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Post by tkw251070 on Apr 16, 2009 20:40:16 GMT -4
Hi All This is my first new thread here. Can anyone explain why this UT video and second part are right or wrong www.youtube.com/watch?v=sgsVHUuNZ7o&feature=channel_pageI don't think the analysis accounts for (a) The fact the LM was moving sideways as it approached the lunar surface, so there wouldn't be some Earth like cone of fire carving into one fixed point of the landing site. (b) In any case, the analysis does not account for the belling of the outlet gases in vacuum. (c) Where does the figure of 3.41 kg per second come from, is this the correct fuel burn for the LM. (d) This fuel burn was used to calculate an exhaust gas speed of 3225 m per second. This seems huge for exhaust gases? However, I think the analysis uses the most basic of momentum equations Force = mass per second * gas velocity This does not seem right, I thought some account had to be taken of exit pressures and free stream pressures. Rocket science is not my strongest area, happy for someone to help me learn some new stuff here. TK |
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Post by JayUtah on Apr 16, 2009 21:39:59 GMT -4
I can't watch the video where I am, but 3.41 kg s-1 sounds like a reasonable estimate for throttled DPS mass flow rate. EGV of 3,225 m s-1 is reasonable for the DPS, but that's mean exhaust velocity at the exit plane. A significant percentage of the actual thrust of the DPS at this point was pressure thrust. The LM was indeed moving significantly to the left and slightly forward at touchdown. The exhaust gases will disperse in a cone of approximately 45 degrees' half-angle.
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Post by fiveonit on Apr 16, 2009 21:47:52 GMT -4
Welcome to the site tkw251070
I find it highly suspicious when a hoaxer posts a video on YT and then *disables* the ability to leave comments. What is this person afraid of? An informative rebuttal perhaps?
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Post by tkw251070 on Apr 17, 2009 1:17:56 GMT -4
I can't watch the video where I am, but 3.41 kg s -1 sounds like a reasonable estimate for throttled DPS mass flow rate. EGV of 3,225 m s -1 is reasonable for the DPS, but that's mean exhaust velocity at the exit plane. A significant percentage of the actual thrust of the DPS at this point was pressure thrust. The LM was indeed moving significantly to the left and slightly forward at touchdown. The exhaust gases will disperse in a cone of approximately 45 degrees' half-angle. I think my question is that the individual concerned has found out the the DPS mass flow rate, and then calculated the exhaust speed by dividing the weight of the LM close to landing (approx 11000 N) by the DPS mass flow rate, this gives an EGV of 3225 m/s. This is where I have a lack of understanding. The thrust equation is given by: Thrust = mv + (P e-P 0)A Is (P e-P 0)A the pressure thrust you talk about. Because if it is, v is less than 3225 m/s. |
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Post by tkw251070 on Apr 17, 2009 1:47:20 GMT -4
Welcome to the site tkw251070 I find it highly suspicious when a hoaxer posts a video on YT and then *disables* the ability to leave comments. What is this person afraid of? An informative rebuttal perhaps? Thanks for the welcome. I tried debating on UT, and after three days realised it was not for the sane. I've met the crazies... |
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Post by slang on Apr 17, 2009 7:23:23 GMT -4
What is "UT" in this context? (youtube is blocked where I am now)
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Post by Obviousman on Apr 17, 2009 7:45:49 GMT -4
I think UT is an abbreviation for U-Tube, meaning YouTube.
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Apr 17, 2009 9:29:55 GMT -4
I don’t know where this individual got his numbers, but there’re not too far off. The method I’d use yields similar numbers.
My method is to use the DPS specific impulse of 311 seconds. This number may vary a little bit based on throttle setting, but my studies have shown that the change is small. To determine the effective exhaust gas velocity we simply multiply the specific impulse by the standard acceleration of gravity:
C = 311 * 9.80665 = 3,050 m/s
Effective exhaust gas velocity takes into consideration both momentum thrust and pressure thrust and is not the actual velocity of the gas.
If we use the video producer’s number of 6,800 kg for the mass of the LM, then the thrust needed to hover the LM is:
F = 6,800 * 1.623 = 11,036 N
where 1.623 m/s2 is the acceleration of gravity at the moon’s surface. The propellant flow rate is the thrust divided by the effective exhaust gas velocity:
q = 11,036 / 3,050 = 3.62 kg/s
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Apr 17, 2009 9:43:15 GMT -4
This is where I have a lack of understanding. The thrust equation is given by: Thrust = mv + (P e-P 0)A Is (P e-P 0)A the pressure thrust you talk about. is the momentum thrust and (Pe-P0)A is the pressure thrust. In this case, v is the actual exhaust gas velocity. Mass flow rate is generally represented by mdot ( m with a dot over it) - I usually use q instead of mdot. Effective exhaust gas velocity, C, combines the effects of momentum and pressure thrust as follows: qC = qV + (P e-P 0)A Because if it is, v is less than 3225 m/s. Yes, V is less than 3,225 m/s, or even the 3,050 m/s that I calculated for effective exhaust gas velocity. We need to calculate the actual exhaust gas velocity, but this requires knowing the characteristics of the engine. I can probably make some assumptions and come up with a pretty good estimate, but I don’t have time right now. My web page goes into some of this stuff if you want to study up on it: www.braeunig.us/space/propuls.htm |
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Post by laurel on Apr 17, 2009 12:34:53 GMT -4
Guilliaume D'Ock has a blog too. pseudonautics.blogspot.com/2008/11/disinformation-techniques.html"But then, if I had a conspiracy to cover up, I'd on one hand encourage or pay promoters of such nonsense, and on the other hand pay debunkers to attack the claims of the former, burying the real issues." NASA's paying disinformation agents and debunkers?  |
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Post by gillianren on Apr 17, 2009 13:12:16 GMT -4
That seems wasteful. And people are complaining about money to be spent for a book?
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Post by JayUtah on Apr 17, 2009 13:41:11 GMT -4
Guilliaume D'Ock has a blog too. pseudonautics.blogspot.com/2008/11/disinformation-techniques.html"But then, if I had a conspiracy to cover up, I'd on one hand encourage or pay promoters of such nonsense, and on the other hand pay debunkers to attack the claims of the former, burying the real issues." NASA's paying disinformation agents and debunkers? "Pseudonautics" isn't even a properly derived term. Pseudoscience and pseudohistory, however, are. Apparently this guy believes that he legitimizes his approach by attaching a formal-sounding name to it. His reasoning is perfectly circular: he characterizes opposition to his ideas only in terms of the "certainty" of a conspiracy. This particular mode of circular reasoning is called "converting the conditional." It is, in fact, he who is burying the real issue. He wants to read some sort of justification into the mere fact that opposition to his ideas has occurred. The real issue is that the accusations and allegations are being made without a proper understanding of the relevant sciences. M. D'Ock doesn't accept the possibility that opposition to his claims is well-founded and motivated purely by a love of truth. |
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Post by JayUtah on Apr 17, 2009 13:57:43 GMT -4
Mass flow rate is generally represented by mdot (m with a dot over it) - I usually use q instead of mdot.
I don't know of any HTML representation of the dot notation. Placing a dot over a standard symbol is a standard annotation for a time-derivative value. Lower-case m is mass, and m with a dot over it (pronounced "em dot") means "mass per unit time." Similarly h is altitude, so h-dot is change in altitude per unit time, or the rise or sink rate. These quantities may be scalar values or functions, depending on the computational needs. But the dot notation is standard and ought to be used were possible.
We need to calculate the actual exhaust gas velocity, but this requires knowing the characteristics of the engine.
In practice this is almost always done by testing and not by computation. Computational models exist that incorporate the underexpansion and other real-life effects of the plume. They may be expressed, as above and elsewhere, into a "effective" gas velocity. But that is useful only in terms of propulsion computations, and then only for mission planning purposes. Actual flight operations merely measure the acceleration as it occurs. In terms of the LM descent programs (where the IMU may be spoofed by lunar gravity effects) a combination of inertial and direct measurements via the landing radar were used to close the loop in the DPS throttle control. This also accommodates changes in LM mass/weight due to DPS propellant depletion.
You cannot use effective gas velocity to drive reasoning about plume effects on impacted structures and surfaces. You must use actual gas velocity, and even then you're still in the realm of first-order approximation. It is a complex fluid-dynamics problem.
We measure thrust in a vacuum rather than to try to compute it because the actual underexpanded exhaust gas velocity at the exit plane isn't even a coherent velocity. It is a "cone" of velocities whose overall imparted momentum is a complex, multivariate problem. As I said, we have models. The equations above are first-order approximations.
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Post by ajv on Apr 17, 2009 16:04:42 GMT -4
I don't know of any HTML representation of the dot notation.ṁ is Unicode character x1e41 (LATIN SMALL LETTER M WITH DOT ABOVE) and can be embedded in HTML with the sequence ṁ It could be used to simulate the mathematical construct. Of course it will depend on whether the forum software and your browser and computer system can display that character. The management is not responsible for how the first character in this paragraph renders in your browser.The right® solution is to use the MathML standard to embed equations but few browsers currently support it either (e.g. a late model Firefox will handle it). In MathML, the embedded code would be like: <mover accent="true"> <mi> m </mi> <mo> . </mo> </mover> which is a bit clunky but you're meant to use a function editor to create it.
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Post by tkw251070 on Apr 17, 2009 16:17:12 GMT -4
Mass flow rate is generally represented by mdot ( m with a dot over it) - I usually use q instead of mdot. Thanks for your answers Bob, my use of m instead of q or em dot was laziness. Sorry if that confused. |
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