Welcome to our tour of the Experimental Impact Laboratory here at NASA's Johnson Space Center, located in Room 1028 in the high-bay area of Building 31. We use specially fabricated guns in this lab to simulate impacts and cratering events that occur on solid planetary bodies, such as the Earth and the Moon, the other terrestrial planets, satellites of the planets, and the asteroids, as well as on spacecraft in orbit around the Earth and in interplanetary space. By studying these impacts and the processes resulting from them, we can learn about the histories and evolution of planetary bodies. We can also use this facility for other purposes, such as the evaluation of materials and methods for collecting tiny cosmic-dust particles that might have originated in the outer reaches of the solar system.
The Experimental Impact Lab (or "Impact Lab," as we typically call it) is in the space once occupied by a hypervelocity impact facility that was used to test spacesuits and other equipment used in the Apollo program. It has a very thick floor to support heavy equipment (like the vacuum chambers we use to contain the experiments) and extra-thick, reinforced walls to protect the rest of the building in the event that something goes very wrong. We follow very strict procedures designed specifically to minimize any potential for accidents, however, and so far (knock on wood!) we've never had to be thankful for this overengineering. Nevertheless, we're always alert for better ways to do specific jobs, and the first topic discussed during our weekly lab meetings is safety.
The first part of the lab we'll visit is the Gun Area. This section is the heart of the laboratory, and it's here that the three guns -- the Flat Plate Accelerator, the Vertical Impact Facility, and the Light Gas Gun -- are located. All three of these guns work essentially the same way: a driving gas is introduced very quickly into the system at high pressure, pushing the projectile down the barrel and toward the target. The general sequence goes as follows. First, we place the target in the impact chamber and the projectile in the gun, and then we load gunpowder into a shell or other receptacle and place it in the breech. The amount of gunpowder that we use depends on the mass of the projectile and how fast we want it to travel. We've calibrated the quantity of gunpowder needed by each gun to launch a projectile at a given velocity by trial and error, shooting projectiles of different compositions and masses, varying the amount of gunpowder until we understood the relationship between powder load and projectile velocity. (A term more scientific-sounding than "trial and error" is usually used, so, if you'd prefer, we can say that the quantity of gunpowder was calibrated empirically. Whatever.) Next, we remove the air from the system with vacuum pumps. This is done for several reasons: (1) any air left in the system would slow down the projectile as it traveled along its trajectory; (2) air left in the system would interact with the gases from the gunpowder, possibly overpressurizing the system with nasty consequences, like blown-out windows; and (3) most of the impacts that we study happen in a vacuum, and air pressure can alter things ranging from the final sizes of craters to the way that material is ejected from a growing crater. We can and have performed experiments with air remaining in the chamber, but such experiments are the exception rather than the rule in our lab. As a safety feature, we've installed pressure-interlock switches on each gun, which prevent the gun from firing unless the pressure in the impact chamber is below a certain value. That way, if a door or window weren't properly sealed, debris from the shot wouldn't be sprayed into the laboratory. Once the system has been evacuated (that is, the air has been pumped out), we ignite the gunpowder, creating high-pressure gases which, in turn, accelerate the projectile toward the target.
Perhaps the two most important quantities that characterize each shot are the projectile's mass and its velocity. We know the mass of each projectile, because we weigh each one before we shoot it. Measuring the velocity is a much more challenging process, and each gun in the lab uses the same basic technique. Light from lasers, spaced at precisely determined intervals, passes across the path that the projectile should take in its quick trip to the target. Each laser shines onto an array of photodetectors, each of which emits a voltage that's proportional to the amount of light hitting it. The more intense the beam, the greater the signal coming out of the photodetector. As it travels downrange, the projectile intercepts the light generated by the lasers, casting a shadow onto the photocells. As the laser light is "eclipsed" by the projectile, the strength of the photocell's signal drops. This change is detected by the logic circuit in each detector assembly, which then sends a signal to high-speed timers. The first laser occulted starts one of these timers in the Electronics Room. The next laser intercepted by the projectile stops that timer and starts another. This continues as many times as there are lasers to occult. (Each gun has at least three.) This way, the time t it takes the projectile to travel between any two lasers can be determined by checking the relevant readout on the timer panel, which we'll visit later in the tour. Since the distance d between any two lasers is known very accurately, we can calculate the velocity v of the projectile (using the equation v=d/t). After the projectile impacts the target, we remove the target from the impact chamber and analyze it using one of many instruments, including a scanning electron microscope, an electron microprobe, a petrographic microscope, regular photography, and/or -- and this is the most important, discriminating instrument of all -- the legendary Mark I Human Eyeball.
While the basic operation all three guns is essentially the same, certain aspects of their configurations make them very different from one another, with one gun more suitable for certain applications than for others. The table below summarizes some of the characteristics of these guns. Diagrams and more detailed information may be viewed by activating the link to the appropriate gun.
The guns' barrels are classified as either "rifled" or "smoothbore." Each type has its own advantages and disadvantages. The rifled barrels are characterized by a spiral pattern of lands (raised ridges) and grooves (the depressed areas between the lands) that start at the breech end of the barrel and continue to the muzzle. The lands actually "grab" the projectile as it travels down the barrel, causing it to spin. In a normal rifle or artillery piece, this spin stabilizes the projectile, permitting accuracy well beyond that of the smoothbore guns and rifles that preceded them. The rifled barrels of our guns also stabilize the projectile, but, perhaps more importantly, the centrifugal force resulting from this rapid spin forces the sabot to fly apart more rapidly after it exits the barrel. This is particularly important in the case of the Vertical Impact Facility, which has little distance between the end of the barrel and the target; quick sabot separation is a necessity to keep the target free of interfering debris. While rifled barrels are useful in imparting stability to the projectile and causing the sabot to separate efficiently, they also impress grooves in the projectile, which can permit the gases behind the projectile to "leak" around the sabot, causing all sorts of complications. Smoothbores don't have this problem, since they are, surprisingly enough, smooth. When the fit of the projectile inside the barrel is tight, it forms a good seal and such "blowby" becomes extremely unlikely. The projectile doesn't spin in a smoothbore gun, however, so a much greater distance must be provided for the sabot to separate from the projectile sufficiently. In cases where the gun doesn't use a sabot, this isn't a consideration.
From the Gun Area, we'll move into the Machine Shop part of the laboratory. If you've worked in a machine shop, you'd feel at home here. We use standard machine tools, including lathes of different sizes, grinders, a band saw, a table saw, a milling machine, and smaller power and hand tools. Bill and Gerry use this equipment to fabricate holders for targets and special parts for the guns and their peripheral equipment. Impacted targets from the Flat-Plate Accelerator are also extracted from their holders using this same equipment. Scientific experimentation like the kind that we do in the lab often requires some strange equipment, such as specialized target holders, weird-looking sabots, and other apparatus that would make Dr. Frankenstein himself wonder. If we need a large number of copies of a single part, we get a dedicated machine shop to make them. When we have to have a custom-fitted part or a special setup, though, we make it in our lab.
The Target Preparation and Analysis Room is located adjacent to the Machine Shop Area and has that name mainly because targets are prepared here for their moments of glory at the business end of the guns. They're also sieved, disassembled, or otherwise subjected to post-experiment treatment in this room. Actually, we use the Sieving Cage, as we usually call it, for other important things, such as weighing powder loads, cutting the samples that are used as targets in the Flat-Plate Accelerator, and using any chemicals that require a vent hood to remove vapors from the area.
You can probably guess the main purpose of the Meeting Room -- we certainly try to be descriptive in our naming system. It's also used as a kitchen for the times the guys eat their lunch or snacks in the lab, and it's a reasonable place for the coffee pot and microwave.
Next comes the Electronics Room. The component of this area related directly to the guns is the panel for the timers that are used in conjunction with the lasers in the velocity-measurement systems. This room also contains computer workstations for recording data and performing various calculations related to the experiments conducted in the lab.
The last stop on our tour is actually located outside the laboratory behind a reinforced concrete wall: the Firing Control Panel for the three guns. Each gun has its own set of firing controls, readouts for the atmospheric pressure inside its impact chamber, and controls for high-speed cameras. When we fire a gun, all personnel must evacuate the laboratory and remain outside for the duration of the shot and, in the case of the light-gas gun, the time it takes the gunner to verify that all of the gun's systems have been safed after a shot. This procedure is designed to protect us from any accident that might result from a misfiring.