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      N-sec of total impulse (8 lb-sec). The Enerjet-8's external delay was
      too cumbersome for unsophisticated users, and was replaced by a
      conventional internal delay when the Enerjet E24, F52, and F67 were
      introduced in 1973 by Enerjet and Centuri, its parent. The F67 was the
      first full 80 N-sec F motor produced in the US.

      FSI is a bit of a cipher in all this. They chose to go with a 21 x 70 mm
      motor for A, B, C, and small D motors, beginning in about 1966 or so.
      Their full D and small E motors are in 21 x 95 mm cases. (D18, D20, and
      E5). The larger FSI motors are in 27mm cases of various lengths. The E60
      is about 95 or 100 mm long, and the F7 and F100 are 125 mm. FSI started
      producing 18 x 70 mm A, B, and C motors in about 1985 or so.
----------------------------------------------------------
5.18   Why don't I just make my own model rocket motors?  Shouldn't I be able
           to custom-make better, more powerful motors, at a cheaper price?

    This subject has been hotly debated on r.m.r.  It is one of those
    'emotional' subjects that find people either firmly for or against.
    The following post from Lawrence Smith probably says it all best.

    From: thesmiths@mv.mv.com (Lawrence Smith)
      In our pre-made, factory-produced society there is a bit of nostalgia
      for doing things the "old-fashioned" way - "home made" carries with
      it the feeling of being somehow "better".  It could be tastier, more
      durable, easier to maintain, or cheaper, it is some quality that we
      don't find in mass-produced items.  It is therefore natural for fans
      of rocketry to look at the prices of the ingredients of an engine and
      think to themselves that such engines really can't be all that difficult
      to make - and that "home made" might be not only cheaper, but
      just as good as factory engines in other ways.

      There is a grain of truth here - _just_ a grain - but that is enough
      to make the idea pretty dangerous.  Most readers of this will not be
      old enough to remember the "Basement Bomber" headlines from the late
      fifties and early sixties.  Making solid-fuel rocket engines _is_ a
      pretty dangerous proceeding, unless you do it _right_.  And even if
      you do it _right_, you must be _consistant_ in following the safety
      precautions.  It really won't matter whether it is the first or the
      fiftieth engine that blows your hand off in the long run.  It is the
      need to watch the safety precautions that is your first "hidden cost",
      something that most people dreaming of home-made engines seldom add to
      the calculation.

      You need _room_, you can't do it in your basement, nor, indeed, in
      your house at all.  Nor in your apartment, nor in your school chem
      lab, nor anywhere else there is something you don't want blown up.
      You must _assume_ the engine _will_ blow up, and ask yourself where
      it would be okay.  Maybe an outbuilding on your property, maybe out
      in some field.  You need _space_ to build engines.  You either need
      to own that space or have the permission of people who do, too.

      In most places in the country, you need some sort of _license_.
      Rocket engines are first cousins to pipe bombs, and there are
      few municipalities that would care to have you building those.  On
      the other hand, there are many municipalities that don't care if you
      reload spent shotgun shells, even without a license.  You need to
      know and understand the local regulations.  You are not building a
      class "C" toy propellant device.  Even if you _think_ you are.  That
      is a legal name, not a descriptive one.

      You _can't_ reuse Estes or other maker engine casings.  Spent casings
      have undergone considerable stress, they can no longer be guaranteed
      to hold if reloaded and fired again.  Yes, they have a fairly consider-
      able safety margin.  It isn't enough to reload them.

      You _can't_ use the same stuff Estes uses, nor can you easily obtain
      composite fuel.  Estes uses black powder - gunpowder - but they are
      using a special formulation, not just mixing the usual ingredients.
      In fact, it's pretty dumb to use any powder at all.  Powder must be
      rammed to make it solid enough for "solid fuel" - that means you have
      to pour the powder into the casing and then take a stiff rod and ram
      it up and down, like one of those old-time muskets.  Ramming will
      compress the powder - which is the objective - and will also heat it,
      which will also make it more shock-sensitive, not to mention the heat
      from the friction of the rod itself moving up and down in the casing.
      It can also raise dust, which is also more easily ignited than a solid
      slug.  A fellow by the name of Jim Flis posted a commentary on creating
      such engines, I recall.  If anyone saved that post, I'd like to see to
      see it inserted here.

      Even if you manage to pull off all the above, the engine may not perform
      like you expect.  Your ingredients may not be up to snuff, or maybe you
      didn't ram enough, or maybe something else got mixed in by mistake, or
      maybe you forgot to add the final layer to the casing - whatever.  There
      is a real good chance any home-made engine is going to do serious damage
      to your rocket, even if it didn't do any to you.  One of the things you
      pay for from Estes and Quest is _quality_control_.  And even with professionals
      with years of experience with A, B, and C engines, Estes has
      problems with D and E engines.  Can you really do better?

      Estes, Quest, and the others have _time_, they have _equipment_, they
      have _space_, they have _quality_control_ and lastly, they have _liability
      insurance_ for when something happens with one of their engines.  _That_
      is what you are paying for, not just a couple teaspoons of black powder
      and a bit of clay and cardboard.  As you can see, there is more than meets
      the eye in an Estes or Quest engine.  The fact of the matter is this: a
      premade, off-the-shelf engine _is_ cheaper, more reliable, and far, far
      less effort to obtain.  There is really no reason to try to duplicate a
      standard engine.

      That said, there will remain a small core of people who still want to
      roll their own.  Maybe they need a non-standard size, or they just are
      more interested in the engine than in the rocket.  Before you proceed,
      you will need a copy of "Rocket Manual for Amateurs" by Bertrand R.
      Brinley, Capt.  It's out of print, so you'll have to do some looking.
      If you aren't patient enough to track down a copy then you aren't
      patient enough to build engines.  Take the hint.

      Brinley gives a good overview of propellant technology up to the mid-
      sixties or so, which is good enough for you, since the advances have
      mostly been in the realm of "real" rockets with incredibly exotic
      chemistry.  He concludes that amateurs should stick to zinc and sulfur.
      Zinc and sulfur can send rockets high enough to need an FAA waiver, so
      this is really not limiting.  Also, though zinc and sulfur can be ram-
      med, they can also be mixed with acetone or alcohol to form a putty
      that can be inserted into a casing and cast into shape.  This he terms
      "micrograin".  I won't go into detail on proportions or other infor-
      mation, you'll have to find the book.  _I_ not telling you how to
      build an engine, _I_ won't do it, and I don't think _you_ should, but
      if you are anyway I want you to be aware of what you need to do to
      succeed, for if you fail the consequences will be horrible.

      While Brinley is helpful for fuel, he is more valuable for the kinds
      of safety precautions you need to take.  You are going to need sand-
      bags.  Take the hint.

      One thing Brinley assumes, though, is using metal cases, including CO2
      cases for small engines.  This will _definitely_ make your rocket require
      an FAA waiver, and the metal content will make it easy to spot your
      rocket on radar, so you better not try to mickey-mouse it.  Of course,
      the FAA will _not_ be forthcoming with a waiver for an experimental
      free-flight rocket, unless you are launching from someplace in the
      southwest desert area, and maybe not even then.  In a way, this is an
      advantage.  A metal-encased engine will fragment when it explodes, and
      is far more dangerous that one in fiberglass or carbon fiber.  Also,
      metal is far more likely to cause a spark somehow than are non-metallic
      components, and so are safer to handle.  This limits the size and
      materials you can use.

      You will need to improvise on Brinley and develop a fiberglass or
      carbon-fiber casing and clay or other material nozzle.  I have an
      ulterior motive for mentioning this, and I admit it freely.  Making
      a casing like that is not trivial to begin with, and you will find
      it's pretty expensive, both monetarily and in terms of time.  If
      that still does not deter, at least you will be encouraged to make
      your engines _small_ - which will make the process safer, for there
      will be less to explode.  I have no idea how thick the casing needs
      to be, nor would I say if I did.  Again, you need to do your home-
      work.

      If you take the advice in this post to heart, you will not make a
      rocket engine.  If you take only some of it, you will build a much
      smaller engine, in a casing less likely to become deadly shrapnel
      (not _un_likely, just _less_ likely), and with techniques less likely
      to cause an explosion.  You will be doing it with the advice of a
      professional (Capt. Brinley, not me) and hopefully will succeed, and
      then either give it up or go legit and start a career with Morton
      Thiokol.  Who are, of course, the makers of the shuttle SRB's used on the
      last flight of the Challenger, just in case you thought being a
      professional meant that you had everything figured out.
---------------------------------------
Copyright (c) 1996, 2000 Wolfram von Kiparski, editor. 
Refer to Part 00 for the full copyright notice. 

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