I'm no physicist, but....
Why does IDW think that a 30-ton CSM docking with the LM at 2 mph would be as catastrophic as a one-ton car hitting a brick wall at 60 mph? Did I miss something a few pages back? #-o

First, he calculated the kinetic energy of the CSM as follows:

KE = mv^2 = (30,000 lb)(2 ft/s)^2 = 120,000 ft-lb

Of course this is incorrect. KE = 1/2mv^2 and the mass needs to be in slugs. The correct calculation should have been,

KE = 1/2mv^2 = 0.5*(30,000/32.2)(2)^2 = 1,863 ft-lb

Second, he said this is the same as a 2,000 lb car hitting a wall at 60 mph (2,000 x 60 = 120,000). Of course this calculation has several errors: (1) the mass of the car needs to be in slugs, (2) 60 mph is the wrongs units, and (3) kinetic energy is proportional to velocity squared. The correct calculation should have been,

v = SQRT(2KE/m) = SQRT(2*1,863/(2,000/32.2)) = 7.75 ft/s

So I count five errors in all. We immediately pointed out the errors but, in typical conspiracist fashion, he has stuck steadfastly the conclusion reached by his flawed calculations saying the it is simply "obvious".

Edited to add:
There's one more problem with the IDW's calculation. When a car hits a brick wall it comes to a complete stop, thus giving up all of its kinetic energy. This doesn't happen in the CSM to LM/S-IVB docking. Working through the calculations we find that only about 1/2 of the CSM's original kinetic energy is dissipated in the "collision". Continuing the car analogy, this means it is more like a 5.5 ft/s crash (also note that 2,000 lb is a very light car).

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I'm not sure either. He keeps changing the assumptions, and when we calculate the results he calls us "inconsistant."

I say "us" only loosely. I'm struggling to follow what Jay and BB are saying. Fortunately for my self-esteem, there seems to be as much of a distance between IDW and me.


One problem with the student who is good with abstraction is they are used to approaching the world with abstraction. Rote learning environments do not suit them well; through the long drills they are always crying " But why is it like this? How does 4 come out of 2 and 2?" School can, indeed, become very unpleasant for a student who is too good at generalizing (and bored too easily to buckle down to memorization).


I haven't encountered quite the same reaction to an honest "I don't know" in my field. I think it is the lack of a real buffer between technician and management, plus the stresses of schedule and budget. When a techie tells another "I don't know" then they'll get respect as being willing to learn. When they say "I don't know" near management, tho, then fecal matter hits the impeller. The technical people correctly interpret doubt as being the understanding that the technique is new, the material is untried, and failure is a distinct possibility. Management interpret doubt as laziness, weaseling, or a shameless bid for more people (which management can only see as a status symbol, not as a tool to get the job done.)

Whoops. Didn't mean to go there. </RANT>

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"You keep using that word. I do not think it means what you think it means."

I just took a look at the Kinetic Energy Calculator at http://www.csgnetwork.com/kineticenergycalc.html, cross-referencing Conversion Online http://www.onlineconversion.com/energy.htm. The kinetic energy of the car is 241,957 Foot-Pounds, while the 30,000 kilogram Apollo CSM measures up as 96,091 Foot-Pounds.
This hypothetical impact would have released the tremendous energy of....
0.9952 ounces of high explosive.

Very good, Jay. Are you going to post that on GLP as well?

No. I don't see much of a need to return to GLP. There is no prayer IDW will understand why he is wrong, much less admit it. And no prayer that he'll shut up and go away. Maybe he has no life, but I do. And it needs attention.

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"Facts are stubborn things." --John Adams
Clavius Moon Base

Jay - have you studied psychology? Your grasp of multple facets of the human mind is well founded in things I remember reading in my clinical psychology courses.

Yes, I have studied psychology, but I'm not sure I would say I'm any kind of expert. Part of what I do for my company is reliability and failure analysis, which -- especially in recent years -- looks harder at human factors and thus draws increasingly from psychology and sociology.

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"Facts are stubborn things." --John Adams
Clavius Moon Base

There were a lot different calculations thrown around over at GLP because we kept altering the scenario. Furthermore, some of the calculations were just examples. Let me try to work though the problem using real numbers.

Let's use the following numbers from Apollo 11:

CM mass = 12,250 lbm = 5,557 kg
SM mass = 51,243 lbm = 23,244 kg
LM mass = 33,205 lbm = 15,062 kg

And from this Web page we have

S-IVB dry mass = 11,750 kg

The LM/S-IVB is the passive vehicle, so we'll assume an initially velocity of zero. The CSM is the active vehicle, so it is closing as some velocity. The design requirement for the docking mechanism was for an axial velocity at initial contact of 0.1 to 1.0 ft/s, so let's use the maximum or about 0.3 m/s.

Let's now define some variables as follows:

m1 = mass of LM + S-IVB = 15,062 + 11,750 = 26,812 kg
m2 = mass of CM + SM = 5,557 + 23,244 = 28,801 kg
v1 = initial velocity of LM/S-IVB = 0 m/s
v2 = initial velocity of CSM = 0.3 m/s
v' = final velocity of combined stack

From conservation of momentum we have,

m1v1 + m2v2 = (m1 + m2)v'

v' = (m1v1 + m2v2) / (m1 + m2)
v' = (26,812*0 + 28,801*0.3) / (26,812 + 28,801)
v' = 0.1554 m/s

The initial kinetic energy is,

KEi = 0.5m1v1^2 + 0.5m2v2^2
KEi = 0.5(26,812)(0)^2 + 0.5(28,801)(0.3)^2
KEi = 1,296 joules

The final kinetic energy is,

KEf = 0.5(m1 + m2)v'^2
KEf = 0.5(26,812 + 28,801)(0.1554)^2
KEf = 672 joules

Thus the change in kinetic energy is,

delta KE = KEi - KEf
delta KE = 1,296 - 672
delta KE = 624 joules

-------------------------------------------------------------------------------

So what can we compare this to? I weight about 190 lb and my desk is 29 inches high. Let's see what my kinetic energy would be upon impact with the floor if I jumped off my desk.

m = 86 kg
d = 0.74 m

We know that,

d = 1/2at^2 ---> t = (2d/a)^0.5
v = at

Therefore, my velocity at contact with the floor is,

v = a(2d/a)^0.5
v = 9.81*(2*0.74/9.81)^0.5
v = 3.81 m/s

Thus my kinetic energy is,

KE = 0.5*86*3.81^2
KE = 624 joules

So, the kinetic energy dissipated during the initial CSM-LM docking is the exact same amount as when I jump off my desk.

Let me go try it ...... hmm, didn't hurt one bit.

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Beautiful.

=D>

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"You keep using that word. I do not think it means what you think it means."

Wow. That's the most painful bit of reading I've done in recent memory.

Kudos to Jay et al -- the serious intestinal fortitude displayed by you folks is indeed worthy of commendation. Even amidst all the egregious GLP behavior, many really informative posts were presented. And, of course, the psychology involved is rather fascinating. Scary, but fascinating nonetheless.

Jay, I'd like to suggest that there's a basic qualitative difference between the example you gave of the obstreperous student and the genuine IDW mentality.

There is. My post was supposed to have a second half. I had to run off to church before getting to it. But you wrote substantially what was going to be in that second half.

The people who monopolize the professor are not always those who simply want to jump ahead of the class. There are also the contentious types, and you just hope they drop the class after failing the first test (which they invariably do).

OTOH, the dogmatic conspiracist type isn't seeking to learn, he's casting about blindly for something to use as a weapon...

Exactly, and you can see the terminology of conflict in IDW's statements. He speaks about "knowing one's enemy" and "taking you guys down a peg". It's not about locating the truth; it's about prevailing (or being seen to prevail). When his ideas are challengedand he realizes the challenge is fatal, he tries to find a happy middle. Phil noticed this when he first debated IDW, and little has changed. IDW tries to find some way in which he can still be right, notwithstanding the newly-presented information.

The conspiracist doesn't need actually to be right. He only has to be perceived as right, or at least not as wrong as he really is. There is the "middle ground" tactic, which IDW used many times. By muddying the waters, he can make it seem like there is enough ambiguity in the problem to allow for his solution also to be true. This, of course, doesn't work for classical physics, where everything has a precise definition. Energy is not force, nor is it power. It has to be victory at all costs. Legitimate experts can suffer admitting an error or two because the substance of their expertise remains untouched. Pretended experts cannot endure even the tiniest error because they live in perpetual fear of discovery, whereafter the ego-reinforcement comes crashing down.

There is also the distraction. IDW pounces upon trivial errors made by others. This counts as a "victory" for him, even if it's not the primary victory he wanted. This fits the notion that resentment is a key motivator. If you can't make yourself look good, make the other guy look bad. Threats about getting people fired further support that theory. IDW hopes to deny to others what he was himself unable to obtain.

The puerility -- profanity, name-calling, etc. -- eventually wears down his critics and they leave. Then he can say they left because they couldn't answer his arguments. The critic is either forced to let that accusation hang in midair or re-engage.

Those seem like truly different motivations to me.

They are, but it's important to remember that both the highly-intelligent and the less intelligent employ these kinds of ego reinforcement constructs. That is, both types of people -- when shorn of appropriate domain knowledge -- will create an ego-support context.

I've also noticed that the willingness to say "I don't know" when appropriate seems, paradoxically, to be more apt to win respect from bosses, clients and co-workers than trying to be a consummate panjandrum all the time.

Of course. Admitting error is a sign of reason. You expect people to defend their egos but where an error is highly apparent then acknowledging it demonstrates that you are driven by facts and reason, even if they don't support your previous beliefs.

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"Facts are stubborn things." --John Adams
Clavius Moon Base

IDW's latest on Apollo 13 (scroll as necessary).

Predictably riddled with errors.

First, the cause of Apollo 13's accident was well understood by the end of the accident, including reconstructing the electrical and mechanical conditions that resulted in the explosion.

Second, the CSM/LM stack already had enough energy to return to Earth. The initial DPS burn was not to "break free" of lunar gravity, but to return the stack to the free-return trajectory. As you recall, free-return transfers are fail-safe. You have to do a lunar orbit insertion burn not to return to Earth.

Third, docked DPS burns were practiced on Apollo 9. The whole stack was engineered to have a common centerline. However, the Apollo 9 scenario used the CSM guidance system to control the burn. The CSM RCS was designed to manuever the entire stack, and it didn't matter whether than was under SPS or DPS power. Losing both the CSM RCS and SPS was not considered a survivable scenario, so the two systems were designed with as much redundancy and reliability as possible. Each CSM RCS "quad" in fact had its own power bus and fuel supply.

Fourth, the astronauts did not do an "orbital injection". In fact, they specifically omitted the planned orbit insertion to allow the trajectory to return them to Earth as designed.

Fifth, there were several manuevers performed on Apollo 13. If you read the actual history instead of watching Ron Howard's movie, you see that the first DPS burn was to re-establish the free-return trajectory. That did not add energy to the orbit; it merely changed its shape. Had they done that and nothing else, they would have returned to Earth. The PC+2 burn (i.e., two hours after pericynthion) added energy to the orbit not because that energy was necessary for orbital mechanics, but because it was prudent to get the crew home as fast as possible because consumables were running out. The subsequent manuevers were corrections only, low-magnitude maneuvers. here the throttlability of the DPS worked to the crew's advantage because they could operate at 10% or 40% thrust and do a lengthy correction burn that achieved the appropriate delta-v for the trajectory correction. That is preferable to a short, intense burn when steering manually.

Sixth, the DPS was indeed gimballed.

Seventh, it is by no means impossible to orient a spacecraft using a badly placed RCS system. In fact, no RCS system is "perfectly" positioned to carry out the programmed manuevers without inducing "residuals" (unwanted rotations or velocities). Translations often create rotations, and vice versa. It's also possible to get a roll residual, for example, from a yaw manuever. Part of flying in space -- whether manually or under computer control -- is dealing with the residuals.

Air pilots are no strangers to the notion of residuals. The rudder on an airplane is used to yaw the vehicle -- turn it side to side. But it also can create a roll rate. Experienced pilots do not expect "pure" control.

If you want to roll the CSM/LM stack along its longest axis, you use the CSM RCS jets. But it's almost impossible to do that without also creating rotations in the other dimensions. The fact is simply that the CSM RCS jets are all piled up at one end of the stack. That's not the best place to put them. So the digital autopilot on the CSM has two modes -- one for the docked stack and one for the CSM alone.

The LM RCS jets cannot possibly be in optimum position for all its flight modes. The ascent stage has to use the RCS for ascent guidance, and the landing employs the RCS with the ascent and descent stages connected and transitioning from fully-fueled to almost empty.

When you consider this, the simple truth distills itself out nicely: the notion that an RCS can only function when it is optimally distributed around the center of mass is pure rubbish. In any real, functioning spacecraft, the center of mass moves around, and you'll always have residuals. it's more likely than not that the RCS for any given spacecraft -- planned or unplanned -- is not optimally positioned.

The issue for Apollo 13 was not that the RCS on the LM was incapable of steering the entire stack, technologically speaking. It was simply that the crew had to learn to do manually what the CSM computer already knew how to do.

This is why you send test pilots. Those are people who know how to teach themselves in a matter of minutes or seconds to fly things that aren't working right. I've helped pilots fly Cessnas using only the doors. Then there is the well-documented story of the DC-10 with the center engine rotor burst, in which the pilots flew the aircraft to a survivable crash landing using only the engines for steering.

I don't plan to go back to GLP anytime soon. If anyone wants to reproduce the substance of this post on GLP, you have my permission. If IDW wants to come here and discuss his material in a forum where he can't use his characteristic name-calling and accusations, and instead concentrate on the facts and reason, I would welcome him. But let it not be said we haven't answered his questions.

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"Facts are stubborn things." --John Adams
Clavius Moon Base

"...using only the doors."

Oh, I DO want to hear the rest of that story!

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"You keep using that word. I do not think it means what you think it means."

Many years ago, my father (a psychiatrist) put it this way: "IQ scores are like the marks on a measuring cup. They give you an indication of capacity, but say nothing about contents!

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Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

Not much to tell -- it's a standard technique, weird though it seems.

Fasten your seat belt tight and push the door out into the slipstream. The aircraft yaws. Do this enough and you produce a slight roll, which gradually turns the aircraft. Personally I'll stick with the ailerons, but the point is that you're never really as screwed as you think.

I don't talk a lot about my personal life, but last week in church I was directing the choir in its weekly anthem. A woman in the choir who has had a history of blackouts had a full-blown grand mal seizure right in the middle of the performance. Now I'm as calm as the next guy in a pinch, but there wasn't a whole lot I knew how to do. But I didn't have to. Within literally about seven seconds she was surrounded by two nurses and a doctor -- members of the congregation. At the first sign of trouble they had instinctively run to her assistance. 911 had already been called.

What's the moral of the story? People who acquire expertise in their chosen field are able to perform astonishing feats, pausing not even a second for indecision. Stimuli that we find unfamiliar or shocking are instinctively acted upon by others who are accustomed to them.

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"Facts are stubborn things." --John Adams
Clavius Moon Base

Not much to tell -- it's a standard technique, weird though it seems.

Fasten your seat belt tight and push the door out into the slipstream. The aircraft yaws. Do this enough and you produce a slight roll, which gradually turns the aircraft. Personally I'll stick with the ailerons, but the point is that you're never really as screwed as you think.

I read an article in Flying many years ago about a guy who had an engine failure in a C-172 or similar light aircraft, at night with fog and clouds preventing him from picking out a good spot to land. He had a parachute with him, and jumped after jettisioning a door. The article described his experience watching the strobes disappear into the clouds... and then come back, as the drag kept the plane in a wide turn. He had two or three close encounters with the bird before they both landed (his landing was much softer).

Bob B., I'm comfortable with 311 s as the specific impulse of the DPS at 100% thrust, but I'm wondering how much it varied depending on throttle setting. I know at least one DPS manuever on Apollo 13 was carried out at about 40% thrust, and I'm trying to decide whether there would be a significant difference in total delta-v. I don't have a table or a curve that shows specific impulse as a function of DPS throttle setting. I would expect the curve to be reasonably flat, and I would remind everyone that the DPS was only discretely and discontinuously throttlable. You could select 10% thrust, or a continuous value between 40% and 90% thrust, or 100% thrust. The ranges (0-10%), (10-40%), and (90-100%) seem to be unavailable.

Also, I wonder if IDW will fall into the standard trap of being misled by g0 in your computations. Delta-v as derived from specific impulse is one of those cases where neither EES nor SI is consistent, since Isp is always given in "dimensionless" (i.e., independent of measuring system) units. If you want delta-v in EES or SI units then you have to apply the standard conversions.

My guess is that if he looks at your computations at all, he'll try to say that the gravitational figures for the moon, not the Earth, should be used. It's the standard layman's mistake when first encountering those formulas.

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Clavius Moon Base

I agree the 311 s number for specifc impulse is probably at the 100% thrust setting. Like you, I've never seen an table or curve showing the change in Isp versus throttle setting, though I'd be very interested in seeing such data.

I've attempted to perform some calculations regarding this in the past. I don't fully remember how I when about it, but I think I started by estimating the chamber pressures and temperatures at lower propellant flow rates. From this everything else calculated easily. As I recall, specific impulse changed very little; there may have been only a few percent drop off at lower settings. For instance, a propellant flow rate of 50% might have produced a thrust of 49%, or somthing like that.

My confidence level in my results is not real high. If you are able to find some real data, could you please share it with me?

Good observation. I'll have to be ready for that response should it occur.

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