Author Topic: Basic Rocket Science Q & A  (Read 270665 times)

Offline vt_hokie

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Re: Basic Rocket Science Q & A
« Reply #80 on: 03/04/2009 04:22 PM »
Digging up old notes on the subject:

The mass of the Earth is not uniformly distributed, and the resultant non-circular shape of the Earth’s equator is described by the term ‘triaxial Earth’ or triaxiality.  The planet’s mass distribution can be roughly approximated by a spherically symmetric Earth with additional masses at 15° West and 165° East longitude.   This is referred to as a ‘triaxial Earth’ because the Earth would have one polar and two equatorial axes.  The additional masses correspond to mid-ocean regions, where the oceanic tectonic plates are heavier than continental plates.

Question for the real experts: So, if you were to place a satellite in geostationary orbit directly over one of the gravity wells, would its natural eccentricity be determined primarily by solar pressure and the satellite's area to mass ratio?  I believe lunar, solar, and other gravitational perturbations are negligible in comparison when it comes to drift.
« Last Edit: 03/04/2009 04:25 PM by vt_hokie »

Offline DMeader

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Re: Basic Rocket Science Q & A
« Reply #81 on: 03/04/2009 05:13 PM »
Concerning the discussion up the thread a bit about nozzle designs... do any of the factors mentioned apply to cold-gas thrusters like some attitude control jets? How about thrusters where a monopropellant like hydrazine or hydrogen peroxide is decomposed across a catalyst bed? Do those concerns not apply so much to units that tend to be that small?
« Last Edit: 03/04/2009 05:15 PM by DMeader »

Offline Herb Schaltegger

Re: Basic Rocket Science Q & A
« Reply #82 on: 03/05/2009 02:27 AM »
Concerning the discussion up the thread a bit about nozzle designs... do any of the factors mentioned apply to cold-gas thrusters like some attitude control jets? How about thrusters where a monopropellant like hydrazine or hydrogen peroxide is decomposed across a catalyst bed? Do those concerns not apply so much to units that tend to be that small?

They apply to any gas thruster if the mass flow is more than a mere trickle.  Regardless of whether the gas is produced via combustion or not, it will expand into the area of low pressure (e.g., the vacuum at one end of the nozzle).  The flow will still choke at the narrowest point.
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Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #83 on: 03/05/2009 09:53 AM »
They apply to any gas thruster if the mass flow is more than a mere trickle.  Regardless of whether the gas is produced via combustion or not, it will expand into the area of low pressure (e.g., the vacuum at one end of the nozzle).  The flow will still choke at the narrowest point.

Is the cold gas in a cold gas thruster cold enough that the nozzle cannot convert much thermal energy to (macroscopic) kinetic energy, or does the gas get appreciably colder and faster as it expands through the nozzle?
« Last Edit: 03/05/2009 09:59 AM by mmeijeri »
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Offline DMeader

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Re: Basic Rocket Science Q & A
« Reply #84 on: 03/05/2009 11:13 AM »
They apply to any gas thruster if the mass flow is more than a mere trickle.  Regardless of whether the gas is produced via combustion or not, it will expand into the area of low pressure (e.g., the vacuum at one end of the nozzle).  The flow will still choke at the narrowest point.

Is the cold gas in a cold gas thruster cold enough that the nozzle cannot convert much thermal energy to (macroscopic) kinetic energy, or does the gas get appreciably colder and faster as it expands through the nozzle?

By "cold gas" I meant just compressed gas like nitrogen. For example the VDU roll thrusters on MIR or the thrusters on the MMU. Not necessarily cryogenic gas.

Offline Herb Schaltegger

Re: Basic Rocket Science Q & A
« Reply #85 on: 03/05/2009 12:47 PM »
Basic physics - when gas expands, it cools.
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Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #86 on: 03/05/2009 01:22 PM »
Basic physics - when gas expands, it cools.

I know, I was wondering how much and how much use the Laval nozzle is in that situation. I am not as dumb as I look. I studied mathematics and computer science. In my first year I also studied physics and passed all exams for that.
« Last Edit: 03/05/2009 01:24 PM by mmeijeri »
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Offline nomadd22

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Re: Basic Rocket Science Q & A
« Reply #87 on: 03/05/2009 01:49 PM »
 I'm exactly as dumb as I look. I'm still trying to figure out why a Geo sat that's a half a degree off the equator (like MSAT1) needs less fuel for station keeping. It's Sband, made for low gain ground antennas so it doesn't have to stay perfectly still.
« Last Edit: 03/05/2009 01:51 PM by nomadd22 »

Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #88 on: 03/05/2009 01:52 PM »
Alas, while I'm not quite as dumb as I look, I am sadly not smart enough to answer that question...
« Last Edit: 03/05/2009 01:53 PM by mmeijeri »
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Offline vt_hokie

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Re: Basic Rocket Science Q & A
« Reply #89 on: 03/05/2009 08:09 PM »
I'm exactly as dumb as I look. I'm still trying to figure out why a Geo sat that's a half a degree off the equator (like MSAT1) needs less fuel for station keeping. It's Sband, made for low gain ground antennas so it doesn't have to stay perfectly still.

That inclination is 10 times what GEO sats are typically held to.  I don't see how that would save much propellant though.  Inclination grows at close to 1 degree per year (I think it might be 0.86 deg/yr, though the rate will decrease as inclination gets greater, and the lunar perturbation varies cyclically with the moon's inclination.  But I'm too lazy to look it up right now).  So, if you started near zero inclination, you could let it go for 6 months or so with no inclination burns.  But then you'd have to prevent the satellite from exceeding 0.5 deg.  So, while with a larger "box" maybe you could do longer burns and reduce the frequency of maneuvers (though that also depends on propulsion limitations), you still need to perform inclination maneuvers.

Edit: Okay, I looked it up...

The moon has a larger effect on inclination than the sun, with the rate of inclination change ranging from 0.48˚ per year to 0.68˚ per year over an 18.6 year period.  This period corresponds to the variation in the moon’s orbital inclination (from 18˚ to 28˚) over the same time period.  The rate of inclination change due to solar effects is approximately 0.27˚ per year.  The solar perturbation is strongest at the solstice, when the sun is furthest from the Earth’s equatorial plane.  In addition to affecting inclination, both the lunar and solar perturbations affect the right ascension of the ascending node (Ω), the angle between the longitude where the satellite crosses the equator in a northerly direction and the vernal equinox direction – i.e. the longitude where the sun appears on the first day of spring.
« Last Edit: 03/09/2009 03:15 AM by vt_hokie »

Offline nomadd22

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Re: Basic Rocket Science Q & A
« Reply #90 on: 03/05/2009 08:31 PM »
MSAT 1 isn't drifting. MSV took it off the equatorial plane on purpose because they were running out of fuel and they say it takes less to stay in place if you're out of place......uh....sort of. Now that I bothered to look it up, it's actually 2 1/2 degrees off.
 The result is that if you track the sat it appears to make a daily figure eight around the point you'd expect it to be.
 It's also one of the Boeing busses that has solar array woes, is several years past it's planned life and has a failed transponder.
 But the little sucker kept our fleet of ships going after Katrina when public utilities were down for months and Iridium had 200 users trying to make a call for every available channel.
 It's replacement, MSV-1 is riding a Proton Briz-M later this year. It'll be a big one at 5600kg.
« Last Edit: 03/05/2009 08:34 PM by nomadd22 »

Offline vt_hokie

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Re: Basic Rocket Science Q & A
« Reply #91 on: 03/05/2009 08:45 PM »
MSAT 1 isn't drifting. MSV took it off the equatorial plane on purpose because they were running out of fuel and they say it takes less to stay in place if you're out of place......uh....sort of. Now that I bothered to look it up, it's actually 2 1/2 degrees off.
 

Okay, so it's nearing end of life and they're just letting inclination go, it would seem.  That's pretty typical.  The satellite might still generate some revenue for a while, or sometimes the "inclined" sats are simply used as placeholders to hold an orbital slot until the operator can get a replacement launched.
« Last Edit: 03/05/2009 08:45 PM by vt_hokie »

Offline nomadd22

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Re: Basic Rocket Science Q & A
« Reply #92 on: 03/06/2009 12:23 PM »
MSAT 1 isn't drifting. MSV took it off the equatorial plane on purpose because they were running out of fuel and they say it takes less to stay in place if you're out of place......uh....sort of. Now that I bothered to look it up, it's actually 2 1/2 degrees off.
 

Okay, so it's nearing end of life and they're just letting inclination go, it would seem.  That's pretty typical.  The satellite might still generate some revenue for a while, or sometimes the "inclined" sats are simply used as placeholders to hold an orbital slot until the operator can get a replacement launched.
OK. It might be simpler than I thought. Maybe the fuel savings comes from maintaining longitude but letting latitude get sloppy. I'd thought they put it where it was on purpose. Wouldn't be the first time I made things more complicated than they were.
 But, it's more than just a placeholder. There are many, many customers using that thing, from sailboats to large ships. (Like mine) We just bought a mess of replacement radios on MSV's promise that the new sats would be compatible. I can even get internet access with them at a blistering 4800bps.
« Last Edit: 03/06/2009 12:26 PM by nomadd22 »

Offline vt_hokie

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Re: Basic Rocket Science Q & A
« Reply #93 on: 03/07/2009 04:34 PM »
OK. It might be simpler than I thought. Maybe the fuel savings comes from maintaining longitude but letting latitude get sloppy.

Yep, that's exactly it!

Quote
I'd thought they put it where it was on purpose.

If only it were possible to center it at a latitude other than zero.  Stupid physics! ;)

Quote
But, it's more than just a placeholder. There are many, many customers using that thing, from sailboats to large ships. (Like mine) We just bought a mess of replacement radios on MSV's promise that the new sats would be compatible. I can even get internet access with them at a blistering 4800bps.

That's cool!  I wonder how much inclination they can tolerate while still providing continuous coverage.

Offline nomadd22

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Re: Basic Rocket Science Q & A
« Reply #94 on: 03/07/2009 08:26 PM »


That's cool!  I wonder how much inclination they can tolerate while still providing continuous coverage.
Most MSAT remotes are autotracking marine or mobile antennas or low gain antennas that you just point in the general direction, so the sat can afford to move around some. It's mostly Ku band birds who might have neighbors on the same frequency and remote, fixed high gain antennas that have to be really precise. We're just starting to get marine Ku band antennas now. Suckers cost a fortune because they have to maintain half a degree precision in rough seas.

Offline vt_hokie

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Re: Basic Rocket Science Q & A
« Reply #95 on: 03/09/2009 04:22 PM »
Most MSAT remotes are autotracking marine or mobile antennas or low gain antennas that you just point in the general direction, so the sat can afford to move around some. It's mostly Ku band birds who might have neighbors on the same frequency and remote, fixed high gain antennas that have to be really precise. We're just starting to get marine Ku band antennas now. Suckers cost a fortune because they have to maintain half a degree precision in rough seas.

Thanks for the explanation, makes sense.  I can see why that would be expensive!

Thought I'd mention that when you do inclination maneuvers, usually the goal isn't to reduce magnitude to zero, but rather to control growth.  As I mentioned, GEO sats are typically maintained to within 0.05 deg of the equator.  To fully reset the inc cycle, you actually want to drive the inclination out to ~0.05 deg.  This is how I explained it a while back...hopefully it makes sense.  (And hopefully I got it right!  I basically just re-worded what was in orbital dynamics texts, but corrections are always welcome. :) )

Ideally, the inclination cycle is controlled such that the inclination vector is set to begin near the inclination deadband (typically 0.05˚) with the right ascension, Ω, near 270˚.  The magnitude of inclination will decrease toward zero and then begin increasing as Ω moves toward 90˚.  North/south stationkeeping maneuvers are required to reset the cycle when the inclination magnitude has increased back to the deadband limit and Ω is near 90˚.  The maneuvers occur at the intersection of the existing orbital plane and the target plane, within 30˚ of the ascending or descending node. 
 
So, in that sense, you do want to purposely take the satellite out of a truly equatorial orbit by a small amount.  Also, you can save some propellant by only correcting for secular perturbations, but that would be a whole new topic that I'm guessing is more than you want to know! :D

It should be easy, but throw in some component malfunctions, unpredictable east/west coupling due to thruster plume impingement and attitude control firings, etc. and it ends up being a real headache! ;) Also, I'm not a huge fan of arcjets!  I don't care how efficient they are, they're a pain from an operational standpoint!  :)
« Last Edit: 03/09/2009 10:03 PM by vt_hokie »

Offline vt_hokie

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Re: Basic Rocket Science Q & A
« Reply #96 on: 03/09/2009 10:56 PM »

Intuition (and simple physics) tell the satellites should be accelerated by the positive anomalies, hence causing an oscillatory motion between two points centered in the gravity well: the strongest gravity source. Are people somehow disoriented by the graphical representations of the geoid, thinking the wells are the "lowest" points in the figure? Or am I missing something entirely and being ridiculously stubborn?

Revisiting this question, I think I see what you're saying here.  You're wondering why the stable longitudes are 90 deg off of the "bulges" rather than directly over them, right?  You're making me think for the first time in months...my brain hurts!  ;)  I'm not sure how to best describe it other than to say that at the low point, the effects of the two additional mass regions cancel each other out and no longer change the shape of the orbit.  (But then that's true at the unstable equilibrium points as well.  The key is how does the shape of the orbit change at all other longitudes.)  Hopefully someone here can explain it better, but I managed to find a figure which might illustrate it better than I'm saying it.  Check out page 260:

Stable longitudes
 
And here's a better explanation:

Pages 137, 138

Figure 5.18 makes perfect sense...I feel dumb now for getting confused over something so obvious!

So, of course you're correct about satellites being accelerated by the positive anomalies.  But it's the old "slow down to speed up, speed up to slow down" paradox of orbital mechanics!  :)   
« Last Edit: 03/10/2009 04:04 PM by vt_hokie »

Offline eeergo

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Re: Basic Rocket Science Q & A
« Reply #97 on: 03/10/2009 07:20 PM »
That's a great explanation, very clear! Stupid orbital mechanics... :)

Thank you very much for the outstanding reference, I hadn't found this resource whey trying to understand the effect.
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Offline Spacenick

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Re: Basic Rocket Science Q & A
« Reply #98 on: 03/10/2009 07:40 PM »
I have a question about heat transfer and cooling/heating of sattelites.
In many sources, especially in main stream media, it sounds like an unbelievably big problem, that in space, the sun side is so much hotter than the shadow. And they talk about huge figures hundreds of degrees celsius of temperature difference.
On the other hand, my common sense tells me, that because of the low pressure there is probably very little convection and therefor there is only radiation for heat transfer.
Therefor, i'd think that though the air moluecules in LEO are technically really hot, they transfer very little energy to a spacecraft.
So how is the cooling/heating acomplished? I'd guess that making the spacecraft reflective on the outside would block most of the energy coming from the sun, an because of little convection the spacecraft should cool very little when provided with basic insulation against heat transfer to the outer hull (which would remove the energy by way of infrared radiation).
Then I'd guess the electronics or other systems (e.g. a human in a spacecraft) would provide enough heat to keep the sacecraft from freezing.
So what, would be left would be to provide a way of radiating exactly as much energy out of the spacecraft as needed o keep constant temperature, so how would I do that?

Offline cgrunska

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Re: Basic Rocket Science Q & A
« Reply #99 on: 03/10/2009 08:54 PM »
I was reading a blurb about the new Merlin Vaccum engine and it showed it on earth with a big flame plume coming out the backend, and an artist rendering of the engine in space with "flares" around the nozzle.

Basically my question is thus, how does an engine, turned on, look in space? There's no fire coming out the backside. Is there anything besides maybe a blue ring around the inner nozzle? Is there even that? Any 'movement field' that can be seen?

Pretty basic question. Thanks for the answer!