From: jsvrc@rc.rit.edu (J A Stephen Viggiano)
Subject: Re: What do I need to get something into orbit?
Organization: RIT Research Corp
Distribution: usa
Date: Wed, 12 Aug 1992 16:13:40 GMT

In article <1992Aug11.170240.28620@SSD.intel.com> cower@SSD.intel.com (Rich Cower) writes:

>Does anyone have any information on getting a model rocket into
>orbit? No payload, just the rocket would be enough. I'm looking
>for specifics on what to buy, and how to put it together.

You _can't_ put a model rocket (see the current definitions) into orbit, at
least not if you launch it from ground level. I guess you could do what Jay
Apt did, and take one aboard a Shuttle flight, but I don't think this is what
you have in mind . . . . :-)

Why you can't do it:

     1.	The LTV Scout, a solid-fueled orbital platform, requires _four_
	stages to acquire orbit. I mention it here because it is probably
	about as close to a model rocket as you are apt to find in an
	orbital platform. Even so, most of its mass is propellant; we
	would say it has a large mass ratio (liftoff mass divided by
	unfueled mass). The mass ratio of a model rocket is
	considerably lower.

     2.	Peter Alway provides a great discussion of the effect of scale
	on efficiency in his impressive _Scale_Model_Rocketry_. In his
	section on the Deacon Rockoon he points out that, assuming a
	constant shape, the mass (and, hence, the amount of propellant,
	if you assume a constant mass ratio) is proportional to the cube
	of the scale, while the cross sectional area (which determines drag)
	is proportional to the square of the scale. Using a smaller scale
	penalizes you severly in drag. In other words, the terminal
	velocity of a body is not independent of its size.

	This is complicated, so let's take a simple example. Let's say I
	build a one-half scale model of the ARCAS. All the lineal dimensions
	on my scale model will be one-half those of the original. If I
	use similar (but scaled) materials, the mass of the scaled version
	will be one-eighth that of the original. I'll be able to cram
	one-eighth the propellant into my scaled version as there is
	in the original. Which seems okay, as I also have one-eighth the
	deadweight -- the drag-free accellerations will be identical.

	But once drag is considered, the situation changes. Assuming the
	same Cd for both versions, the scaled version will have one-fourth
	the drag product (Cd times cross sectional area) of the original.
	As it only has one-eighth the propellant with which to compensate
	for this, it is at a disadvantage.

     3.	Acquiring orbit means more than simply accellerating a body to
	orbital velocity. The velocity must be vectored relatively tangent
	to the Earth's surface. And it has to happen 150Km or so up. You'd
	need guidance equipment to do this at the right time. (You can
	have horizontal powered flight with FSI E5 "steam engines," but
	they only seem to do this about 50m up on windy days. :-)

Perhaps a better challenge is supersonic flight; the excellent composite
engines available at attractive prices make this more within reach. They
really offer an advantage over black powder for this sort of thing: the
exhaust velocities are about 2,5 times as great (ca. Mach 6).