Gravity Air Launching of Earth-To-Orbit Space Vehicles

Transcription

1 Gravity Air Launching of Earth-To-Orbit Space Vehicles Marti Sarigul-Klijn, Ph.D. *, Nesrin Sarigul-Klijn, Ph.D., Gary C. Hudson and Livingston Holder AirLaunch LLC, Kirkland, Washington, Gregg Liesman **, Major, USAF and Dale Shell Space and Missile Systems Center, Detachment 12, Kirtland AFB, and Chris Webber Air Force Flight Test Center, Edwards AFB, CA, AirLaunch LLC is developing small launch vehicle called QuickReach that is carried by and launched from an existing military cargo aircraft for the DARPA and USAF Falcon program. One purpose of the QuickReach development program was to demonstrate AirLaunch s Gravity Air Launch (GAL) method. For this demonstration, a single inert Drop Test Article (DTA) was successfully dropped from a C-17 aircraft onto the Edwards flight test center range. The DTA was a water filled steel tank with the same outer mold line, stiffness, and mass properties as the actual launch vehicle and was the longest object ever dropped from the C-17 aircraft. In addition, this paper describes AirLaunch s future development program for GAL. * Chief Engineer for Airdrop. AIAA Member. Professor, University of California at Davis, nsarigulklijn@ucdavis.edu (530) AIAA Associate Fellow. CEO & Program Manager, 5555 Lakeview Drive, Suite 201. AIAA Senior Member. Chief Program Executive, 5555 Lakeview Drive, Suite 201. AIAA Member. ** Chief, Space Vehicles Test and Evaluation Division. Deputy Chief, Space Vehicle Test and Evaluation Division. Mission Systems Airdrop Engineer, 418 th Flight Test Squadron. Sarigul-Klijn, Sarigul-Klijn, Hudson, Holder et al 1 AIAA Space 2006 Copyright 2006 AirLaunch LLC. Published by AIAA with permission. Approved for Public Release, Distribution Unlimited.

2 I. Introduction In June 2003, the Defense Advanced Research Projects Agency (DARPA) held an open competition and selected 9 companies for six month Phase 1 studies for a Small Launch Vehicle (SLV)1. The Falcon SLV program objectives are to develop and demonstrate technologies that will allow the country to execute time-critical small satellite launch missions. In September 2004, DARPA held another open competition and selected 4 companies for further Phase 2A studies and demonstrations. In September 2005, DARPA selected AirLaunch LLC to continue with further Phase 2B studies and demonstrations. AirLaunch LLC has proposed a SLV called the QuickReach that is carried by and launched from an existing military cargo aircraft. AirLaunch has also proposed a new launch method called Gravity Air Launch (GAL) that greatly improves simplicity, safety, and reliability of air launching from an unmodified cargo aircraft as compared to existing methods that rely on standard heavy equipment airdrop procedures and equipment. Unlike the standard heavy equipment airdrop method, GAL imparts much of the launch carrier aircraft's altitude and airspeed onto the rocket, which in turn improves payload mass to orbit. This paper describes the Phase 2A studies and demonstrations that were completed to demonstrate the feasibility of the GAL launch method. It also describes the studies and demonstrations that will be completed during Phase 2B. II. Overview of QuickReach TM The QuickReach will be an air-launched, two-stage liquid fueled rocket that will be capable of placing over 1,000 pounds into low earth orbit (LEO). The carrier aircraft will be flown to a drop box, an approximately 1 x 1 mile area over the open ocean for an operational mission. Several minutes prior to launch the aircraft deck angle is established at approximately 6 degrees nose up, cabin pressure is equalized with local atmospheric pressure, and the aft cargo door is opened. Fifteen seconds before launch, a small drogue parachute attached to the first stage nozzle is deployed. Upon launch command, the rocket is released and gravity pulls the launch vehicle out of the aircraft, assisted by the small stabilizing drogue chute. The launch vehicle pitches up after leaving the cargo bay due to cresting the end of the cargo ramp. The drogue chute damps this pitch rate and after about 3 seconds the launch vehicle s pitch attitude is 70 to 80 degrees above the horizon. At this point, the first stage is ignited and the parachute is released by the simple mechanism of having its risers burned off. The Stage One engine is ignited when the launch vehicle is over 200 feet from the aircraft, with the launch vehicle descending at 100 feet per second (fps) down and traveling 50 fps aft relative to the aircraft. Because of the relatively low thrust to weight of its Stage One engine (compared to airto-air missiles), the launch vehicle needs another 500 feet of altitude to arrest its descent. The rocket flies to a vertical heading and continues in this attitude until it re-crosses the launch altitude more than 1,000 ft behind the launch aircraft 15 seconds after extraction. At this point, the rocket s angle-of-attack decreases as the launch vehicle accelerates into a standard gravity turn for a trajectory into LEO. T = 0 seconds T - 3 seconds 750 ft 1300 ft T + 6 seconds T + 12 seconds Figure 1: QuickReach Launch Sequence Air launch should minimize range costs and maximize operational flexibility by allowing deployment from any 4,000 foot runway without need for fixed installations. Air launching simplifies operations compared to ground launch from a fixed range in several ways: required coordination is minimized with other users of the range, weather constraints can be avoided by flying to open sky, and there will be fewer delays waiting for specific launch windows (to match desired orbits) because the launch vehicle can be flown to an alternate launch point that is better aligned with the desired orbit. In addition, ground launches are often postponed when ships enter the ocean zones near the coastal launch sites or where rocket stages are expected to drop. The QuickReach carrier aircraft can avoid such delays by flying to a different release point. Public safety is greatly enhanced since the launch takes place over the open ocean, far away from any populated areas. Sarigul-Klijn, Sarigul-Klijn, Hudson, Holder et al 2 AIAA Space 2006

3 Launching from altitude provides a unique opportunity to operate a simple, low-cost rocket system while maintaining the high performance characteristics of most two stage launch vehicles. Air launching takes the advantage of the low outside atmospheric pressure at altitude (30,000 feet or greater) which allows a pressure-fed launch vehicle to use high area ratio nozzles while operating at relatively low engine chamber pressures. This approach provides weight and specific impulse (Isp) performance that is competitive with high-pressure turbopumpfed systems without the associated safety, cost, or complexity issues. Launch vehicle tank pressures do not have to exceed 200 pounds per square inch (psi) and the engines can run at a maximum chamber pressure of 150 pounds per square inch (psi). This compares to a typical ground launch vehicle that has chamber pressures from 700 psi (Delta II) to 3700 psi (Atlas V). Altitude launch also allows the use of vapor pressure (VAPAK) propellant feed. VAPAK is based on using the internal energy of a liquid stored in a closed container to provide the pressure and to perform the work required to expel the liquid from the container. This method of propellant feed was successfully used in the X-Prize winner, SpaceShipOne. VAPAK eliminates costly components such turbopumps and gas generators used in a typical pumpfed launch vehicle. It also eliminates the high-pressure gas storage vessels and pressure regulators or heated gas systems (Tridyne) normally associated with pressure-fed rockets. Launching from altitude reduces the size of the launch vehicle required to deliver a given size payload to orbit. Air launching provides a modest performance gain of about 1,100 to 1,800 feet per second of the velocity (delta V) required to achieve orbit 2. The QuickReach launch system is designed for use with a C-17A military transport aircraft. It is also compatible with and therefore may also be employed with other transports with comparable or greater capacity such as the An- 124 or C-5A. Satellite launch missions will be conducted with a single QuickReach vehicle loaded in the aircraft along with range support equipment. For an actual launch mission, the design goal is to be able to load QuickReach TM onto a C-17A in 20 minutes, and then fly to a nominal launch point ~200 or more miles off the coast. The aircraft will climb to ~30K 35K feet altitude where it cruises to the desired drop point. The C-17A can loiter before launching for several hours. If a dangerous situation arises on-board, emergency extraction may be triggered by the loadmaster anytime during flight. During an emergency extraction all rocket components and propellants would be Figure 2: Drop Test Article (DTA) extracted in less than 30 seconds. III. Description of Gravity Air Launch (GAL) Hardware The GAL hardware consists of three main components: a Storage and Launch Carrier (SLC), a Chain Release System (CRS), and a transporter. For both the Phase 2A and Phase 2B tests, a Drop Test Article (DTA) is used to simulate the operational rocket. The DTAs have the same aerodynamic properties and can be ballasted to have the same mass properties as the operational rocket. They are intended to be dropped from the C-17A to demonstrate the feasibility of GAL. The DTA is 790 inches long (65.83 ft), 87 inches in diameter, and weighs 22,000 pounds empty. For the first Phase 2A drop the DTA was ballasted to 50,000 lbs. During Phase 2B airdrops will be demonstrated with DTA weights of up to 72,000 lbs, the same as the operational rocket. The DTA has a steel body and a fiberglass and foam nose. Figure 3: Storage and Launch Carrier (SLC) Sarigul-Klijn, Sarigul-Klijn, Hudson, Holder et al 3 AIAA Space 2006

4 The DTA is equipped with a Data Acquisition System (DAS) and a telemetry (TM) system to transmit positional pitch, yaw, and roll attitude, rate, and acceleration data to both the C-17A aircraft and to a chase aircraft. A ship and shoot Storage and Launch Carrier (SLC) concept is used for QuickReach. For the operational rocket, the SLC will be mated to the rocket at the factory and the SLC and QuickReach will form a self-contained system with a long shelf life, minimal field assembly, and minimal inspection before use. The SLC consists of two columns of tires, on which the rocket rolls. The tires are readily available 17.5 inch diameter business aviation tires. Tire pressure is at 135 psi, less than the launch vehicle s tank internal pressures, thus avoiding concentrated loads, and the tires are designed for continuous operation at the atmospheric pressures and temperatures at 45,000 feet altitude. 84 tires and wheel assemblies are supported by I-beam structures that uniformly distribute the loads to the C-17A cargo floor. The final three rows of wheels are doubled and have 12 tires to distribute the load as the vehicle crests the end of the ramp. The empty weight of the SLC is approximately 8,200 lbs. The SLC is compatible with both the 88 inch wide C-17A logistic rails and, with the installation of special adapter rails, the 108 inch wide C-17A centerline Aerial Delivery System (ADS) rails. Each adapter rail weighs less than 40 lbs and attach in less than 10 seconds without the use of any tools. The SLC is compatible with the C-5A and An-124, by removing the forward 4 rows of wheels and tires of the ramp section of the SLC to fit on their shorter ramps. The SLC is self contained and palletized so that existing transport aircraft can be used without any modification. Since no modification to the aircraft is required, the aircraft can be reconfigured between launch support and standard cargo missions within a few hours. A dedicated aircraft is not required. The SLC concept also eliminates the need for expendable support cradles. The DTA is held in position on the SLC with a Chain Release System (CRS). The CRS in conjunction with the SLC restrains the rocket in accordance with Mil-Handbook 1791, Designing for Internal Aerial Delivery in Fixed Aircraft, to 4.5 g down, 2.0 g up, 1.5 g laterally, and 3.0 g fore and aft. There are no explosive cut-a-ways or pyrotechnic cutters inside either the SLC or CRS. Instead pneumatic pressure is used to release the CRS. The CRS can be released either automatically via the C-17A mission computer or manually by the loadmaster. As its name implies the CRS is tied down to the C-17A cargo deck with standard C-17 25,000 lb load chains. A total of 12 chains are used, 6 on each side. Crushable cardboard on the floor prevent damage of aircraft floor or the CRS when it is released. The CRS was tested under flight loads using special ground test simulator and it completed over 90 consecutive tests without failure. The SLC is stored on top of a custom Transporter. The Transporter width is approximately 102 inches and is 53 feet long, which means it can be loaded onboard a C-17A with a SLC, i.e., the Transporter loads the unfueled rocket and SLC on one side of the aircraft, and then the Transporter is loaded on the other side. This feature permits carriage of a Transporter, SLC, and SLV as a unit. A single C-17A can forward deploy an entire QuickReach launch system. The Transporter also meets highway requirements, i.e., taillights and remote brakes, so that rockets and SLCs can be moved to and from the factory or depot by truck, ship, or aircraft. The transporter with an empty rocket can be moved over the highway without needing a permit in all 50 states. Figure 4: Chain Release System (CRS) Sarigul-Klijn, Sarigul-Klijn, Hudson, Holder et al 4 AIAA Space 2006

5 The transporter is equipped with two separate lifting systems capable of lifting the transporter and a fully fueled rocket to the C-17A deck height of 64 inches in less than 1 minute. In the event of a required offload from the C-17A, one electric winch is located at the very front and center of the trailer deck and is capable of towing the SLC onto the trailer from the C-17A. The floor of the transporter has two columns of rollers installed to allow rolling the SLC and launch vehicle directly from the transporter to the C-17A. All transporter controls are located near the rear end of the transporter. Figure 5: DTA on Transporter For environmental protection, a roller top cover is installed and can convert the transporter in less than 5 minutes from covered or uncovered. The roller top can be operated by one person. The transporter can be disassembled quickly so as to allow carriage of the transporter alongside the SLC onboard the C-17. The Phase 2A testing was conducted on a C-17A production aircraft. For this test, the C-17A was equipped with a digital data system to gather flight condition data from several of the aircraft MIL-STD-1553 data busses and an internal DTA INS with triple redundant recording methods of DTA dynamics. Six video cameras were added in the cargo bay of the aircraft, along with airborne chase video and multiple ground video. Also, the cargo ramp s aft most roller conveyors were replaced by a total of 32 load cells to measure the forces imparted on the ramp floor by the DTA drop, and a cargo ramp accelerometer was added to measure accelerations. IV. Scope of Tests The first GAL test from the C-17A was conducted on 29 September, It was limited to a 50,000 pound DTA dropped from approximately 8,800 feet above mean sea level (MSL) at 145 knots calibrated airspeed (KCAS). The conditions for the initial test were chosen to demonstrate the feasibility of GAL without requiring the crew to be on oxygen, to minimize the extent of structural analysis on the C-17A cargo ramp, and to stay within the airspeed limits of current drogue parachutes. The next GAL test, planned for the summer of 2006, will be a 65,000 pound DTA dropped from 29,500 feet MSL at approximately 202 KCAS (Mach 0.56). This test will then be followed by up to three 72,000 pound DTAs (same weight as the operational rocket) dropped from approximately 31,600 feet, also at approximately 202 KCAS. V. Results and Discussion A. Extraction Simulation. The extraction and orientation of the QuickReach SLV was studied using analytic studies, dynamic simulation, and model testing. These studies revealed that the QuickReach rocket without its orientation parachute and without fins was unstable. As the rocket exits the aircraft, it picks up a nose up pitch rate of about 30 degrees per second (5 rpm) due to cresting the aircraft s ramp edge. The force from a standard 15 ft drogue chute reefed to about 6 ft diameter was found to be sufficient to arrest this pitch up rate in approximately three seconds. The launch vehicle pitch 580 fps at 29,500 ft 550 fps 536 fps 1 second interval between images Chute force = 9,400 lbf for both 65 and 72K lb DTA 518 fps 360 ft from ramp edge to nose tip at chute release 100 ft 498 fps Chute Release Figure 6: Example of Extraction Simulation (2 nd DTA drop shown) Sarigul-Klijn, Sarigul-Klijn, Hudson, Holder et al 5 AIAA Space 2006

6 up maneuver does not change very much with expected variations in aircraft launch conditions, parachute drag, or with unsteady aerodynamics. The desired launch vehicle attitude at engine start is with the nose pointed to the vertical. However, like a submarine-launched ballistic missile (SLBM), nose attitude at engine start can be 20 to 25 degrees from the vertical in either pitch or yaw, and should not present a problem to the rocket s 1st stage thrust vectoring control (TVC) system. A Y-bridle attachment with the parachute risers attached to the nozzle exit at the 4:30 and 7:30 o clock positions was found to provide the best combination of pitch and yaw stability. The load from the parachute is small (about 6,000 lbf for the Phase 2A drop at 8,800 feet) when compared to the engine thrust (151,500 lbf). Figure 7: Computational Fluid Dynamics B. Aerodynamic Analysis. An aerodynamic analysis using Air Force Missile DATCOM and computational fluid dynamics (CFD) was completed to evaluate the aerodynamic characteristics during extraction. A Reynolds- Averaged Navier-Stokes (RANS) flow solver was used to determine the details of the flow field around the QuickReach configuration and to extract realistic flight loads throughout its trajectory. Predicted CFD results were close to the DATCOM results. The data from the aerodynamic analysis was used in the extraction simulation discussed above. C. Storage and Launch Carrier (SLC) Analysis and Tests. The SLC structural design was completed using classical hand calculations. These calculations were then verified using two methods. First, Quartus Engineering of San Diego, CA completed an independent structural analysis using finite element analysis with the program NASTRAN. Second, Space Vector Corporation of Chatsworth, CA completed a series of ground static load tests to prove structural adequacy and airworthingness of the SLC and DTA. An 87 inch diameter (same as QuickReach TM ) and 8 foot long tank (called the Ground Test Article or GTA) was used to statically load a single SLC conveyor station to duplicate a 4.5 g down condition and to statically load a single SLC teeter station to maximum load allowed by the C-17A ramp. The SLC successfully passed these tests. D. C-17 Analysis and Studies. Air launch of a QuickReach launch vehicle involves dropping ~72K lbs at above 30,000 ft altitude. However, only 60,000 lb unit airdrop had been flight-tested from the C-17A at low altitude. Boeing Long Beach completed analytic studies that have shown that the C-17A should have an airdrop capability of 72,000 lbs unit air drop using GAL. The main reason for this is that GAL spreads the teeter loads over the last 5 feet of the cargo ramp instead of concentrating the loads on the aft most aircraft rollers as done with normal heavy equipment airdrop methods. E. Mojave Ground Test. A ground test was conducted at Mojave, CA on 2 April 2005 to verify the simulations and analysis of the GAL method and to test the SLC. At the Mojave test site a simulated C-17A cargo deck was constructed from two 40 ft long Sea Cargo Containers equipped with two 5,000 pound capacity winches. A simulated C-17A cargo compartment roof was constructed from 2 x 4 wood pieces. The DTA was dropped into a 60 foot deep hole lined with plywood and crushable cardboard. The drop test and release system operations proved to be almost exactly as predicted by the simulations and analyses. The precision of the predicted dynamics allowed the test team to recover the ground test article with such minor damage that the test article will be the used in the second DTA airdrop. Figure 8: Ground Test at Mojave Sarigul-Klijn, Sarigul-Klijn, Hudson, Holder et al 6 AIAA Space 2006

7 F. t/lad Flight Tests. To prove drop dynamics, three drops of a 250 inch long (20.83 ft) and 37.4 inch diameter DTA (approximately 1/3 scale of the DTA used in the C-17A) weighing 2,018 lbs were completed from Scaled Composites Proteus aircraft during May and June 2005 to test a configuration for Transformational Space Corporation (t/space) as part of a space exploration contract with NASA 3. The DTA was attached to the Proteus via a modified F-4 Phantom fighter bomb rack that was installed in the Proteus s centerline pylon fairing. A trapeze and lanyard mechanism (called t/lad or Trapeze Lanyard Air Drop) installed in the pylon fairing was used to impart an initial pitch up rate that duplicated the pitch up that occurs in GAL as the rocket crests the edge of the C-17A ramp. After the 12 foot long lanyard was released, then the pitch up dynamics were the same as GAL. The DTA s exhibited very little sideslip or yaw during the pitch up maneuver. The three t/lad drops validated the drop simulations that were later used in the C-17 GAL demonstration, confirmed that the DTA would rotate mostly in pitch without much yaw or roll instability, and showed that the post ignition aerodynamic stability of the booster was sufficient so that a typical rocket thrust vector control system should be able to control the launch vehicle. G. Flight Test Results and Discussion. The overall flight test objective of the AirLaunch drops was to demonstrate the feasibility and safety of the GAL launch method. This test objective was met on 29 September 2005 with a successful drop from the C-17A. The drogue parachute was stable Figure 9: t/lad Flight Test and trailed the C-17A aircraft in an acceptable position. Once the pneumatically operated CRS was activated, the DTA rolled aft supported by the SLC tire/wheel assemblies and cleared the cargo ramp in about 4 seconds. The DTA began to pitch upward as its center of gravity passed over the SLC teeter. Vertical clearance below the forward edge of cargo door and the DTA was 38 inches. This compared to the absolute clearance physically obtainable of 40 inches; which was demonstrated during horizontal loading in ground operations. As the DTA left the aircraft, it continued to rotate but with its pitch angle and pitch rate attenuated by the action of the drogue parachute. The drogue parachute mechanically released as planned when the DTA s pitch rate reversed from a positive nose up to a negative nose down. The DTA reached a maximum pitch attitude of 67 degrees (above horizontal). This compares to the preflight prediction of 65 degrees, determined by using a 3 degree of freedom (3DOF) dynamic simulation program using aerodynamic parameters obtained from computational fluid dynamics (CFD) (described above) an excellent match. The DTA began to yaw to the left approximately 1 second prior to the release of parachute. The DTA s yaw angle was approximately 18 degrees to the left at parachute release. It was later determined that Figure 10: QuickReach TM First DTA Drop Test Sarigul-Klijn, Sarigul-Klijn, Hudson, Holder et al 7 AIAA Space 2006

8 this yaw was caused by a combination of three factors: (1) the DTA s nozzle was installed slightly bent so that it acted as a left turning rudder at low angle of attacks, (2) a large access hole was left open on the forward right side of the cylindrical portion of the DTA, this generated a left turning force at high angle of attacks (> 70 degrees), and (3) an asymmetrical vortex located at the tip of the DTA nose caused a turning moment from about 40 to 70 degrees angle of attack. Causes 1 and 2 were relatively easy to fix, but Cause 3 required an extensive CFD and poor mans wind tunnel test campaign conducted on a truck to design nose strakes to eliminate the unstable yaw moment caused the asymmetrical vortex located at the DTA s nose. The aircraft pitch attitude increased from 6 to 9 degrees during the airdrop due to the aft ward center of gravity shift as the DTA rolled out, and then returned to the original pitch attitude. This is well below the 17 degree pitch attitude required to stall the C-17A in the high altitude airdrop configuration (slats down and flaps up). The cargo ramp load cell data reached a combined maximum of 41,900 pounds from the 50,000 pound DTA when the tare weight of the ramp section of the SLC was subtracted. Video data showed that a portion of the DTA s weight was being supported by several sets of SLC tires forward of the tires that were instrumented by the load cells. These tires will be instrumented during the next drop and the C-17A Aerial Delivery System (ADS) ramp links will also be instrumented. The ADS links help support the C-17 cargo ramp when it is open and their structural capacity is the limiting factor for heavy airdrops. VI. Conclusions AirLaunch s QuickReach Drop Test Article (DTA) flight test program shows that Gravity Air Launch (GAL) method works as intended. This new launch method promises to contribute to improving the simplicity, safety, cost, and reliability of launching small satellites into low earth orbit (LEO). A GAL launch simplifies operations compared to ground launch, allows launch from any 4,000 foot runway, eliminates weather constraints, minimizes coordination with other users of a launch range, allows direct trajectories into a desired orbit, and allows the use of a simple and safe vapor pressurization (VAPAK) engine cycle for the launch vehicle. The next step to further develop the GAL launch method will be to complete the Phase 2B DTA s test program, followed by a Phase 2C launch of a live rocket into LEO. VII. Acknowledgements The organizations responsible for the conducting the GAL tests were the 412 th Test Wing and the Global Reach Combined Test Force, both located at the Air Force Flight Test Center, Edwards Air Force Base (AFB). Other participating government agencies include DARPA; the Space and Missile Systems Center, Detachment 12, Kirtland AFB, New Mexico; and the C-17 Systems Group (SG) and Air Force Research Lab / Air Vehicles Directorate, Wright-Patterson AFB. The GAL hardware was designed and built by the AirLaunch Limited Liability Corporation (LLC), Kirkland, Washington. Some of AirLaunch s subcontractors involved with the GAL tests include the Space Vector Corporation, Chatsworth, California, Scaled Composites, Inc, Fiberset, and Mojave Airport and Spaceport, all located in Mojave, California; and Western Trailer, Boise, Idaho. VIII. References 1 Sarigul-Klijn, M., Sarigul-Klijn, N., Hudson, G., McKinney, B., Menzel, L., and Grabow, E., Trade Studies for Air Launching a Small Launch Vehicle from a Cargo Aircraft, 43 th AIAA Aerospace Sciences Meeting and Exhibit, AIAA Sarigul-Klijn, N., Noel, C., and Sarigul-Klijn, M.., Air Launching Earth-to-Orbit Vehicles: Delta V gains from Launch Conditions and Vehicle Aerodynamics, 42 th AIAA Aerospace Sciences Meeting and Exhibit, AIAA Sarigul-Klijn, M., Sarigul-Klijn, N., Morgan, B., Tighe, J., Leon, A., Hudson, G., McKinney, B., and Gump, D., Flight Testing of A New Air Launch Method for Safely Launching Personnel and Cargo into Low Earth Orbit, Journal of Aircraft, vol. 43 no. 3, pp , Sarigul-Klijn, Sarigul-Klijn, Hudson, Holder et al 8 AIAA Space 2006