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Ask a Rocket Scientist

Discussion in 'Off-Topic' started by Crezth, Oct 23, 2011.

  1. Crezth

    Crezth Idiot Savant

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    I'm a student in aerospace engineering at the University of Cincinnati, currently pursuing a double-major in both a Bachelor's of Science and a Master's Degree in the aforementioned.

    I have some experience in the field and a lot of experience in academia, and I can field questions on anything from basic Newtonian mechanics to computational fluid dynamics, and I have a lot to say on the value of space exploration and applied science in general. You can also ask me about what it's like being a student in engineering, if you're really curious.

    Any more esoteric questions are also welcome. Do your best to trip me up; after all, it's just rocket science. Oh, wait. ;)
     
  2. Quackers

    Quackers The Frog

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    How does a plane fly? (I think i have a good idea, but it would be nice to hear it in precise terms) :)
    How does a helicopter fly?
     
  3. Winner

    Winner Diverse in Unity

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    When will we have a Single-Stage-to-Orbit spaceplane that uses airbreathing dual-mode engines? :mischief:
     
  4. Perfection

    Perfection The Great Head.

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    How far are you in your major?

    Have you done any actual research?

    Is there any good free CFD modelling software that you know of? I got forced into learning about aerodynamics for a flowmeter I was working on awhile back, it would have been helpful if I knew of some good free simulation software.
     
  5. asbestos

    asbestos Chieftain

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    what do you say instead of "its not rocket science"?

    for example, "I don't get rocket science, but at least it's not < x >".
     
  6. Quackers

    Quackers The Frog

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  7. Crezth

    Crezth Idiot Savant

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    This is the million-dollar question. As an interesting point of fact, most aerospace engineers - graduated and everything - will have a hard time answering this question for you because even Wikipedia gets it wrong.

    That link actually goes to the Equal-Transit Time theory, which is the popular explanation for how airplanes generate lift. It is completely false for more than one reason, not least of which includes that there is no physical reason for the two particles to be linked, and that, in fact, experimentally it is often seen to not be the case.

    The other reason that theory is false is the same reason why Wikipedia is wrong and why most aero's struggle to answer the question: Bernoulli's theorem is plain unapplicable here, mainly because you can only compare pressures across one streamline. When you get into radically different streamlines, above and below the wing? And what about high-speed airflows that don't even obey the conditions under which Bernoulli's theory works? Then you're in for a world of hurt.

    But not all hope is lost, because we actually have a pretty good explanation for lift that doesn't rely on Bernoulli's equations of velocity and density and what-have-you. The basic explanation is that air enters the trailing edge moving horizontally, and exits the trailing edge smoothly, and at an angle, like so:

    True (look at the second diagram here, the one "with circulation")

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

    False:


    The air exiting off of the trailing edge pushes back on the airfoil, adding momentum in the vertical direction. Conservation of momentum* teaches us that, in order for the momentum of this vertically-traveling airflow to be equalized, a force must be generated. That force is commonly understood as the lifting force, and is equal and opposite to the downwash of the flow.

    Now then, why does the air come off the trailing edge smoothly? That introduces a whole new concept called circulation, which can be basically understood as the tendency of the airflow to change its direction around the wing. Think of the wing as a hardened vortex that gradually swirls the air around it.

    Helicopters basically have wings as well - the helicopter blades are each symmetrical wings, good for going up and down depending on what you need. They lift for all of the same reasons that an airplane lifts, although that is the end of the similarities between them. Because of the orientation of the blades, the calculations involved in the airflows above and below a helicopter are very complicated. Basically, if you can describe why an airplane flies, you can describe why a helicopter flies.

    * I know that the Bernoulli equations are a derivation of Newton's equations, but they are nonetheless simplified in extremum. They add a lot of other terms because air, unlike water, is a complicated fluid. When you start dealing with "real" air and 3-dimensional wings, along with the vortices that get formed off of the trailing edge and at the wing-tips, the Bernoulli explanation doesn't even start to work mathematically anymore. Hence why we describe the lift as happening because of the dramatic circulation of the airflow.

    Hah, I have no idea! I do know that a big area of research right now is VTOL planes that transform into jets after they're done being helicopters.

    Spaceplanes aren't receiving a lot of focus since the space shuttle program got slashed, as most of the concentration is going into big rockets for traveling to other celestial bodies and whatnot. Which is fine by me, but it means cool toys, like spaceplanes, are put on the backburner.

    I am three years into a six-year program.

    I am currently in the process of doing some research for NASA regarding information dissemination; or, in other words, nothing very cool.

    I have done some research on fuzzy logic systems and modeling and hope to do some research on more advanced methods of computational fluid dynamics, the true subtleties of which are only known to me due to excessively talking with my professors about it.

    On the question of free CFD software, that is an excellent question. All snark aside, if you're interested in more "popular" applications, something like this is bound to impress a girl or two. If you want something more powerful, however, the sad fact is you're either going to need to shell out or model the airflows yourself, with a program like Tecplot. That answer isn't really helpful to somebody outside the field, but I don't have any personal knowledge of really good, free CFD software.

    "At least it's not like talking to girls!"
     
  8. El_Machinae

    El_Machinae Colour vision since 2018 Retired Moderator

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    What's the vibe on the Space Elevator? I used to be a big fan, but now I'm hearing that the required strength is not only impractical, but potentially impossible?

    Additionally, in units of Sun-seconds, how much energy would it require to propel 1.7 million metric tons up to 0.8c and then back down to zero? Please assume that the fuel is onboard, and is a matter/anti-matter mix being used at 50% efficiency (or, since you're a rocket scientist, whatever efficiency you believe is reasonable. As well, since all the hard work has been done, what is energy needed if the target speed was 0.9c? Thanks!
     
  9. Crezth

    Crezth Idiot Savant

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    I have heard conflicting opinions on the space elevator, too. It is theoretically possible, if built closer to the true equator, because of coriolis effect, but the structure would need to withstand intense stresses to survive the process if it were repeated.

    And hohoho, no holds barred, eh? ;) I'll bring this with me and work it out in my free time. And yes, I'll even show my work. :mischievous:
     
  10. El_Machinae

    El_Machinae Colour vision since 2018 Retired Moderator

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    Thanks, I've done my own back-of-the-envelope, but I am worried that I am wrong. The fuel is in addition to the 1.7 million tons, btw
     
  11. ParkCungHee

    ParkCungHee Chieftain

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    Do you ever catch yourself saying "It's not rocket science?"
     
  12. NedimNapoleon

    NedimNapoleon Weird Little Human

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    How does a bike work (its not that simple).
     
  13. Perfection

    Perfection The Great Head.

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    Why would you use fuzzy logic for computational fluid dynamics?

    I despise that link and lose respect for anyone who uses it except when someone asks a really obvious question. I have poked around the web for this kind of stuff quite a bit, thank you very much.

    Yeah, that's sort of what I expected, still I like to ask because something might be out there I don't know of.

    The app is kind of cool though, and I do have an iOS device.
     
  14. uppi

    uppi Chieftain

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    If I may chime in on that: The required strength is larger than any material available right now offers. Theoretically it is possible, as graphene and carbon nanotubes have the required strength, but as the "nano" in the name implies these materials exist only on the nano- to microscale. So in theory one would just have to scale this up to the required 40000 kilometers. But this scaling is extremely hard because you have to ensure that there are no defects in your not-so-nano-anymore-tube. So you need to build a perfect molecule from like 10^18 atoms and that's far beyond your current capabilities.

    And then nobody knows, whether at some point the tube becomes unstable and collapses for some reason or another. An ab initio simulation of a large molecule is impossible (at least on a classical computer), so nobody knows whether such a long tube can exist.

    So at the moment we know nothing that would theoretically exclude a space elevator, but in practice there are huge hurdles to overcome.
     
  15. 7ronin

    7ronin 海軍少佐

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    Do you think that space programs funding is justified in this time of dire economic troubles? Can we afford to pay for a Mars mission or even just going back to the moon again?
     
  16. Midgard Eagle

    Midgard Eagle Chieftain

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    Given a city with 17 , should I build a Laboratory before building the SS Thrusters?
     
  17. Crezth

    Crezth Idiot Savant

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    Not anymore. I have friends who say it to me, though, in particular a friend who is majoring in neuroscience. We have a little back-and-forth that goes with that, so I'll let you imagine what the exchange entails. ;)

    I dunno, really. From what I know of basic mechanics, it's like a drivetrain that uses chains to rotate the bike wheels, right? I don't have much experience with bicycles. :p

    I'm sorry if I wasn't clear: we used fuzzy logic for general problem-solving, developing programs that gradually change the conditions to dynamically solve complex problems.

    The ultimate implications of this are A.I.'s that are capable of governing all of the automated operations of a space mission or research base with efficiency and near-intelligence.

    Haha, I'm sorry! I didn't mean to offend you. The short answer is and was that I don't know of any, so you're better off looking by yourself. ;)

    Fair enough! And the app is very nifty.

    Absolutely! Now, I am a bit (read: considerably) biased, so let me explain this.

    I view space exploration as the single most important task in front of humanity today. I think it is of utmost necessity that we spread out among the stars as quickly as we can, and given the relative stagnation the entire space industry has suffered for the past 40 years, I think the space program can afford to have it up with any more patience.

    Now, I understand that this is, at the current juncture, not necessarily practicable. However, it is my position that space programs, such as the Mars mission, are so inexorably linked to the progress and advancement of humanity as a whole, that the cost of the endeavor cannot possibly exceed the value of its accomplishment.

    I also think the financial situation is mostly the result of bad regulation of financial markets and insufficient taxation of the upper classes, but that's an entirely different can-of-worms. Long story short, with the highest marginal income tax rate of 60% in the 1960's, we (America) achieved the greatest technological achievement of mankind, and it isn't like we couldn't also fight a war and oversee economic growth at the same time.

    But I will stress that I don't want this thread to be about politics, so I'd like any further discussion to be on the merits of space exploration and the space program, even the Apollo program if you like. (I have met many, many people who say and think that the Apollo program was not important in any way whatsoever).

    Dunno! :lol:
     
  18. 7ronin

    7ronin 海軍少佐

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    Thanks for your thoughts on the space program. My own view is that the Challenger and International Space Station projects were unnecessary diversions. The space probes and the Hubble telescope were more modest endeavors but had a bigger payoff and much more impact. I am wholy in favor of going back to the Moon and for making a trip to Mars. Even though it has been forty years or so since the Apollo missions, I'm not sure what a reasonable target date might be. Perhaps it should be an interntional effort. One thing that amazes me is that we were able to go to the Moon several times and return successfully using technology and methodology (e.g. slide rules) which seems absolutely primitive when compared to what we have today. The rockets themselves don't seem to have changed much though.
     
  19. Souron

    Souron The Dark Lord

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    What level of protection does it take for an electronic device to function in open space?
     
  20. Patroklos

    Patroklos Chieftain

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    Why do you think you are a rocket scientist?
     

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