As recent press articles have discussed, the OSIRIS-REx team has been busy performing a special study related to our launch mass and propellant load. The objective of this study is to look at ways to increase the OSIRIS-REx spacecraft delta-V capability as much as possible without impacting the current spacecraft or launch vehicle designs. Delta-V is a common term in astrodynamics. It literally means “change in velocity”. It describes the amount of “effort” that is needed to change our trajectory in space by making orbital maneuvers using our propellant and rocket engines to produce a thrust that accelerates the spacecraft.
A Balance Among Four Factors
Our special study focuses on a careful balance of four factors: the dry mass of the spacecraft, the wet mass of the spacecraft, the launch vehicle capability, and the delta-V budget of the mission. The dry mass of the spacecraft is the total mass of the OSIRIS-REx spacecraft when the propellant tanks are empty. The wet mass is the total mass once the hydrazine propellant is loaded into the fuel tank. The launch vehicle capability is determined by the total amount of thrust produced from the rocket based on our required hyperbolic escape velocity (5.41 km/s). The delta-V budget summarizes the total amount of energy needed to get to Bennu and back.
Our Design Constraints
Throughout most of the design cycle for OSIRIS-REx, we have designed the mission using a launch vehicle capability of 1955 kg with a characteristic energy (C3) of 29.3 km2/s2. The C3 is a measure of the excess specific energy over that required to escape from the Earth’s gravitational pull. It is exactly equal to the square of our hyperbolic escape velocity. The total dry mass available (860 kg) is calculated by subtracting the propellant needed to achieve the delta-V budget (1095 kg) from the total launch vehicle capability (1955 kg at the time of mission selection in 2011). However, as the design of our spacecraft, launch vehicle, and mission have matured, it has become apparent that we may have the opportunity to fly with more propellant in our tank than originally planned.
The Design Matures
Throughout most of the design period, we have carried an uncertainty on the total dry mass of the flight system. We call this allocated mass our “mass margin”. This margin is needed because unexpected changes to the design may increase the total dry mass of the system. However, many components of the flight system are now built and their mass has been measured. For example, the spacecraft structure is complete and the propellant tank has been installed. Our current best estimate for the as-built dry mass of the system is 828 kg. History tells us that, even at this late stage in development, it is prudent to allow some room for a small amount of mass growth, on the order of 4% (33kg). Even with this margin, we are on target to maintain our projected dry mass of 860 kg. The maximum capacity of our propellant tank is 1245 kg of hydrazine. Adding this mass to our dry mass results in launch wet mass of 2110 kg, which is a 155-kg increase over our current launch vehicle design capability. If we are limited to this capability, then we will launch with a propellant tank that is only 88% full.
Enter the ULA Team
Fortunately for OSIRIS-REx, the launch-vehicle team at the United Launch Alliance has been busy studying the capability of our Atlas V 411 rocket. The evaluation of our available launch mass is underway. Preliminary results for our baseline 30 minute-per-day 39-day launch window already suggests that an additional 123 kg are available, yielding a 2078-kg launch mass. Work is ongoing to determine if the additional 32 kg needed to top off the propellant tank will be available. Thus, the final decision on whether to fuel OSIRIS-REx with more propellant is still a couple of months away. The good news is that we can accommodate any increase over the current 1955-kg estimate. We will simply add the extra fuel until the wet mass equals the launch vehicle capability.
The Benefits of Heavy Launch
One possible outcome of having the extra fuel on board is an “Early Return” option, which returns the sample to Earth one year sooner. This trajectory requires a delta-V of 935 m/s for the departure burn from Bennu on January 3, 2020. Earth return occurs on September 24, 2022. The re-entry velocity is 12.24 km/s, which is still below the 12.4 km/s requirement. This scenario reduces mission risk by shortening the flight mission timeline. It also enables us to begin our primary science of sample analysis one year earlier than currently planned. For comparison, our nominal mission uses a departure burn with a delta-V of 328 m/s on March 3, 2021. Earth return occurs on September 24, 2023. Thus, the Early Return option cuts 425 days off of our Bennu encounter. However, with the Approach Phase planned to begin on August 17, 2018, we still have 505 days to survey Bennu, select a sample site, and retrieve the sample from Bennu’s surface.
The Heavy Launch option offers even more flexibility. In addition to Early Return, we also open some possibilities for a late departure from Bennu. Detailed astrodynamic analysis by the OSIRIS-REx Flight Dynamics team has revealed multiple trajectory options for late departure that require additional delta-V to enable. All of these options maintain an Earth-return date in September 2023. The latest return trajectory identified has OSIRIS-REx departing Bennu on April 10, 2022, resulting in a whopping 1,333 days spent at the asteroid. This flexibility increases mission robustness and tolerance to unforeseen events or conditions at Bennu.
The good news is that we don’t have to decide on an Early Return or a Late Departure until after we launch. In fact, we don’t have to make the decision on when to depart until after our arrival at Bennu. Our plan is to stick to our present baseline mission design until after launch. In the meantime we will continue to pursue opportunities to maximize the amount of delta-V available to OSIRIS-REx. This strategy ensures that we have the most resources available and greatly improves our chances of mission success.