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THE BULLET PROJECT'S SILVER BULLET RV-1 ROCKET LAND SPEED CAR
By Franklin Ratliff (Team Member, Bullet Project)
INTRODUCTION
Our goal with the Bullet Project is to design and build the Silver Bullet RV-1, a 30 foot four-wheel steerable liquid oxygen and kerosene rocket propelled land vehicle with a top speed in excess of 1,000 mph. The RV-1 will incorporate the latest knowledge in vehicle aerodynamics and chassis design, as well as break new ground in wheel design and driver safety. New ground being broken by the Bullet Project team includes a wheel with an internal suspension system where only the outer rim rotates, and a fireproof blast-resistant ejectable driver capsule that maintains the driver in a breathable atmosphere.
The Bullet Project, in comparison to Australian, British, and American teams which all have vehicles now in the build or testing phase 44 feet in length or longer and weighing six tons or more, is taking a radically different approach to the problem of building a supersonic car. The target dry weight of RV-1 is three tons with a rocket propellant capacity of approximately two tons. Bloodhound SSC, a British project lead by Richard Noble, is a combination jet and rocket vehicle 44 feet long with a fully fueled weight of approximately seven tons. Aussie Invader 5R, a pure rocket vehicle under construction by Australian land speed record holder Rosco McGlashan, is 50 feet long with a dry weight in excess of seven tons. The North American Eagle, a jet vehicle converted from an F-104 Starfighter, is 56 feet long and weighs 6.5 tons. The other American project, the Sonic Wind LSRV rocket car now under construction by Waldo Stakes, is 47 feet long with a dry weight of 3.5 tons.
Originally, we also contemplated a design for a vehicle 60 feet long. However, by next examining various iterations of much lighter vehicles, beginning with the smallest one possible then working up until we felt we arrived at the minimum size that would still hold enough rocket propellant for 1,000 mph, we were able to arrive at our current design.
POWER PLANT
Rocket engines were selected due to their inherent compactness and light weight as well as eliminating the need for a large and destabilizing air inlet duct. The drawback to rocket engines is the large mass of fuel and oxidizer they require, and the potential for explosion. The amount of rocket propellant needed for RV-1 is minimized by keeping the size and weight of the vehicle as small as possible, while still storing enough energy onboard to achieve the target speed. RV-1 will have a capacity for approximately 4,200 lbs combined weight of liquid oxygen (LOX) and kerosene. This will be sufficient to support a continuous thrust of 35,000 lbs for a minimum of 30 seconds or 50,000 lbs for a minimum of 17 seconds. The thrust will be variable so that it can be tailored to match the planned trajectory for each run. Trajectories will be plotted based on drag curves calculated through detailed computational fluid dynamics (CFD) analysis. To avoid the mechanical complexity of a pump-fed system, a blowdown system will be used with nitrogen pressurizing the LOX and kerosene tanks. The explosion hazard is minimized by placement of the LOX and kerosene tanks in separate compartments, divided by bulkheads and further separated with the cockpit and the nitrogen tanks.
CHASSIS DESIGN
A conventional four-wheel layout was chosen to avoid stability problems that have been demonstrated in the dynamics of ultranarrow front track/ultrawide rear track configurations, such as the Budweiser rocket car (1979) and Craig Breedlove's third jet car (1996). The primary structural element for the RV-1 chassis will be a large diameter tube that houses the main LOX tank, the nitrogen tanks, the cockpit, the kerosene tank, and the rocket motors. The tube will also provide the attachment points for the forward LOX tanks, the wheels, the braking ballutes, and the body shell. It is currently planned for the tube to be of carbon fiber composite construction, although consideration is also being given to fabricating it using rolled and welded magnesium. Magnesium has the same density as carbon fiber, but is not as stiff and so would have to be thicker, as well as protected against corrosion from salt. The cockpit will be a sealed and self-contained capsule that drops in a compartment located near the mid-point of the vehicle.
DRIVER SAFETY
Once it was determined that the cockpit would be a sealed fireproof and blast-resistant capsule, inspired by the example of the Scott Crossfield incident in the X-15 rocket plane, consideration was then given to making the capsule ejectable. After studying examples from both aviation and 250 mph drag boat racing of ejectable or separable capsules, it was decided to pattern the capsule separation for RV-1 largely after the drag boat model, in which the capsule separates only after a crash has already initiated. However, due to the much higher speeds and different motions expected in comparison to a drag boat if RV-1 were to crash, it was decided to incorporate both a mechanism for actively ejecting the capsule and an aerodynamic decelerator. Banks of compressed air rams on each side of the capsule will serve as the ejection mechanism, with tilt switches mounted in the horizontal and vertical planes acting to initiate ejection without driver intervention. In the ejection sequence currently envisioned, if the RV-1 pitches or rolls beyond preset limits, a tilt switch will open a solenoid valve that releases compressed nitrogen into the banks of rams. As the capsule is launched outward, the tension from a steel cable lanyard attached between the capsule and the chassis will pull a mechanism in the capsule that releases speed brakes and telescoping booms to slow and stabilize the capsule.
WHEELS AND SUSPENSION
At 1,000 mph, a 30 inch diameter wheel will be turning over 10,000 rpm, generating a centrifugal force of 50,000g at the rim. When it was found that even with a hollow all carbon fiber wheel, based on a rim sandwiched between two disks, the lightest that could be expected for a 30 inch diameter 8 inch wide wheel was still 230 lbs, the decision was made to explore reducing rotating mass by going to a design where only the outer rim rotates. This had the ripple effect of making an internal suspension system practical, which in turn freed up the volume that would otherwise be occupied by a conventional suspension system. To generate a stable and consistent steering response, tireless wheels must plane on the surface of the track instead of plowing ruts. The wheel width of 8 inches for RV-1 was determined by reviewing the experience of other teams to run vehicles with tireless wheels on a salt flat. Based on the experiences of these other teams, it was felt 8 inches should be wide enough to support a five tons gross weight vehicle running on dry salt without cutting ruts. The wheel rim will rotate on air bearings placed around the perimeter of a bearing block that also houses the slide which serves as the suspension system. Rubber cushions on the top and bottom of the slide permit an up and down movement of +/- 1 inch. The sides of the slide will serve as adjustable friction dampeners to control bounce.
AERODYNAMICS
To assure yaw stability, the center of pressure must be behind the center of gravity. The RV-1 design accomplishes this through two large tail fins and moving the center of gravity as far forward as possible through placement of the heaviest components in the front half of the vehicle. To counteract any lift that might be generated as shockwaves form at transonic and supersonic speeds, various methods of generating downforce are being incorporated, principally the shape of the nose and the use of canard wings. However, the use of ground effects tunnels on the underside of the nose is also being considered.
The channel formed between the tail fins is intended to augment vehicle downforce at speeds in excess of Mach 1 by taking advantage of a phenomenon called compression lift. To generate compression lift, air is forced outwards by a center body, in this case the tunnel on top of the vehicle housing the rocket propellant feed lines, against two vertical aerodynamic surfaces.
Downforce is further enhanced through the two canard wings located beside the cockpit. The canard wings for the RV-1 are based on studies of disk wings. The shape permits a large amount of surface area while maintaining a short span. So that the wing angle of attack can be adjusted while the vehicle is in motion, the wings pivot from the front moved hydraulic actuators attached at the rear. This combination of a short span and two-point attachment provides a very rigid system which reduces the potential for flutter, a destructive aerodynamic phenomenon that results in rapid uncontrolled motion which can lead to structural failure.
CONCLUSION
Setting the World Land Speed Record requires two runs in opposite directions within one hour through a mile speed trap located in the middle of the course. There is no minimum course length. We believe that minimizing the size of the vehicle also minimizes other problems, such as building the vehicle, transport of the vehicle to the Lake Gairdner salt flats, and the length of the course needed. A smaller vehicle can accelerate quicker and stop in a shorter distance. Minimizing the stopping distance maximizes the margin for error, and means we don't have to search all over the world for a track that allows a twelve or fourteen mile course because we can do it in nine miles on Lake Gairdner.
By Franklin Ratliff (Team Member, Bullet Project)
INTRODUCTION
Our goal with the Bullet Project is to design and build the Silver Bullet RV-1, a 30 foot four-wheel steerable liquid oxygen and kerosene rocket propelled land vehicle with a top speed in excess of 1,000 mph. The RV-1 will incorporate the latest knowledge in vehicle aerodynamics and chassis design, as well as break new ground in wheel design and driver safety. New ground being broken by the Bullet Project team includes a wheel with an internal suspension system where only the outer rim rotates, and a fireproof blast-resistant ejectable driver capsule that maintains the driver in a breathable atmosphere.
The Bullet Project, in comparison to Australian, British, and American teams which all have vehicles now in the build or testing phase 44 feet in length or longer and weighing six tons or more, is taking a radically different approach to the problem of building a supersonic car. The target dry weight of RV-1 is three tons with a rocket propellant capacity of approximately two tons. Bloodhound SSC, a British project lead by Richard Noble, is a combination jet and rocket vehicle 44 feet long with a fully fueled weight of approximately seven tons. Aussie Invader 5R, a pure rocket vehicle under construction by Australian land speed record holder Rosco McGlashan, is 50 feet long with a dry weight in excess of seven tons. The North American Eagle, a jet vehicle converted from an F-104 Starfighter, is 56 feet long and weighs 6.5 tons. The other American project, the Sonic Wind LSRV rocket car now under construction by Waldo Stakes, is 47 feet long with a dry weight of 3.5 tons.
Originally, we also contemplated a design for a vehicle 60 feet long. However, by next examining various iterations of much lighter vehicles, beginning with the smallest one possible then working up until we felt we arrived at the minimum size that would still hold enough rocket propellant for 1,000 mph, we were able to arrive at our current design.
POWER PLANT
Rocket engines were selected due to their inherent compactness and light weight as well as eliminating the need for a large and destabilizing air inlet duct. The drawback to rocket engines is the large mass of fuel and oxidizer they require, and the potential for explosion. The amount of rocket propellant needed for RV-1 is minimized by keeping the size and weight of the vehicle as small as possible, while still storing enough energy onboard to achieve the target speed. RV-1 will have a capacity for approximately 4,200 lbs combined weight of liquid oxygen (LOX) and kerosene. This will be sufficient to support a continuous thrust of 35,000 lbs for a minimum of 30 seconds or 50,000 lbs for a minimum of 17 seconds. The thrust will be variable so that it can be tailored to match the planned trajectory for each run. Trajectories will be plotted based on drag curves calculated through detailed computational fluid dynamics (CFD) analysis. To avoid the mechanical complexity of a pump-fed system, a blowdown system will be used with nitrogen pressurizing the LOX and kerosene tanks. The explosion hazard is minimized by placement of the LOX and kerosene tanks in separate compartments, divided by bulkheads and further separated with the cockpit and the nitrogen tanks.
CHASSIS DESIGN
A conventional four-wheel layout was chosen to avoid stability problems that have been demonstrated in the dynamics of ultranarrow front track/ultrawide rear track configurations, such as the Budweiser rocket car (1979) and Craig Breedlove's third jet car (1996). The primary structural element for the RV-1 chassis will be a large diameter tube that houses the main LOX tank, the nitrogen tanks, the cockpit, the kerosene tank, and the rocket motors. The tube will also provide the attachment points for the forward LOX tanks, the wheels, the braking ballutes, and the body shell. It is currently planned for the tube to be of carbon fiber composite construction, although consideration is also being given to fabricating it using rolled and welded magnesium. Magnesium has the same density as carbon fiber, but is not as stiff and so would have to be thicker, as well as protected against corrosion from salt. The cockpit will be a sealed and self-contained capsule that drops in a compartment located near the mid-point of the vehicle.
DRIVER SAFETY
Once it was determined that the cockpit would be a sealed fireproof and blast-resistant capsule, inspired by the example of the Scott Crossfield incident in the X-15 rocket plane, consideration was then given to making the capsule ejectable. After studying examples from both aviation and 250 mph drag boat racing of ejectable or separable capsules, it was decided to pattern the capsule separation for RV-1 largely after the drag boat model, in which the capsule separates only after a crash has already initiated. However, due to the much higher speeds and different motions expected in comparison to a drag boat if RV-1 were to crash, it was decided to incorporate both a mechanism for actively ejecting the capsule and an aerodynamic decelerator. Banks of compressed air rams on each side of the capsule will serve as the ejection mechanism, with tilt switches mounted in the horizontal and vertical planes acting to initiate ejection without driver intervention. In the ejection sequence currently envisioned, if the RV-1 pitches or rolls beyond preset limits, a tilt switch will open a solenoid valve that releases compressed nitrogen into the banks of rams. As the capsule is launched outward, the tension from a steel cable lanyard attached between the capsule and the chassis will pull a mechanism in the capsule that releases speed brakes and telescoping booms to slow and stabilize the capsule.
WHEELS AND SUSPENSION
At 1,000 mph, a 30 inch diameter wheel will be turning over 10,000 rpm, generating a centrifugal force of 50,000g at the rim. When it was found that even with a hollow all carbon fiber wheel, based on a rim sandwiched between two disks, the lightest that could be expected for a 30 inch diameter 8 inch wide wheel was still 230 lbs, the decision was made to explore reducing rotating mass by going to a design where only the outer rim rotates. This had the ripple effect of making an internal suspension system practical, which in turn freed up the volume that would otherwise be occupied by a conventional suspension system. To generate a stable and consistent steering response, tireless wheels must plane on the surface of the track instead of plowing ruts. The wheel width of 8 inches for RV-1 was determined by reviewing the experience of other teams to run vehicles with tireless wheels on a salt flat. Based on the experiences of these other teams, it was felt 8 inches should be wide enough to support a five tons gross weight vehicle running on dry salt without cutting ruts. The wheel rim will rotate on air bearings placed around the perimeter of a bearing block that also houses the slide which serves as the suspension system. Rubber cushions on the top and bottom of the slide permit an up and down movement of +/- 1 inch. The sides of the slide will serve as adjustable friction dampeners to control bounce.
AERODYNAMICS
To assure yaw stability, the center of pressure must be behind the center of gravity. The RV-1 design accomplishes this through two large tail fins and moving the center of gravity as far forward as possible through placement of the heaviest components in the front half of the vehicle. To counteract any lift that might be generated as shockwaves form at transonic and supersonic speeds, various methods of generating downforce are being incorporated, principally the shape of the nose and the use of canard wings. However, the use of ground effects tunnels on the underside of the nose is also being considered.
The channel formed between the tail fins is intended to augment vehicle downforce at speeds in excess of Mach 1 by taking advantage of a phenomenon called compression lift. To generate compression lift, air is forced outwards by a center body, in this case the tunnel on top of the vehicle housing the rocket propellant feed lines, against two vertical aerodynamic surfaces.
Downforce is further enhanced through the two canard wings located beside the cockpit. The canard wings for the RV-1 are based on studies of disk wings. The shape permits a large amount of surface area while maintaining a short span. So that the wing angle of attack can be adjusted while the vehicle is in motion, the wings pivot from the front moved hydraulic actuators attached at the rear. This combination of a short span and two-point attachment provides a very rigid system which reduces the potential for flutter, a destructive aerodynamic phenomenon that results in rapid uncontrolled motion which can lead to structural failure.
CONCLUSION
Setting the World Land Speed Record requires two runs in opposite directions within one hour through a mile speed trap located in the middle of the course. There is no minimum course length. We believe that minimizing the size of the vehicle also minimizes other problems, such as building the vehicle, transport of the vehicle to the Lake Gairdner salt flats, and the length of the course needed. A smaller vehicle can accelerate quicker and stop in a shorter distance. Minimizing the stopping distance maximizes the margin for error, and means we don't have to search all over the world for a track that allows a twelve or fourteen mile course because we can do it in nine miles on Lake Gairdner.
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