### Author Topic: Basic Rocket Science Q & A  (Read 271806 times)

#### Lee Jay

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##### Re: Basic Rocket Science Q & A
« Reply #520 on: 08/09/2010 02:30 PM »
You could go faster than what Jim said if you could apply continuous delta-V directed radially inward.  For example, if you had a hypothetical aircraft that flew in the upper atmosphere upside down generating lift toward the center of the Earth instead of away from it like a usual airplane, you could in theory go faster.  Practice and theory in this regard are still quite a ways apart.

#### JohnFornaro

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##### Re: Basic Rocket Science Q & A
« Reply #521 on: 08/09/2010 03:16 PM »
Bizzarre minds need great accuracy.  I'm there with that.  Great answer.  Two more points for Jim!  Hufflepuff?
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#### Lee Jay

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##### Re: Basic Rocket Science Q & A
« Reply #522 on: 08/10/2010 07:58 PM »
Orbital mechanics question.

Let's say you're in a perfectly circular orbit around a gravitation point source (no gravitational gradients or anomalies).  Let's move our frame of reference into your frame and call your now-fixed location "A".  If you were to apply a small delta-V in either direction along the velocity vector, what would be the shape of your flight path relative to point "A"?

#### kevin-rf

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##### Re: Basic Rocket Science Q & A
« Reply #523 on: 08/11/2010 01:49 AM »
It could be even faster if you built a craft that could orbit inside the lithosphere!

Again, given enough time and money someone would say could turn the theory in practice.
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#### gospacex

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##### Re: Basic Rocket Science Q & A
« Reply #524 on: 08/11/2010 03:04 AM »
It could be even faster if you built a craft that could orbit inside the lithosphere!

No problem, as long as you are ok with very small craft: LHC already pushed the speed pretty much to the limit.

#### Urvabara

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##### Re: Basic Rocket Science Q & A
« Reply #525 on: 08/11/2010 06:16 AM »
So, is it easy to vary the thrust force in chemical rockets? How does that change the propellant flow rate? If I halve the thrust, does the propellant flow rate halve too?

#### simonbp

##### Re: Basic Rocket Science Q & A
« Reply #526 on: 08/11/2010 06:31 AM »
Interestingly enough, you don't find many natural orbital systems faster than a few hours. The closest binary stars have periods of greater than ~3 hours, as do the closest asteroid satellites...

Now, for very massive systems, that's still pretty fast; e.g. PSR B1913+16, with a period of 7.5 hours and orbital velocity of the secondary of 450 km/s...

http://en.wikipedia.org/wiki/PSR_B1913%2B16
« Last Edit: 08/11/2010 06:31 AM by simonbp »

#### mdo

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##### Re: Basic Rocket Science Q & A
« Reply #527 on: 08/11/2010 07:42 AM »
Interestingly enough, you don't find many natural orbital systems faster than a few hours. The closest binary stars have periods of greater than ~3 hours, as do the closest asteroid satellites...

Although slightly OT, a quick search in the ADS database reveals that some exotic star systems manage an even faster pace. For instance, Stella et al. claimed the discovery of a so called low mass X-ray binary with an 685 s orbital period.

#### ugordan

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##### Re: Basic Rocket Science Q & A
« Reply #528 on: 08/11/2010 07:56 AM »
Interestingly enough, you don't find many natural orbital systems faster than a few hours.

I don't find that very surprising. Such systems tend to decay fast (on cosmic timescales) so statistically we don't see many of them.

#### Antares

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##### Re: Basic Rocket Science Q & A
« Reply #529 on: 08/11/2010 03:20 PM »
So, is it easy to vary the thrust force in chemical rockets? How does that change the propellant flow rate? If I halve the thrust, does the propellant flow rate halve too?

No.  You have to look at combustion stability and turbodynamics.  The motor may not be able to operate at that point without tearing itself up.  Even if those two items are still healthy, you have to do the detailed aerothermal analyses to see how the nozzle flow works to determine what mass flow effects half thrust.  It would probably be within a few percent of half the mass flow, assuming the nozzle was still choked.
If I like something on NSF, it's probably because I know it to be accurate.  Every once in a while, it's just something I agree with.  Facts generally receive the former.

#### Citabria

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##### Re: Basic Rocket Science Q & A
« Reply #530 on: 08/11/2010 03:54 PM »
Bizzarre minds need great accuracy.  I'm there with that.  Great answer.  Two more points for Jim!  Hufflepuff?

At LEO velocity, relativistic effects are about 3 in 10 billion. I doubt that calculator includes relativity, so any digit after the tenth in Jim's answers is wrong. One point deducted! Slytherin.

(Seriously, one must also consider the accuracy of the constants used in the calculation, as well as the precision of the variables.)
« Last Edit: 08/11/2010 05:56 PM by Citabria »

#### KelvinZero

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##### Re: Basic Rocket Science Q & A
« Reply #531 on: 09/25/2010 12:38 AM »
Laymans' Orbital mechanics question. I hope this is the right thread.

I have heard that there are low energy but slow orbital transfers for example to get from the earth to the moon, or to get back. I looked on line but mainly found papers with very scary titles. I could not find a good summary of what savings in fuel are plausible, at what cost in increased travel time. Does anyone know of such a summary or just some interesting specific examples? Im just trying to get an idea of whether taking 6 months saves 25% of your propellant or is an order of magnitude improvement etc.

#### Antares

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##### Re: Basic Rocket Science Q & A
« Reply #532 on: 09/25/2010 02:15 AM »
Google interplanetary superhighway and/or low energy transfers.  That'll get you fairly layman, or at least technical generalist, answers.  Here's a Discover article that says Lunar Observer could have saved 25% of its fuel and 30% for a generic mission.

http://discovermagazine.com/1994/sep/gravitysrim419/article_view?b_start:int=1&-C=
If I like something on NSF, it's probably because I know it to be accurate.  Every once in a while, it's just something I agree with.  Facts generally receive the former.

#### mmeijeri

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##### Re: Basic Rocket Science Q & A
« Reply #533 on: 09/25/2010 09:21 AM »
For L1/L2 it's the difference between 3.8km/s and 3.2km/s delta-v, which is substantial. Useful for cargo and (well-insulated or space storable) propellant that don't mind taking a long detour.
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#### KelvinZero

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##### Re: Basic Rocket Science Q & A
« Reply #534 on: 09/25/2010 11:02 AM »
Ah ok.. yes those were the sort of numbers I found online. For some reason I thought there were bigger eg x10 potential savings for a 6 month trip. I guess I was mistaken.

#### mmeijeri

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##### Re: Basic Rocket Science Q & A
« Reply #535 on: 09/25/2010 11:08 AM »
That may be true for interplanetary routes.
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#### MP99

##### Re: Basic Rocket Science Q & A
« Reply #536 on: 09/25/2010 02:29 PM »
Google interplanetary superhighway and/or low energy transfers.  That'll get you fairly layman, or at least technical generalist, answers.  Here's a Discover article that says Lunar Observer could have saved 25% of its fuel and 30% for a generic mission.

http://discovermagazine.com/1994/sep/gravitysrim419/article_view?b_start:int=1&-C=

Quote
“You have to lose 600 miles per second of velocity to get off the Earth orbit and allow the spacecraft to be captured by the moon’s gravity,”

That should be metres per second, I reckon. Defo not "miles per second".

cheers, Martin

#### KelvinZero

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##### Re: Basic Rocket Science Q & A
« Reply #537 on: 09/26/2010 04:57 AM »
It occurs to me that perhaps whatever I read initially was also assuming some more efficient but lower thrust form of propulsion that becomes practical if the timeframe is six months?

#### ycs86

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##### Re: Basic Rocket Science Q & A
« Reply #538 on: 10/05/2010 01:41 AM »
What exactly will happen when a thrust, that is perpendicular to the orbital plane, is applied to an orbiting spacecraft? It would no doubt change the inclination of the orbital plane, but is that all that's gonna happen? Will it somehow affect the altitude?

#### Proponent

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##### Re: Basic Rocket Science Q & A
« Reply #539 on: 10/05/2010 01:52 AM »
The application of thrust perpendicular to the orbital plane will increase the speed, although if the magnitude of the burn is small compared to orbital speed (about 7800 m/s in low earth orbit), then this effect will be very small.  It's just the Pythagorean theorem.  If the initial orbit is circular, then the new orbit will be in a different plane, as you surmise, and elliptical, with perigee at the point where the burn was performed.