Author Topic: Basic Rocket Science Q & A  (Read 271755 times)

Offline tnphysics

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Re: Basic Rocket Science Q & A
« Reply #400 on: 01/12/2010 12:33 AM »
Can one use a lunar gravity assist to aid TMI?

Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #401 on: 01/12/2010 10:27 AM »
Yes, Farquhar proposed a double gravity assist from both the Earth and the moon from SEL-2. If you want to launch straight from LEO it might be different.
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Offline tnphysics

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Re: Basic Rocket Science Q & A
« Reply #402 on: 01/14/2010 01:38 AM »
From LEO

Offline yinzer

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Re: Basic Rocket Science Q & A
« Reply #403 on: 01/14/2010 03:01 AM »
Theoretically you could, but the moon's gravity is not that strong so it won't help that much.  There may not be many times when the Moon is in a useful place relative to the outbound trajectory to Mars, either.
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Offline Cbased

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Re: Basic Rocket Science Q & A
« Reply #404 on: 01/18/2010 10:25 AM »
When do you usually stop using the term "altitude" and start using "distance from/to a planet"?
Thank you.

Offline clongton

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Re: Basic Rocket Science Q & A
« Reply #405 on: 01/18/2010 11:28 AM »
When do you usually stop using the term "altitude" and start using "distance from/to a planet"?
Thank you.


Generally, "alitude" is used to identify an orbital path around a planet or moon and is repeating with each succeeding orbit while "distance" generally describes, for spacecraft anyway, how far it is from the planet/moon as it is approaching or departing.
« Last Edit: 01/18/2010 11:29 AM by clongton »
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Offline thomson

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Re: Basic Rocket Science Q & A
« Reply #406 on: 01/19/2010 05:17 PM »
Very basic question.

In one message (http://forum.nasaspaceflight.com/index.php?topic=13206.msg526703#msg526703) Robotbeat stated that 195 second hover (a recent feat accomplished by Masten team) is enough for a trip from L2 to high Mars orbit. My question is how does one convert hover time to delta-v? Is it a matter of simple multiplying hover time by G? So you get:

dV= t*G = 195s*9.81m/s^2 = 1912m/s

Is this right?

My second question is built on top of the first one. If this is correct, then one could calculate required hover time for a vehicle capable of reaching orbital velocity. I've assumed v0=9300m/s, so required hover time is 948s. Ignoring atmosphere drag, it is valid to say that if Masten vehicle would be able to achieve ~20% (195s/948s) of orbital velocity?


Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #407 on: 01/19/2010 05:29 PM »
Generally, "alitude" is used to identify an orbital path around a planet or moon and is repeating with each succeeding orbit while "distance" generally describes, for spacecraft anyway, how far it is from the planet/moon as it is approaching or departing.

There's also the difference between distance to the surface (altitude) and distance to the center of mass of the planet/moon. This is the terminology the Orbiter flight simulator uses. Altitude is more useful close to the surface, distance from the center of mass is more useful when you're farther away and when integrating the equations of motion.
« Last Edit: 01/19/2010 05:31 PM by mmeijeri »
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Offline sdsds

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Re: Basic Rocket Science Q & A
« Reply #408 on: 01/19/2010 08:18 PM »
Very basic question.

In one message (http://forum.nasaspaceflight.com/index.php?topic=13206.msg526703#msg526703) Robotbeat stated that 195 second hover (a recent feat accomplished by Masten team) is enough for a trip from L2 to high Mars orbit. My question is how does one convert hover time to delta-v? Is it a matter of simple multiplying hover time by G? So you get:

dV= t*G = 195s*9.81m/s^2 = 1912m/s

Is this right?

My second question is built on top of the first one. If this is correct, then one could calculate required hover time for a vehicle capable of reaching orbital velocity. I've assumed v0=9300m/s, so required hover time is 948s. Ignoring atmosphere drag, it is valid to say that if Masten vehicle would be able to achieve ~20% (195s/948s) of orbital velocity?

The assumptions underlying your second question probably don't lead to accurate results.  For a first approximation, try thinking about it another way.  If a vehicle was launched straight up, and the top of its ascent was at an altitude where the velocity required to reach orbit was 7500 m/s, and if the vehicle could then instantaneously propel itself sideways to gain 7500 m/s, it would be in orbit.  If instead that propulsion were distributed over 948 seconds, it would likely fail to make orbit because during those 15+ minutes the vehicle would have fallen to an altitude where its trajectory enters the atmosphere.  Does that way of thinking about it help at all?  (There are better ways to think about it involving the rocket equation and thrust/weight ratios, but those ways are "rocket science."  They require <shiver> logarithms!)

P.S.:  It would be interesting to calculate the altitude to which Xoie would have to be lifted so that she could make orbit under her own propulsion!
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Offline jongoff

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Re: Basic Rocket Science Q & A
« Reply #409 on: 01/19/2010 10:54 PM »
Very basic question.

In one message (http://forum.nasaspaceflight.com/index.php?topic=13206.msg526703#msg526703) Robotbeat stated that 195 second hover (a recent feat accomplished by Masten team) is enough for a trip from L2 to high Mars orbit. My question is how does one convert hover time to delta-v? Is it a matter of simple multiplying hover time by G? So you get:

dV= t*G = 195s*9.81m/s^2 = 1912m/s

Is this right?

That's basically right.  You're providing a constant 1g=9.81m/s^2 worth of acceleration for most of the flight (except for brief accelerations and decelerations at the ascent/descent/translation tipping), so if you multiply that by time that is how much velocity change you could have done *assuming that throttling profile, and those atmospheric conditions*.  Our mission averaged Isp is actually a lot worse than our full-throttle Isp, which is less than our vacuum Isp, which is a lot less than our Vacuum Isp with a high expansion ratio nozzle extension (I can't remember what our mission-averaged Isp was, but I think it was in the 180-190s range, our full throttle launch altitude Isp is around 215-220s range, our vacuum Isp with the short nozzle is probably in the 250s range, and with a full altitude-optimized nozzle is in the 300-310s range).

Quote
My second question is built on top of the first one. If this is correct, then one could calculate required hover time for a vehicle capable of reaching orbital velocity. I've assumed v0=9300m/s, so required hover time is 948s. Ignoring atmosphere drag, it is valid to say that if Masten vehicle would be able to achieve ~20% (195s/948s) of orbital velocity?

Only sort of.  One of the big challenges for long hovers is that you have to have a very wide throttling range, and your backpressure losses at low-throttle start eating your lunch.  Designing for a long hover that has to get down to low throttles really killed our mission-averaged Isp, and it only gets worse the longer the duration you have to do.

Without some sort of altitude compensation, multiple engines (that you can shutoff in series), or flow-separation control, we'd be hard pressed to get anywhere near even half of that.

~Jon

Offline kyle_baron

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Re: Basic Rocket Science Q & A
« Reply #410 on: 01/23/2010 09:56 PM »
I'm guessing for stability, but are there other reasons? 
What would be the result in air pressure, if an SRB were placed between 2 fins?
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Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #411 on: 01/23/2010 10:04 PM »
I'm guessing for stability, but are there other reasons? 
What would be the result in air pressure, if an SRB were placed between 2 fins?

No other reason.  Too many variables to answer the last question.

Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #412 on: 01/23/2010 11:21 PM »
Assuming their angle of incidence is zero, fins always move the center of pressure toward the fin.  Assuming the center of pressure is aft of the center of gravity, they also act like dampers in a spring, mass, damper system.
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Offline Fequalsma

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Re: Basic Rocket Science Q & A
« Reply #413 on: 01/24/2010 01:45 AM »
Yep, positive static stability is a GOOD thing!  Gotta
keep CG in front of the AC, or bad things will happen...
F=ma

Assuming their angle of incidence is zero, fins always move the center of pressure toward the fin.  Assuming the center of pressure is aft of the center of gravity, they also act like dampers in a spring, mass, damper system.

Offline kyle_baron

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Re: Basic Rocket Science Q & A
« Reply #414 on: 01/24/2010 02:45 PM »
I'm guessing for stability, but are there other reasons? 
What would be the result in air pressure, if an SRB were placed between 2 fins?

  Too many variables to answer the last question.

Oh, I don't know about that.  Let's think this thing through.  The fin is parallel with the SRB (roughly speaking).  And the SRB skirt would be at a 45 deg. angle to the fin (roughly speaking).  Wouldn't a column of air comming straight down from the SRB get caught between the fin and skirt (45 deg. pointing down), and get thrusted beneath the base of the rocket?  Might be another base heating variable.
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Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #415 on: 01/24/2010 02:54 PM »
I'm guessing for stability, but are there other reasons? 
What would be the result in air pressure, if an SRB were placed between 2 fins?

  Too many variables to answer the last question.

Oh, I don't know about that.  Let's think this thing through.  The fin is parallel with the SRB (roughly speaking).  And the SRB skirt would be at a 45 deg. angle to the fin (roughly speaking).  Wouldn't a column of air comming straight down from the SRB get caught between the fin and skirt (45 deg. pointing down), and get thrusted beneath the base of the rocket?  Might be another base heating variable.

What skirt?  Whose says the skirt and fin are on the same level? 

No, too many variables and you even say it.

Offline kyle_baron

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Re: Basic Rocket Science Q & A
« Reply #416 on: 01/24/2010 03:56 PM »
I'm guessing for stability, but are there other reasons? 
What would be the result in air pressure, if an SRB were placed between 2 fins?



  Too many variables to answer the last question.

Oh, I don't know about that.  Let's think this thing through.  The fin is parallel with the SRB (roughly speaking).  And the SRB skirt would be at a 45 deg. angle to the fin (roughly speaking).  Wouldn't a column of air comming straight down from the SRB get caught between the fin and skirt (45 deg. pointing down), and get thrusted beneath the base of the rocket?  Might be another base heating variable.

What skirt?  Whose says the skirt and fin are on the same level? 



The SRB skirt.  Of all the pictures I've seen of Ares V (or Direct) the SRB skirt is at the base of the rocket.  Fins could also be placed at the base of the rocket, unless you can think of a reason why they shouldn't be.  All the rocket nozzles are located beneath the base of the rocket.  Now that I think of it, the air flow between the fin and SRB skirt, MIGHT flow directly on to the nozzles!  And with 2 SRB's, that would be at 4 locations, near the perimeter of the base of the rocket.  Thanks, Jim.

Not all of the flow would go beneath the base, to the nozzles.  Maybe 1/2 of the flow, nearest the 1st stage core.  This is because the flow goes between the skirt (which is round) and the fin section, which is flat, and the base core (which is round, but we can consider it flat).
« Last Edit: 01/24/2010 04:41 PM by kyle_baron »
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Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #417 on: 01/24/2010 04:37 PM »
Fins could also be placed at the base of the rocket, unless you can think of a reason why they shouldn't be.

No need, only extra weight and drag.

This is what is used.  The scoops are on right and left of the base of the stage

Offline kyle_baron

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Re: Basic Rocket Science Q & A
« Reply #418 on: 01/24/2010 04:50 PM »
Fins could also be placed at the base of the rocket, unless you can think of a reason why they shouldn't be.

No need, only extra weight and drag.

This is what is used.  The scoops are on right and left of the base of the stage

Thank you, I see the scoops with the holes in them.  Now the question is, are the fins more aerodynamic than the scoops?  IIRC, the fins on Apollo, only had an internal tubing (conduit) structure.

The scoops are a pretty clever idea.  But another question, would be the quantity of air flow.  And which method would give more.
« Last Edit: 01/24/2010 05:03 PM by kyle_baron »
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Offline Art LeBrun

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Re: Basic Rocket Science Q & A
« Reply #419 on: 01/25/2010 01:49 AM »
Fins could also be placed at the base of the rocket, unless you can think of a reason why they shouldn't be.

No need, only extra weight and drag.

This is what is used.  The scoops are on right and left of the base of the stage

Jim's image is launch of GT-9.

Attached is an image of GT-2 12557. Notice scoops are not in place but a hole pattern seems to be part of the mounting. Any comments that will help? Did these scoops fly on all Gemini-Titans?
« Last Edit: 01/25/2010 03:04 AM by Art LeBrun »
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