Clearing the Marsh Fog About OEZ Charts vs Individual Risk Analysis

As part of our FAA launch site operator license application, we are required to provide a fight safety analysis which was conducted by the Aerospace Corporation. The Aerospace Corporation was founded in 1960 to serve the Air Force in the scientific and technical planning and management of missile-space programs.  Since that time, it has expanded its scope to technical guidance and advice on all aspects of space missions to military, civil, and commercial customers.

The detailed methodology and algorithms for preparing the flight safety analysis are proprietary to The Aerospace Corporation.  Key details of any flight safety analysis that address rocket design, performance, operations, and accident evaluation may be restricted from public disclosure under the International Trafficking in Arms Regulations (ITAR) and the Missile Technology Control Regime (MTCR). Therefore, Camden County is unable to share such proprietary information as it pertains to the Spaceport Camden safety analysis.

Camden County strives for transparency in the project development process. Therefore, to help you better understand how Spaceport Camden passes the rigorous regulations in place to protect people and property, we’ve prepared the following blog post that addresses some of the questions we’ve heard in recent weeks.

“Overflight Exclusion Zone” Using 30% Launch Failure Rate (10% per phase of flight) for 100 Degree Trajectory, Medium-Large Launcher [14 CFR §420]
What is this image and where did it come from?

The image is from an early “stress test” analysis of the “Overflight Exclusion Zone” (OEZ). The Aerospace Corporation produced this image to determine the extent of an extremely conservative (i.e., excessively large) OEZ for the largest sized rocket proposed to launch from Spaceport Camden.

What does the image actually show? What is an OEZ?

An OEZ is the area where members of the uninvolved public are not allowed to be present during a launch. It is calculated by modeling a rocket launch and simulating between 100,000 and 500,000 random failures of that same rocket. From this large statistical analysis, individual grids are evaluated assuming a person is standing in the grid, and if they would sustain an injury (FAA regulations call this a “casualty”) from those hundreds of thousands of random rocket failures. If the grid produces an injury probability that is greater than the defined threshold, the grid is marked red, and the analysis continues until all of the grids are analyzed. Different colors represent different levels of risk. The red grids comprise the OEZ for that analysis case.

The red grids in the diagram look like they cover a large area, are all those people at risk of injury?

No, not at all.  In fact, the red area, where the probability of injury is 1 in 10,000 or higher, is unpopulated by members of the uninvolved public either as part of Spaceport Camden or the surrounding marsh.  It does not cover any areas that typically have population around Spaceport Camden, such as Little Cumberland or Cumberland Island.

What makes this “conservative”? Does it have something to do with it being a “stress test”?

Yes, that is right. The image shows a conservative estimated OEZ area based on a simulation of 300,000 random failures for a medium-large rocket (e.g., a SpaceX Falcon 9 or United Launch Alliance (ULA) Atlas V).  A rocket this size can launch satellites the size of a school bus to orbits 20,000+ miles above the Earth’s equator.  It is the largest possible rocket that is proposed to launch from Spaceport Camden. Second, as a stress test, it was assumed that the failure rate of the proposed rocket was three times greater than that required by the Federal Aviation Administration (FAA) for a launch site operator license.  The FAA requires a launch site operator license applicant to assume a minimum 10 percent failure rate for the proposed rocket used in the license application.  In this simulation, the rocket was assumed to have a 30 percent failure rate, which is greater than the failure rate experienced by first time experimental rockets in the United States.  Even with these conservative assumptions, the OEZ for such a rocket is still far from the islands and passes the FAA requirements for a launch site operator license.

Other things that make this a conservative analysis are some of the underlying assumptions about the rocket itself, and how it performs when it suffers a failure.  For example, the worst type of failure that would potentially cause an injury (or catastrophic destruction of property) is if the rocket falls intact and hits the ground, resulting in an explosion of the remaining fuel.  This simulation assumes that the rocket is quite strong and can survive a larger than normal amount of mid-air tumbling before it breaks up into smaller pieces – this assumption increases the probability that the hypothetical rocket will hit the ground intact, explode and hence increase the probability of an injury, so is a more conservative assumption.

How does this compare to actual rocket failure rates here in the United States for similar rockets?

This is a good question.  The assumed failure rate for the image above is 30 percent.  First time experimental rockets have a failure rate of about 28 percent.  The FAA requires an assumed failure rate of 10 percent for the license application.  The actual failure rate for the ULA Atlas V and SpaceX Falcon 9 medium-large rockets that fly in the United States is between 0 percent and 1.5 percent, respectively.

Some people have said that the image shows Camden may receive a spaceport license, but that launches cannot happen in Camden – is that true?

No, as with many items related to rockets and government regulations, things can be complicated and confusing. The FAA regulations require a launch site operator (i.e., a spaceport) to show the OEZ in which the probability of injury to an individual is greater than 1 in 10,000 (i.e., 1 x 10-4or 1E-4).  However, the FAA requires the launch operator (the company who builds and launches the rocket) to show the mission (and its given assumptions of failure rate and other factors) has a probability of injury to an individual (a.k.a., “individual risk”) that is 1 in 1 million (i.e., 1 x 10-6, 1E-6, or “ten to the minus six”).

If you only look at the image above, it may appear that a rocket cannot be launched from Spaceport Camden because the 1 x 10-6  (yellow area) extends well past Cumberland and Little Cumberland Island. But keep in mind, this image is for a medium-large rocket with a “stress test” 30 percent failure rate (greater than the failure rate experienced by the first tests of United States-based brand-new rockets that have never flown).

Camden County is not planning to launch orbital test vehicles or other unproven rockets, so the 30 percent failure rate for a medium-large rocket is not practical for an evaluation of individual risk in accordance with FAA regulations.  When the individual risk image is produced using a more reasonable failure assumption the probability of casualty area for the “ten to the minus six” area (the yellow area in the first image) shrinks dramatically. The updated analysis of individual risk with higher failure probability than is required, 17.5 percent versus the 10 percent required by FAA, is shown below (notice in the legend, red is now 1 in 1 million, or 1E-6, or “ten to the minus six”). It passes; the red area does not touch locations where people are typically located.  And again, this is for a medium-large rocket, the biggest and most powerful rocket that could be flown from Spaceport Camden.  It is noted that smaller rockets produce smaller OEZs and individual risk areas, given similar assumptions.

“Individual Risk” Area For Launch Using 17.5% Launch Failure Rate (5%, 5%, & 7.5% for 1st stage launch, 2nd stage to orbit, & 1st stage landing) for 100 Degree Trajectory, Medium-Large Launcher [14 CFR §417] 
How long will a rocket launched from Spaceport Camden be over Cumberland Island or Little Cumberland Island? And how high will the rocket be as it crosses the islands?

The quick answer to these two questions is very quickly and very high. For a medium-large rocket flying a typical trajectory, overflight time is between about 6-10 seconds, and overflight altitude is between 70,000 and 100,000 feet.

To be clear, every vertical launch trajectory will be different, but generally, all trajectories follow a similar path … First, straight up to clear the heavier parts of the atmosphere as quickly as possible (less drag on the rocket and its payload). Then, the rocket generally travels horizontal, but gaining altitude, to achieve the desired orbit.

Most of the initial vertical ascent (e.g., 45-75 seconds) takes place almost directly over the launch pad and within the Spaceport Camden boundary.  By the time a medium-large rocket flying a typical trajectory crosses the Cumberland River and first enters the airspace above Cumberland Island, it will be at an altitude of approximately 72,000 feet.  At this height, the rocket is flying at about twice the cruising altitude of a commercial airliner.   Between 6-10 seconds later, the rocket departs the airspace above Cumberland Island at an altitude of approximately 97,000 feet, roughly 1/3 of the way to the boundary of space called the Kármán Line.

Representative Trajectory for a Medium-Large Launcher

An interesting thing, though, is that if the rocket fails while over the island, it is highly unlikely any debris will land there due to the speed and altitude it is travelling. With high probability, it will follow the same path and fall into the ocean. A different question is how long will the islands be exposed to potential debris should the rocket fail along its trajectory before reaching the edge of Cumberland Island?  As it turns out, the answer is similar to the overflight of the island itself, the potential exposure of the island to debris in the case of an accident is about 6-9 seconds.

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