 lt-min.

Jonathan_S wrote: (Now in actuality I didn't work it out like this until now, I simply plugged the numbers into a spreadsheet I already had for missile accelerations; treating the ship like a very slow, very long endurance, missile)
Just what I did! If you compare, you'll see the author's error isn't plot-critical; missiles run slower, but they run longer so they actually have longer ranges than advertised.
----
Imagined conversation:
Admiral [noting yet another Manty tech surprise]:
XO, what's the budget for the ONI?
Vice Admiral: I don't recall exactly, sir. Several billion quatloos.
Admiral: ... What do you suppose they did with all that money?
Top
Re: Cheatsheet for the relativistic rocket (was Re: the Dest
Post by Jonathan_S   » Sun Jul 12, 2015 2:13 pm

Jonathan_S
Fleet Admiral

Posts: 8306
Joined: Fri Jun 24, 2011 2:01 pm
Location: Virginia, USA

Bill Woods wrote:
Jonathan_S wrote: Since RFC seems to ignore relativity for wedge acceleration, we can just use Newtonian formulas for all this.
I didn't, since 0.3c is mildly relativistic. Here's the set of formulas I've got:

[snipped the formulas]
Jonathan_S wrote: (Now in actuality I didn't work it out like this until now, I simply plugged the numbers into a spreadsheet I already had for missile accelerations; treating the ship like a very slow, very long endurance, missile)
Just what I did! If you compare, you'll see the author's error isn't plot-critical; missiles run slower, but they run longer so they actually have longer ranges than advertised.
Cool, thanks for showing the real math.

And it's kind of amusing that we both had spreadsheets for missiles to reuse for the ship accel.
Top

Return to Honorverse