تجزیه و تحلیل سیستم حرارتی برودتی برای انبار محرک اوربیتال

In any manned mission architecture, upwards of seventy percent of all payload delivered to orbit is propellant, and propellant mass fraction dominates almost all transportation segments of any mission requiring a heavy lift launch system like the Saturn V. To mitigate this, the use of an orbital propellant depot has been extensively studied. In this paper, a thermal model of an orbital propellant depot is used to examine the effects of passive and active thermal management strategies. Results show that an all passive thermal management strategy results in significant boil-off for both hydrogen and oxygen. At current launch vehicle prices, these boil-offs equate to millions of dollars lost per month. Zero boil-off of propellant is achievable with the use of active cryocoolers; however, the cooling power required to produce zero-boil-off is an order of magnitude higher than current state-of-the-art cryocoolers. This study shows a zero-boil-off cryocooler minimum power requirement of 80–100 W at 80 K for liquid oxygen, and 100–120 W at 20 K for liquid hydrogen for a representative Near-Earth Object mission. Research and development effort is required to improve the state-of-the-arts in-space cryogenic thermal management.

In 2009, the Review of United States Human Space Flight Plans Committee, also known as the Augustine Committee, was charged by the Office of Science and Technology Policy to review the multitude of options for human spaceflight and human exploration after the planned retirement of the Space Shuttle Program. The result of the 6 months review was a 150 page report documenting various recommendations for the future of United States space policy [1]. The committee׳s primary finding was that achieving the ultimate goals of human space exploration to other planets would require both “physical and economic sustainabilities.” The committee judged the Constellation Program [2] to be so far behind schedule and underfunded that meeting any of its objectives would be impossible. In response to the review, President Obama canceled the Constellation Program and enacted the 2010 National Space Policy of the United States of America [3]. The National Space Policy set forth new and far-reaching exploration milestones that include a crewed mission to a near Earth object (NEO) by 2025 and a crewed mission to Martian orbit by the mid-2030s. The exploration milestones follow the “Flexible Path towards Mars with alternatives to the Moon” strategy outlined by the Augustine committee. The goal of the flexible path strategy is to take incremental steps towards Mars, allowing astronauts to learn, live, and work in free space under similar conditions to those found on the way to Mars. Following the flexible path strategy allows humanity to gain ever-increasing operational experience in-space, growing in duration from a few weeks to several years in length, and moving from close proximity to the Earth to as far away as Martian orbit. These incremental steps involve several intermediate destinations to explore before Mars. Fig. 1, reproduced from the Augustine committee׳s report [1], shows a notional representation of the flexible path leading to an eventual Mars mission.The Augustine committee also identified several technologies that it deemed critical for sustainable space exploration and thus a priority for NASA to develop. One of these technologies identified was the storage and transfer of cryogenic propellant in-space. In any exploration mission architecture, upwards of seventy percent of all payload delivered to orbit is propellant [4], and propellant mass fraction dominates every stage. Without significant breakthroughs in propulsion or materials technology [5], a more efficient and economical method to deliver and store propellant in orbit is needed. Many studies by NASA and commercial companies have examined and noted the tremendous benefits of the utilization of orbital propellant depots [6] and [7] coupled with the emerging commercial launch market as a solution.

A system-level thermal and mass model of an orbital cryogenic propellant depot was developed to investigate the effects of two chosen thermal management systems on overall system mass. The model was used to investigate the system level trade off between the passive thermal insulation (MLI) and the active thermal mitigation (cryocoolers). The results show that for a 225 mT propellant depot at an OF ratio of 5.5, the minimum boil-off rate achievable for insulation only depot is roughly 2.5% per month for hydrogen and 0.5% per month of oxygen. This translates to boil-off losses of approximately 10,000 kg of hydrogen and 13,000 kg of oxygen per year. The sum of the boil-off losses is roughly the payload capability of a current generation Delta IV heavy launch vehicle [48]. The use of cryocoolers can reduce the boil-rate significantly, though at a price of increased dry mass required for the propellant depot. The current low efficiency, power limited cryocoolers have a distinct mass advantage over MLI only scenarios for both LO2 and LH2 when the mission duration is greater than 6 months. For long duration missions, the increase in dry mass is beneficial compared to the mass saving from elimination of boil-off losses. Minimum cryocooler power requirements to achieve ZBO for a LEO depot are on the order of 80–100 W at 80 K for oxygen and 100–120 W at 20 K for hydrogen. Current state-of-the-art in-space cryocoolers are an order of magnitude less powerful than those required to achieve zero boil-off for hydrogen, but are within reach for oxygen. The efficiencies of current cryocoolers are also very low, with the most efficient coolers operating at around 20% of Carnot efficiency, resulting in high electrical power requirements. The limitations of the current space based cryocooler are typically due to the lack of requirement to operate at such high capacity. Effort to improve the cooling capacity should be a top priority for NASA to enable long duration space exploration missions. In line with the recommendation of the Office of the Chief Technologist, this study has shown importance of cryogenic thermal management technology on long term human space exploration. Active cryocoolers are an integral part of this technology investment portfolio. Research and development of all long term cryogenic management technology is critical to make long duration human space exploration feasible. Technologies such as vapor cooled shields, broad area cooling, and para–ortho hydrogen conversion are all potential candidates that can further improve the performance of cryogenic fluid management in orbit. The utilization of an orbital propellant depot and/or long duration cryogenic propulsion stage can serve as a testbed to advance cryogenic thermal management techniques and provide a more efficient method of performing space exploration. The research and development of these systems should be a top priority for the United States and NASA.