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Comment: News? (Score 5, Interesting) 33

People have been launching sounding rockets into the aurora borealis for something like 60 years - in the many hundreds, if not thousands. The facilities in AK and Canada are far and away the most active sounding rocket sites in the world because of it.

    How is this news?

Comment: Re:What's the term for a prophylactic prediction? (Score 1) 677

by Brett Buck (#49041639) Attached to: Empirical Study On How C Devs Use Goto In Practice Says "Not Harmful"

Whether he was right or wrong, people have taken his statements, as with most religious zealots, to the ultimate ridiculous end.

      I heard the capper in about 1988 or so, when one of our customers asked, during a code review, if an unconditional branch in assembly code wasn't just like a GOTO, which was prohibited.

 

Comment: Re:why? (Score 1) 677

by Brett Buck (#49041609) Attached to: Empirical Study On How C Devs Use Goto In Practice Says "Not Harmful"

You win! I was showing some newbies an example of that yesterday. With a lot of error checking, and indenting that Wirth would have approved of, the inner statements in the deepest loop wouldn't have fit on the screen.

      The alternative chosen was *multiple subroutine exits*, which is arguably much worse than a few GOTOs.

      Brett

Comment: Re:Yeah! (Score 1) 282

by Brett Buck (#48952959) Attached to: NASA Looking At Nuclear Thermal Rockets To Explore the Solar System

That's compared to a few hundred pounds for an equivalent chemical rocket. The point of post you are replying to is absolutely correct, it only makes sense if the rocket using it is large and carries a large amount of propellant.

      There are effectively two factors in rocket design - the engine ISP and the mass ratio. The mass ratio is a measure of how much propellant is carried VS the dead weight (engine, tanks, payload). Those two things can tell you the velocity change of the rocket (see: "rocket equation"). Note that you have to recognize that the ISP and exhaust velocity are one and the same to make sense of it.

  The ISP is twice as good as a chemical rocket, but the dead weight is very high, too, so for this to make sense, you need a large amount of propellant. The difference between 250 lbs (chemical rocket weight) and a few thousand (practical lower end of the NTP and associated shielding, etc) could be critical.

    For example, using hydrogen as a working fluid increases the ISP (the lighter the exhaust products, the better) but reduces the mass ratio because the density is so low, the tanks have to be gigantic and therefore heavy. reducing the mass ratio. If it used Xenon, it might have lower ISP but the dead weight would be smaller due to much smaller fuel tanks. It's a trade-off, and NTP engines don't care very much what fuel they use.

    Someone has already figured all this out, there was a perfectly sound design for a rocket upper stage using a NERVA engine, I would suggest that as a point for further research.

Comment: Re:Yeah! (Score 2) 282

by Brett Buck (#48950255) Attached to: NASA Looking At Nuclear Thermal Rockets To Explore the Solar System

In this case, it's absurdly beside the point as well. The expenditure of nuclear materials is utterly irrelevant to the problem.

      The way to rocket works it to use nuclear-generated heat to expand and accelerate a working fluid (usually hydrogen) and shoot it out the nozzle. What matter is the mass of the working fluid expended per impulse (force x time) - the specific impulse (lb-sec/lb or kg-sec/kg, for units of seconds) or ISP. T

      housands and thousands of lbs of the working fluid will be consumed, the fact that it also consume a few ounces of nuclear material, too, is utterly in the noise.

    A very good chemical rocket will have an ISP of 450-460 seconds. A nuclear thermal rocket will have an ISP of around 900-1000, or roughly twice as "good". "Good" is defined by the amount of impulse/momentum change you get for a given amount of fuel consumed.

A nuclear thermal engine can be built to provide almost any desired thrust level, with 25000 lb thrust engines actually built and tested.

      By comparison, a Hall Current or other ion engine will have an ISP of around 1800, but use vast amounts of electrical power for extremely feeble thrust of far less than a pound in typical cases.

Comment: Ion Thruster (Score 4, Informative) 282

by Brett Buck (#48949251) Attached to: NASA Looking At Nuclear Thermal Rockets To Explore the Solar System

An Ion thruster (of any variety) is not *remotely* a replacement for a nuclear thermal engine. The ISP is great but the thrust levels are (and always will be, at rational sizes) feeble. And it's very likely that massively clustering them to get the thrust up will required a nuclear reactor to power them. 6/10ths of an *ounce* of thrust for 4 kW power input.

Ion thrusters have their uses, like in gently nudging things over long periods. They are not going to replace chemical rocket or NTP engines for any sort of high-thrust application.

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