Zero-boil-off tank experiments to enable long-duration space exploration

Do now we have sufficient gas to get to our vacation spot? That is in all probability one of many first questions that involves thoughts each time your loved ones will get able to embark on a highway journey. If the journey is lengthy, you’ll need to go to gasoline stations alongside your path to refuel throughout your journey.

NASA is grappling with comparable points because it will get able to embark on a sustainable mission again to the moon and plans future missions to Mars. However whereas your automobile’s gas is gasoline, which may be safely and indefinitely saved as a liquid within the automobile’s gasoline tank, spacecraft fuels are unstable cryogenic liquid propellants that should be maintained at extraordinarily low temperatures and guarded from environmental warmth leaks into the spacecraft’s propellant tank.

And whereas there’s already a longtime community of economic gas stations in place to make refueling your automobile a cinch, there aren’t any cryogenic refueling stations or depots on the moon or on the way in which to Mars.

Moreover, storing unstable propellant for a very long time and transferring it from an in-space depot tank to a spacecraft’s gas tank underneath microgravity conditions is not going to be simple because the underlying microgravity fluid physics affecting such operations isn’t effectively understood. Even with at present’s expertise, preserving cryogenic fuels in space past a number of days isn’t doable, and tank-to-tank gas switch has by no means been beforehand carried out or examined in space.

Warmth carried out by way of assist buildings or from the radiative space atmosphere can penetrate even the formidable multi-layer insulation (MLI) techniques of in-space propellant tanks, resulting in boil-off or vaporization of the propellant and inflicting tank self-pressurization.

The present apply is to protect towards over-pressurizing the tank and endangering its structural integrity by venting the boil-off vapor into space. Onboard propellants are additionally used to chill down the new switch traces and the partitions of an empty spacecraft tank earlier than a gas switch and filling operation can happen. Thus, valuable gas is constantly wasted throughout each storage and switch operations, rendering long-duration expeditions—particularly a human Mars mission—infeasible utilizing present passive propellant tank stress management strategies.

Zero-boil-off (ZBO) or lowered boil-off (RBO) applied sciences present an revolutionary and efficient means to exchange the present passive tank stress management design. This methodology depends on a fancy mixture of energetic, gravity-dependent mixing and power elimination processes that permit upkeep of secure tank stress with zero or considerably lowered gas loss.

Zero boil-off storage and switch: A transformative space expertise

On the coronary heart of the ZBO stress management system are two proposed energetic mixing and cooling mechanisms to counter tank self-pressurization. The primary is predicated on intermittent, pressured, subcooled jet mixing of the propellant and entails complicated, dynamic, gravity-dependent interplay between the jet and the ullage (vapor quantity) to regulate the condensation and evaporation phase change on the liquid-vapor interface.

The second mechanism makes use of subcooled droplet injection through a spraybar within the ullage to regulate tank stress and temperature. Whereas the latter choice is promising and gaining prominence, it’s extra complicated and has by no means been examined in microgravity the place the phase change and transport conduct of droplet populations may be very completely different and nonintuitive in comparison with these on Earth.

Though the dynamic ZBO strategy is technologically complicated, it guarantees a formidable benefit over the at the moment used passive strategies. An evaluation of 1 nuclear propulsion idea for Mars transport estimated that the passive boil-off losses for a big liquid hydrogen tank carrying 38 tons of gas for a three-year mission to Mars could be roughly 16 tons/yr.

The proposed ZBO system would offer a 42% saving of propellant mass per yr. These numbers additionally indicate that with a passive system, all of the gas carried for a three-year Mars mission could be misplaced to boil-off, rendering such a mission infeasible with out resorting to the transformative ZBO expertise.

The ZBO strategy supplies a promising methodology, however earlier than such a fancy technological and operational transformation may be totally developed, applied, and demonstrated in space, necessary and decisive scientific questions that influence its engineering implementation and microgravity efficiency should be clarified and resolved.

The zero-boil-off tank (ZBOT) microgravity science experiments

The zero boil-off tank (ZBOT) Experiments are being undertaken to kind a scientific basis for the event of the transformative ZBO propellant preservation methodology. Following the advice of a ZBOT science assessment panel comprised of members from aerospace industries, academia, and NASA, it was determined to carry out the proposed investigation as a sequence of three small-scale science experiments to be carried out onboard the Worldwide House Station. The three experiments outlined beneath construct upon one another to deal with key science questions associated to ZBO cryogenic fluid administration of propellants in space.

The ZBOT-1 experiment: Self-pressurization and jet mixing

The primary experiment within the sequence was carried out on the station within the 2017-2018 timeframe. The second picture above exhibits the ZBOT-1 {hardware} within the microgravity science glovebox (MSG) unit of the station. The primary focus of this experiment was to research the self-pressurization and boiling that happens in a sealed tank as a consequence of native and international heating, and the feasibility of tank stress management through subcooled axial jet mixing.

On this experiment, the difficult interplay of the jet movement with the ullage (vapor quantity) in microgravity was rigorously studied. Microgravity jet mixing knowledge was additionally collected throughout a variety of scaled movement and warmth switch parameters to characterize the time constants for tank stress discount, and the thresholds for geyser (liquid fountain) formation, together with its stability, and penetration depth by way of the ullage quantity. Together with very correct stress and native temperature sensor measurements, particle picture velocimetry (PIV) was carried out to acquire whole-field movement velocity measurements to validate a computational fluid dynamics (CFD) mannequin.

Among the attention-grabbing findings of the ZBOT-1experiment are as follows:

• Supplied the primary tank self-pressurization charge knowledge in microgravity underneath managed circumstances that can be utilized for estimating the tank insulation necessities. Outcomes additionally confirmed that classical self-pressurization is kind of fragile in microgravity and nucleate boiling can happen at hotspots on the tank wall even at average warmth fluxes that don’t induce boiling on Earth.
• Proved that ZBO stress management is possible and efficient in microgravity utilizing subcooled jet mixing, but additionally demonstrated that microgravity ullage-jet interplay doesn’t comply with the anticipated classical regime patterns.
• Enabled commentary of surprising cavitation throughout subcooled jet mixing, resulting in huge phase change at either side of the screened liquid acquisition system (LAD). If any such phase change happens in a propellant tank, it could possibly result in vapor ingestion by way of the LAD and disruption of liquid movement within the switch line, doubtlessly resulting in engine failure.
• Developed a state-of-the-art two-phase CFD mannequin validated by over 30 microgravity case research. ZBOT CFD fashions are at the moment used as an efficient software for propellant tank scaleup design by a number of aerospace firms taking part within the NASA tipping level alternative and the NASA Human Touchdown System (HLS) program.

The ZBOT-NC experiment: Non-condensable gasoline results

Non-condensable gases (NCGs) are used as pressurants to extract liquid for engine operations and tank-to-tank switch. The second experiment, ZBOT-NC will examine the impact of NCGs on the sealed tank self-pressurization and on stress management by axial jet mixing. Two inert gases with fairly completely different molecular sizes, xenon and neon, will likely be used because the non-condensable pressurants. To attain stress management or discount, vapor molecules should attain the liquid-vapor interface that’s being cooled by the blending jet after which cross the interface to the liquid aspect to condense.

This research will concentrate on how in microgravity the non-condensable gases can decelerate or resist the transport of vapor molecules to the liquid-vapor interface (transport resistance) and can make clear to what extent they could kind a barrier on the interface and impede the passage of the vapor molecules throughout the interface to the liquid aspect (kinetic resistance). By affecting the interface circumstances, the NCGs may change the movement and thermal buildings within the liquid.

ZBOT-NC will use each native temperature sensor knowledge and uniquely developed quantum dot thermometry (QDT) diagnostics to gather nonintrusive whole-field temperature measurements to evaluate the impact of the non-condensable gases throughout each self-pressurization heating and jet mixing/cooling of the tank underneath weightlessness circumstances. This experiment is scheduled to fly to the Worldwide House Station in early 2025, and greater than 300 completely different microgravity exams are deliberate. Outcomes from these exams may also allow the ZBOT CFD mannequin to be additional developed and validated to incorporate the non-condensable gasoline results with bodily and numerical constancy.

The ZBOT-DP experiment: Droplet phase change results

ZBO energetic stress management may also be achieved through injection of subcooled liquid droplets by way of an axial spray-bar straight into the ullage or vapor quantity. This mechanism may be very promising, however its efficiency has not but been examined in microgravity. Evaporation of droplets consumes warmth that’s provided by the new vapor surrounding the droplets and produces vapor that’s at a a lot decrease saturation temperature. Because of this, each the temperature and the stress of the ullage vapor quantity are lowered.

Droplet injection may also be used to chill down the new partitions of an empty propellant tank earlier than a tank-to-tank switch or filling operation. Moreover, droplets may be created throughout the propellant sloshing brought on by acceleration of the spacecraft, and these droplets then bear phase change and warmth switch. This warmth switch may cause a stress collapse that will result in cavitation or a large liquid-to-vapor phase change. The conduct of droplet populations in microgravity will likely be drastically completely different in comparison with that on Earth.

The ZBOT-DP experiment will examine the disintegration, coalescence (droplets merging collectively), phase change, and transport and trajectory traits of droplet populations and their results on the tank stress in microgravity. Specific consideration may also be dedicated to the interplay of the droplets with a heated tank wall, which might result in flash evaporation topic to issues brought on by the Liedenfrost impact (when liquid droplets propel away from a heated floor and thus can not cool the tank wall).

These difficult phenomena haven’t been scientifically examined in microgravity and should be resolved to evaluate the feasibility and efficiency of droplet injection as a stress and temperature management mechanism in microgravity.

Again to planet Earth

This elementary analysis is now serving to business suppliers of future touchdown techniques for human explorers. Blue Origin and Lockheed Martin, contributors in NASA’s Human Touchdown Programs program, are utilizing knowledge from the ZBOT experiments to tell future spacecraft designs.

Cryogenic fluid administration and use of hydrogen as a gas aren’t restricted to space purposes. Clear inexperienced power supplied by hydrogen could one day gas airplanes, ships, and vans on Earth, yielding monumental local weather and financial advantages. By forming the scientific basis of ZBO cryogenic fluid administration for space exploration, the ZBOT science experiments and CFD mannequin growth may also assist to reap the advantages of hydrogen as a gas right here on Earth.