Rocket Science for Earthlings
a continuing series for the gravitationally impaired. Rocket Science for Earthlings 4

Section IV finding your way to orbit, Aliens rule the fourth World!

You walk up a stairs and through a door. Simple for you and me, yet the requirements from a guidance systems point of view are the same as those of a rocket flying into orbit.

We depend on three systems to get ourselves up the stairs. The first is our attitude control system, located in our ears strangely enough. The three semicircular canals of our inner ear sense any changes in the attitude of our head. they provide the signals that keep us upright and prevent us from falling on our faces. If you close your eyes you will be more sensitive to their signal. Now nod your head slowly. This is the motion called pitch. Nod your head if you understand. Now look from side to side, or shake your head no. This is called yaw. Now tilt your head from side to side. This is the roll motion. You may notice that you can interchange the axis of these motions by laying on your back, so they are arbitrary as to how you want to orient them. Astronauts find that frequently rearranging your perception of pitch, yaw, and roll, while in zero gee causes nausea however, so try to stick to one set of axis's.

Our rocket uses the exact same system to keep itself pointed in the right direction, and therefore to keep it's thrust pushing it into orbit. The latest technology uses laser light traveling through coils of fiber optic cable. The coils are oriented exactly like the canals of our inner ear, in the three plans of pitch, yaw, and roll. The rocket has the added task however of slowly changing it's attitude, to tilt over as it ascends, to orient the engine thrust to attain orbital velocity.

Another sense we get from our inner ear and from the rest of our body is acceleration. We feel each step rising us up the stairs. We also feel our selves moving forward when we start the climb. And we also feel it when we move from side to side. Unfortunately we humans do not have a good sense of acceleration. If the change in velocity occurs slowly and smoothly, we may not notice the change. Rocket guidance systems have a much more sensitive and accurate system for sensing acceleration. Tremendous strides have been made in this area. The latest technology uses micro machined integrated circuit systems developed originally for auto airbags. They are sensitive, accurate, and CHEAP! Five bucks gets you an accelerometer that 10 years ago would have cost $1000.

Now try an experiment, stand at the bottom of a long and narrow set of stairs. With your eyes closed, walk up the stairs, WITHOUT touching anything. If you are careful and put in some practice you can do it. You are depending entirely on what is called inertial guidance. Military rockets often use this system because it is independent of outside support. It is also very expensive to do accurately. You notice that if you are not careful and sensitive to the signals from your ears, you tend to drift, and bump into the hand rails. That's the problem with inertial guidance, to minimize the drift is very expensive. We humans overcome this problem by using our eyes. We NAVIGATE. We constantly update our position in space and modify our attitude, to correct our course, to take us to our destination. The new technology of GPS (Global Positioning Satellites) allows rockets to update their position and correct their flight path into orbit. This is a tremendous improvement over the old system of using expensive government owned radar systems to provide flight path updates. It is also MUCH MUCH CHEAPER. The whole idea behind a navigation system is to reduce the cost and complexity of your attitude control system. Sort of like using your eyes to make your ears cheaper.

The Instrument Unit or guidance system of the Saturn V was 3 ft high, 22 ft in diameter, weighed 4,492 pounds, and cost God only knows how much. The latest military guidance systems are a cube 5 inches on a side weighing 5 pounds and costing about $15,000. With a little innovation it should be possible to reduce that cost to about $500 for a commercial system. High technology has made the guidance system of a rocket a minor problem instead of a major headache.

BIG DUMB BOOSTERS

I did a little playing around with some math and came up with a chart which better explains the concept of big dumb boosters. The big dumb booster concept can be credited to one Arthur Schnitt, a government think tank employee who originally started out studying rocket fuel tanks. He discovered that a low tech approach called fracture resistant design allowed very large common steel rocket propellant tanks to be cheaper than smaller high tech tanks made from high test aerospace alloys. When combined with a large simple pressure feed rocket engine, make it big enough and simple enough and it will cost much less than a small high tech rocket. The problem arises when the big dumb concept is applied to launch vehicle design, or more specifically when people who are used to seeing sleek NASA designed launch vehicles see the huge size of big dumb boosters. They are very BIG and very DUMB and at first seem overly large.

The first step in rehabilitating your thought process is to go back to the basic performance equations of a rocket vehicle.

CHANGE IN VELOCITY = NATURAL LOG(MASS RATIO) * SPECIFIC IMPULSE*G

MASS RATIO = TOTAL MASS / EMPTY MASS

The big dumb booster concept deals with the Mass Ratio part of the equation. For a certain Mass Ratio and Specific Impulse your rocket will gain a certain amount of velocity. Now consider what a payload does to the Mass Ratio of a rocket. (note that I will now be using propellant mass/ total mass = mass ratio.) If we have a perfect booster rocket engine with a Mass Ratio of 1.0, that is it's all propellant, and it's weight is 8, (let's keep the math simple here!) and we put a second stage and payload with a mass of 8 on top of that booster, we get an actual Mass Ratio of 8/16=0.5. This is the actual working Mass Ratio of the booster, and the point labeled 1 on graph below. The bottom scale of the graph is the Mass Ratio of the booster. The upper line is the weight of the booster, and the lower line is the weight of the propellant. For this exercise we will keep the actual working Mass Ratio of the booster at a constant value of 0.5, so that the vehicle performance ALWAYS STAYS EXACTLY THE SAME.

Note that as the booster Mass Ratio is reduced, the mass of the booster increases, becoming infinite as we approach 0.5. Unfortunately I can only estimate the cost data of the booster to plot on this graph to allow a comparison with the Mass Ratio. The cost of the perfect booster with it's Mass Ratio of 1.0 is infinite, and off scale at the right side of the chart. The region between 0.6 and 0.7 is of primary interest. The total mass has not risen too high, yet cost have dropped to an "industrial" level.

 In this region lies those vehicles which can be called BIG DUMB BOOSTERS. Their tanks are made of steel, their engines are pressure fed with no turbopumps, they are massive and simple and cheap. These are the beasts which we will be building. As we increase the Mass Ratio, the materials used to build the rocket become more high tech and more expensive. The usual progression is steel, maraging steel, aluminum alloy, aramid fiber composite, graphite fiber composite, and unobtainum.

It should be noted that this exercise applies only to the booster. Upper stages need very high mass ratios to attain orbital velocities, so not everything can be cheap.

UNOBTAINUM

Unobtainum is the perfect space engineering material. It can function as both a structural material and as a propellant. As a structural material it is far lighter than Titanium, infinitely stronger than Tungsten, and has NO MELTING POINT. Unobtainum is also total resistant to corrosion, radiation, fatigue, and it is mistake proof. It can even be prepared as fine cuisine. As propellant it is infinitely dense, requiring no large storage tanks. It's specific impulse and thrust can be specified at any level without complex engine systems, the stuff literally pulls itself into the combustion chamber and spontaneously ignites. Unobtainum has been successfully used in nearly every proposed new NASA launch vehicle or space mission design effort. It's low cost and high performance is an essential element in obtaining government contracts. Best of all, a life time supply of Unobtainum can be mailed in a simple first class postage envelope. ORDER YOURS TODAY!

CAUTION - UNOBTAINUM CAN DEGENERATE RAPIDLY AND UNEXPECTEDLY INTO COSTOVERUNIUM WHEN EXPOSED TO THE ENGINEERING STAFF.