For Questions That Don't Merit Their Own Thread

[KG3]

 

Hello, I have a question about this video.  It's about getting the exploration rovers to mars from way back in 2006.  About a minute and 15 seconds in it shows the vehicle separating from the upper stage of the rocket.  Then it fires more rockets to make the vehicle spin presumably to stabilize it for the last engine to fire and send it on it's way to Mars.  Then it looks like two weights on the ends of strings spool out from the sides to slow down the vehicles spin.  Does anyone know why they used weights on strings for this?  I saw a video recently where they were testing a new parachute in the Earths upper atmosphere in which they used rocket motors to spin the vehicle then just used another set of rocket motors pointing the opposite direction to stop the spin after the main engine shut off.      

[Steel]

45 minutes ago, KG3 said:

 

Hello, I have a question about this video.  It's about getting the exploration rovers to mars from way back in 2006.  About a minute and 15 seconds in it shows the vehicle separating from the upper stage of the rocket.  Then it fires more rockets to make the vehicle spin presumably to stabilize it for the last engine to fire and send it on it's way to Mars.  Then it looks like two weights on the ends of strings spool out from the sides to slow down the vehicles spin.  Does anyone know why they used weights on strings for this?  I saw a video recently where they were testing a new parachute in the Earths upper atmosphere in which they used rocket motors to spin the vehicle then just used another set of rocket motors pointing the opposite direction to stop the spin after the main engine shut off.      

Perhaps because it's simpler, cheaper and (potentially) more reliable than fitting more rocket motors.

[Racescort666]

@KG3 They call it yo-yo de-spin I wasn't able to find a specific advantage to this particular technique . Steel's hypothesis about it being simpler and cheaper is probably correct though.  This paper from Goddard also discusses the yo-yo method and suggests that it's more tolerant to variations in initial spin rate. 

[magnemoe]

19 hours ago, Steel said:

Perhaps because it's simpler, cheaper and (potentially) more reliable than fitting more rocket motors.

Yes the transfer stage has an rocket engine so it makes sense to use it for spinning up, the decent module is smaller and have minimal or none rockets so you use the jojo method. 
Know this has been used for spin and de-spin during mars landings to. 

Another question, the animation showed staggered SRB drops, 3 of the 9 SRB burned far longer than the first 6, this is very common in KSP as we can not adjust SRB trust curve so you often use two SRB with 100% trust and two with 70%, another is 100%, 80% and 60%, this let you get away with an small engine for core stage. 
Is this real, looked like an Atlas but it only have 5 SRB 

[Dfthu]

1 hour ago, magnemoe said:

Is this real, looked like an Atlas but it only have 5 SRB 

The rockets real, its a Delta II, but the video looks kinda fake

[KG3]

12 hours ago, Racescort666 said:

@KG3 They call it yo-yo de-spin I wasn't able to find a specific advantage to this particular technique . Steel's hypothesis about it being simpler and cheaper is probably correct though.  This paper from Goddard also discusses the yo-yo method and suggests that it's more tolerant to variations in initial spin rate. 

Never heard of the yo-yo de-spin.  Looks like a very elegant solution though.  The paper talks about a stretch yo-yo system that compensates "to a reasonable degree"  for variations spin rates.  The faster the vehicle spins the more centripetal force there is on the yo-yo, the longer the chords stretch and more inertia is removed from the vehicle.  Clever and simple! 

I can't quite picture how the yo-weight works on the spent solid rocket boosters.  It somehow makes the spent booster tumble and not crash into the upper stage from residual thrust.    

Edited January 17, 2018 by KG3
forgot to mention...

[shynung]

4 hours ago, KG3 said:

I can't quite picture how the yo-weight works on the spent solid rocket boosters.  It somehow makes the spent booster tumble and not crash into the upper stage from residual thrust.    

For that purpose, only a single yo-weight is used. The asymmetric yo-weight release would induce the spent booster to tumble, provided it was positioned correctly.

[ChrisSpace]

So I have some questions about this video. If we assume that the ship's timeline (0.5c at T+2 years, 0.9c at T+4 years and 0.99c at T+6 years) is relative to Earth, and taking the effects of relativity into account:

1. Assuming no Dv remains after T+6 years, what is the initial and final acceleration of the vessel?

2. From the mass ratio (easily calculatable from question 1) and the Dv required to reach 0.99c (which is a lot more than 0.99c), what is the Isp of the propulsion system?

[Green Baron]

1.) we need wet and dry mass and thrust or exhaust velocity to do an estimation. Also relativity comes into play at higher speeds, meaning you can throw out as much as you want and as fast as you want, if there is dry mass left it will not accelerate much more Edit: Lorentz term results to ~7 at 0.99c. Frankly: the video is is just color and sound, no specific information (or i scrolled over it because i wanted the time back :-)), and a little bit blabla. And, oh, it makes a sound in space. What does that tell us ? :-)

2.) [mechanic voice] "too many variables, cannot calculate" :-)

 

If you can specify a few cornerstones like hypothetical handwavial propulsion system and type of fuel we could play with some numbers ...

Edited January 22, 2018 by Green Baron

[Green Baron]

4 hours ago, ChrisSpace said:

1. Assuming no Dv remains after T+6 years, what is the initial and final acceleration of the vessel?

Second thought: assuming linear acceleration from 0 to 297,000,000m/s in 1,892,216,000s that is a moderate ~1.57m/s² acceleration. Hope i didn't forget or add a 0 on the way. 0s are important sometimes :-)

[justidutch]

Anybody want to take a crack at figuring out the ISP and/or thrust of a vacuum cleaner?

http://www.bbc.com/news/av/science-environment-42774229/iss-cosmonaut-does-test-flight-on-a-vacuum-cleaner

Perhaps a new mod?

[WinkAllKerb'']

why theyr'e not much thread about neuro science around here ? and why each time there's one post it's fastly bashed and underated ? because  more toaster ; ) /vostokconcordia&etc&etcpalm

i'd like to remind that you need a "first step"achievement , a second step achieve, a third step "a.", &etc ...

Edited January 22, 2018 by WinkAllKerb''
[ 3/8 free taunt on off] 3/8 fair enough ? , )

[ChrisSpace]

15 hours ago, Green Baron said:

1.) we need wet and dry mass and thrust or exhaust velocity to do an estimation. Also relativity comes into play at higher speeds, meaning you can throw out as much as you want and as fast as you want, if there is dry mass left it will not accelerate much more Edit: Lorentz term results to ~7 at 0.99c. Frankly: the video is is just color and sound, no specific information (or i scrolled over it because i wanted the time back :-)), and a little bit blabla. And, oh, it makes a sound in space. What does that tell us ? :-)

Okay, here's what I meant:

The Dv required to reach 0.5c is 0.5775c

The Dv required to reach 0.9c is 2.0646c

The Dv required to reach 0.99c is 7.01811c

Thus we can take relativity out of the equation by first reducing velocities by, say, 100,000 to the following:

0.0000 km/s at T+0 years

1.7313 km/s at T+2 years

6.1900 km/s at T+4 years

21.0398 km/s at T+6 years

Now you can see more clearly how the acceleration gradually increases. From this, it should be possible to calculate the initial and final acceleration, then multiply those by 100,000 again.

[YNM]

@ChrisSpace : That's not quite how they work.

dV is just a way to express change in kinetic energy. Or in relativistic case, relativistic energy. Initial and final (rest) mass would help in determining those respective energies. For acceleration, I'd think that the proper way to do them is as work, which then could be turned into mass flow rates.

[Green Baron]

8 hours ago, ChrisSpace said:

Okay, here's what I meant:

The Dv required to reach 0.5c is 0.5775c

The Dv required to reach 0.9c is 2.0646c

The Dv required to reach 0.99c is 7.01811c

Thus we can take relativity out of the equation by first reducing velocities by, say, 100,000 to the following:

0.0000 km/s at T+0 years

1.7313 km/s at T+2 years

6.1900 km/s at T+4 years

21.0398 km/s at T+6 years

Now you can see more clearly how the acceleration gradually increases. From this, it should be possible to calculate the initial and final acceleration, then multiply those by 100,000 again.

 

Yep, i fear that is not the correct way. The Lorentz factor describes the change of relativistic mass, contraction and time dilation at speeds. Speed is a variable, not the outcome.

I'd rather think the other way round: acceleration at 0.9c requires double reaction mass or exhaust velocity compared to 0c, from the reference frame of start. As the ship leaves the acceleration in reference to the start goes down as time dilation kicks in at high speeds (1s on board is 7s at the start, ship's mass is 7 times its rest mass Edit: @0.99c).

If you want the exact acceleration on board at a certain time you need the rest masses and thrust or exhaust v to do the math. Once you have that you can then calculate the effect of relativity as you did via the Lorentz factor and apply the result to the masses and times to obtain the acceleration in reference to the start.

Hope that was correct ....

Edited January 23, 2018 by Green Baron

[kerbiloid]

T, y	v/c	average acceleration, m/s2
0..2	0..0.5	5
0..4	0..0.9	7
0..6	0..0.99	8.4

And their calculation of the distance is wrong (4.3 ly after 4 years), it corresponds to average acceleration 3 m/s².

[Green Baron]

50 minutes ago, kerbiloid said:

T, y	v/c	average acceleration, m/s2
0..2	0..0.5	5
0..4	0..0.9	7
0..6	0..0.99	8.4

And their calculation of the distance is wrong (4.3 ly after 4 years), it corresponds to average acceleration 3 m/s².

I get averages of 2.37m/s² to 0.5c, 2.14 to 0.9c and 1.57 to 0.99c simply by dividing the speed in m/s with the time in seconds. Is that wrong ?

Edited January 23, 2018 by Green Baron

[kerbiloid]

17 hours ago, Green Baron said:

Is that wrong ?

Quoting Shklovskiy, who quoted Sagan, who quoted (can't remember two researchers right now).

t = (2c/a) arcch (1 + aS/c²)

v = c (1 - (1 + aS/c²)^-2)^1/2

***

Substituting these formulas together,

t = (2c/a) arcch (1 + aS/c²)
S = c² (ch(at/2c) - 1) / a
v = c (1 - (1 + aS/c²)^-2)^1/2 = c (1 - (1 + a(c² (ch(at/2c) - 1) / a)/c²)^-2)^1/2 = c (1 - (1 + c² (ch(at/2c) - 1)/c²)^-2)^1/2 = c (1 - (1 + (ch(at/2c) - 1))^-2)^1/2 = c (1 - (1 + ch(at/2c) - 1)^-2)^1/2 = c (1 - (ch(at/2c))^-2)^1/2 = c (1 - ch^-2(at/2c) )^1/2.

v = c (1 - ch-2(at/2c) )^1/2
(v/c)² = 1 - ch^-2(at/2c)
arcch((1 - (v/c)²)^-1/2)= at/2c

a = 2c arcch((1 - (v/c)²)^-1/2) / t

duration = t, s
acceleration = a, m/s²
speed = v, m/s
distance = S, m

Upd.
Forgot about the test samples.
With a = 9.81,
30000 ly = 21 y
2 mln ly = 28 y

Edited January 24, 2018 by kerbiloid

[K^2]

^ This. The reason formulae for acceleration get so very weird, is because of the relationship between coordinate acceleration and proper acceleration. Proper acceleration is what the ship's crew experiences. Consequently, it's also the same as rocket's actual thrust divided by its rest mass. Coordinate acceleration is the acceleration relative to the inertial observer, that's not accelerating along with the rocket. So long as rocket's moving much slower than c w.r.t. the inertial observer, the frame acceleration is almost identical to proper acceleration. But as the rocket gains speed, the two values become quite different. Indeed, coordinate acceleration drops to almost zero as rocket gets close to c.

The general relationship is quite complex, but for a special case of acceleration along the velocity vector, the relationship is a_p = γ³a_c. Note the cubed power on the Lorentz factor. Because distance traveled by rocket is the second integral of coordinate acceleration over time, even if proper acceleration is kept constant, maintaining constant thrust-to-mass ratio, the distance traveled over time ends up being an ugly formula involving hyperbolic functions. I've derived them on paper once, and do not want to repeat the experience.

The upshot is that if you can maintain acceleration of 1 Earth gravity on a torch ship, you can go bloody far within a single lifetime. This relationship is so absurd, in fact, that it eventually catches up with the Rocket Equation, and you can travel anywhere within visible universe on a finite amount of fuel. It's still a huge amount of matter you'd have to convert into light to travel the distance, but we're talking planetary mass scale. Not mass of the universe times lots, as you might have expected. For conventional rocket, even a nuclear-pulse powered, it goes back to mass of the universe times lots, so it's definitely a photon drive or bust, but hey, I'll take 'physically possible' here.

[Racescort666]

3 hours ago, K^2 said:

if you can maintain acceleration of 1 Earth gravity

I've maintained 1 Earth gravity my entire lifetime but I've only gone in a bloody circle.

Spoiler

Sorry, couldn't resist the joke. 

 

[p1t1o]

39 minutes ago, Racescort666 said:

I've maintained 1 Earth gravity my entire lifetime but I've only gone in a bloody circle.

  Reveal hidden contents

Sorry, couldn't resist the joke. 

 

And because of that you are travelling into the future 0.0219seconds per year faster than people in deep space

(Thats on top of the 0.15seconds or so that you gain per year due to Earths orbital motion around the sun.)

 

[WestAir]

Hey guys. Quick question I didn't know where else to ask:

While on paper it's impossible for an object to keep pace with the propagation of causality, is it possible for an electron to do just that if said propagation can be quantized?

I'll try to explain my question as clearly as I can. Imagine we have a photon and an electron. The electron is traveling at a speed of 0.999[...]99C. In simpler terms, let's say this electron will move 2,999,999,999 Planck lengths in 3,000,000,000 Planck times. If we slow time and observe a race between the electron and the propagation of information, wouldn't it appear like the electron is keeping pace with propagation up until it doesn't? If the electron will only lose ground on the 3 billionth Planck length - and not a single Planck length sooner - won't it appear that the electron is in fact moving at the speed of light?

I imagine the answer lies in whether or not space can be meaningfully quantized, but correct me if I'm wrong that stuff like electromagnetism and gravity shouldn't work below the Planck scale because Bosons collapse as soon as they interact with anything (which means they can't be force carriers?) on this scale because they're too energetic, so if we're comparing the Electron to a nearby Photon, then quantization is a thing?

Assuming the prior points stand (and that's a big assumption),If there is no way to tell the difference between our energetic electron and the speed of light, prior to the passage of time showing us who the winner is, then how does the Universe know which is faster? Can an electron even have that much energy?

Thanks for bothering to read. I have an Elementary School understanding of Physics so don't feel too badly if you have to talk slow or be a little condescending in your response.

[K^2]

23 minutes ago, WestAir said:

If we slow time and observe a race between the electron and the propagation of information, wouldn't it appear like the electron is keeping pace with propagation up until it doesn't?

That's not how quantization works. If you measure distance traveled during interval, you can only measure an integer number of some unit, which depends on how you measure it. In the most ideal case, the smallest you can get that unit down to is Plank length. However, as you increase measurement time, you are still only limited to an integer number.

Lets give concrete example. Say, expectation of the distance traveled in 1 time interval is 2.5 length intervals. That means, you will randomly measure 2 or 3 intervals traveled. However, if you now measure over 2 time intervals, you'll consistently be seeing 5 length intervals traveled. Not 4 or 6.

Quantization is always the result of interaction between system and observer. Of course, "observer" is a very loose term, here. So is what constitutes measurement.

 

This is a very common misunderstanding, by the way, and doesn't require one to go into depths of Quantum Field Theory. The energy levels of electron in hydrogen atom are discrete. And I keep seeing a lot of otherwise very educated people imply that electron instantly jumps from one energy level to another when excited. Nothing can be further from the truth! It takes finite amount of time for electron to go from ground state to an excited one, as it absorbs electromagnetic radiation. During that time, electron probability distribution around the atom gradually changes shape, say, from a 1s orbital to a 2p orbital.

What's notable, that half-way through absorption, the distribution is a superposition of 1s and 2p states, while photon that was being absorbed is at half-amplitude, which corresponds to fifty-fifty odds of it being detectable and not. If I was to generate a very low energy laser beam at the right frequency, I can actually time this, and leave atom in this half-excited state. However, in order for me to see what energy state it ended up in, my best bet is to put a detector next to it and wait for the photon to be emitted again, as atom's state decays back from 2p down to 1s. The emitted photon would also be at half-amplitude, *entangled* with the original photon. Which means that while odds of me detecting either are fifty-fifty, if I detected one, I would not be able to detect the other. So I either measured the atom to absorb a full photon and re-emit it, or completely fail to absorb a photon. This is where quantization actually comes in.

Can we detect these in-between states? In some cases, yeah. A photon at 45° polarization will pass through horizontal filter 50% of the time. But it will pass through a 45° filter 100% of the time. Similarly, I can prepare a pair of two half-excited atoms, and with the right coupling, get them into a state where one is 100% excited, and the other is in ground state. This lets me verify that these intermediate states really do exist. But they cannot be measured directly.

 

So now we can get back to Plank length. While it's certainly a thing, and while it certainly puts a bottom limit on measurements we can make, on the grand scale, universe will always keep working as if that limit isn't there. There is a separate note that can be made here about ultraviolet catastrophe, but that's also largely a matter of how you integrate things over bulk.

There's also a little caveat about crystal lattices, where similar quantizations arise due to periodicity of the lattice, and that has really fun side-effects, but it also comes with rather noticeable anisotropy of space, and no such thing has been observed.

[WestAir]

Beautifully written, K^2. I haven't visited these forums in some time but I always remember that one Particle Theorist who tirelessly tries to instill decades of intense research into the minds of casual Kerbonauts.

I didn't realize that in intervals of the 3 billion Planck lengths I'd get a randomized outcome which will always be less than (instead of equal to) the same interval of lightspeed.

13 minutes ago, K^2 said:

So now we can get back to Plank length. While it's certainly a thing, and while it certainly puts a bottom limit on measurements we can make, on the grand scale, universe will always keep working as if that limit isn't there.

Correct me if I'm wrong but surely there is a macro scale where things break apart? Because of the metric expansion of space, isn't there a point where interaction also breaks down?

[K^2]

No. Because all of space expands, any two points of it can interact over finite time, even if they are receding FTL from each other. That felt counterintuitive to me too, untill I checked the math. It does mean gravity gets weaker over distance rather faster than invere square across great spans, but no interaction is ever quite severed.