Avoiding the 7 minutes of terror: the next generation of space technology to head to Mars
Just before lunchtime on the 14th November 2024, sounding rocket S-520-34 launched into the sky. Onboard this brief flight were experiments testing the next generation of space technology for planetary exploration.
Far below on Earth, the Communication Hall at the Institute of Space and Astronautical Science was showcasing space history to a stream of visitors. Many would linger beside a backroom display case, gazing at four items that look suspiciously like sections of a flying saucer. The scarred top and bottom are heat shields that wrapped around a celestial sample container in the inner cavity, while the inner components include the parachute that was released via pyrotechnics.
Assembled, this saucer is the re-entry capsule that protected the asteroid grains collected by the Hayabusa2 spacecraft from the intense aerodynamics heating that occurs when a solid object falls at high speed through the Earth’s atmosphere. The crisscross of jagged lines on the charred shields are evidence of the success of a design that returned both of humanity’s first two asteroid samples. But something is bothering Associate Professor Yamada Kazuhiko.
“It wouldn’t work for Mars,” he says thoughtfully. “The atmosphere is too thin to act as a sufficient brake.”
The atmosphere of Mars has about one hundredth of the density of that of the Earth. This thinner cloak of gases reduces the dangerously hot aerodynamical heating experienced by an incoming object, but is far poorer at slowing the descent. One solution is to greatly increase the parachute size to create more drag. However, the thin atmosphere and rapid fall of the descending object means that the parachute must risk opening as supersonic speeds, and thrusters are still needed to further reduce the velocity to safe levels near the ground. Landing on Mars is therefore considered one of the most technologically challenging parts of Martian exploration.
(NASA/JPL-Caltech)
Those who have achieved the Martian landing, such as the NASA Mars rover teams, refer to this as the “seven minutes of terror”. But what if there was an entirely new technology that could avoid the seven minutes of terror, and be compact and light enough for low cost missions to the red planet?
Across campus in the Department of Space Flight Systems sits a large circular membrane. The brown fabric feels silky and light to the touch, and is stretched across an outer inflatable ring. It looks like something that would be fun on a beach, but this particular inflatable is a new generation of atmospheric re-entry equipment.
The “Re-entry and recovery module with deployable Aeroshell Technology for Sounding rocket experiment” or “RATS”, swaps the rigid saucer design of the Hayabusa2 sample return capsule for an aeroshell of flexible material that can be folded for storage and expanded for use by injecting gas into the inflatable ring.
“A small high-pressure gas bottle is mounted on the rocket,” explains Yamada, who leads the development team for RATS. “When the aeroshell needs to be deployed, the metal cover of the packed aeroshell is discarded, and a hole is punched in the gas bottle which injects the gas into the inflatable ring. Since the surrounding pressure is almost zero in space, the aeroshell expands rapidly right after gas injection.”
While inflation is an easy task in space, ensuring that the flexible aeroshell does not become damaged is a challenge. The RATS qualification tests require that the aeroshell and the inflatable ring remain intact through multiple rounds of folding, changing pressure, and high levels of vibration before eventual expansion. The aeroshell material also needs to be highly heat resistant, flexible, and have sufficient structural strength for atmospheric entry.
RATS is currently being used in sounding rocket experiments, which reach altitudes of about 300km to perform tests in the environment of space before splashing into the ocean. The inflated RATS separates from the sounding rocket before it re-enters the atmosphere, floating through the air to carry the data from the onboard experiments safely to Earth.
In 2021, a 1.2m diameter RATS successfully returned data collected from sounding rocket S-520-31, splashing down into the ocean where it bobbed on the waves until collection. The weight of the RATS return module was less than 5kg, and the buoyant ring allows landing even on water. As the membrane itself slows the descent through the atmosphere, there is no need for an additional parachute.
The size of the deployed RATS aeroshell can be adjusted depending on the mass of the payload to be delivered to the planet surface. A heavier payload will require a larger aeroshell to prevent high speeds during atmospheric entry, and thereby reduce both the aerodynamic heating that occurs during the rapid passage through the atmosphere, and reduce the force with which the payload hits the ground. On the other hand, a very large aeroshell risks structural weakness so a balance between these competing effects is required.
The adjustability of the aeroshell size also makes it possible to adapt RATS for landing on different planets, including the notoriously difficult Mars. Yamada hopes that RATS can eventually be used to deliver a small Mars lander to the red planet, opening a pathway for lighter and cheaper Martian exploration. To efficiently leverage the thin atmosphere of Mars, Yamada’s team experimented with a RATS-L design with a larger diameter of 2.5m. Unlike a large parachute that must wait until lower altitudes (and higher atmospheric density) to deploy the canopy, the RATS aeroshell inflates prior to atmospheric entry. This release at high altitude decelerates even in the thin atmosphere, avoiding intense aerodynamic heating and the need for supersonic deployment or additional thrusters to attain sufficiently slow speeds for landing.
“There is then nothing to be done during the flight,” says Yamada. “Instead of the autonomous sequence that makes up the seven minutes of terror, which must perform flawlessly, the RATS just waits to land.”
The RATS-L design successfully flew on the sounding rocket S-520-33 and landed safely in the ocean. At present, the maximum payload that can be carried by RATS is a few kilograms, but this could change by an order of magnitude with future development in heat resistance and structural strength. Although Yamada suspects that in order to transfer a big payload like the large Mars rovers developed by NASA, a technical breakthrough would probably be required for RATS technology.
RATS also is not suitable for a deep space sample return to Earth, as the aerodynamic heating from the ultra-high speed plunge through our thick atmosphere would still be too great for RATS to withstand. For that, the tried-and-tested capsule shape, heat shield and parachute system which is designed to withstand intense heating is the right choice.
“They’re complementary technologies,” explains Yamada. “RATS is well designed for sounding rocket experiments that need to land on water, but do not reach as high velocities during the descent, and for Mars with the challenges incurred by the thin atmosphere.”
The maiden space voyage for RATS onboard S-520-31 in 2021 had not been the only demonstration of future space technology onboard the sounding rocket. RATS also returned to Earth a data chip recording the first successful space flight the Rotating Detonation Engine System (known as RDE or DES).
A Rotating Detonation Engine ignites rocket propellent in a rapidly moving shockwave that loops around the walls of a cylindrical combustion chamber to continuously burn the injection of fresh propellent. The design has the potential to create far smaller rocket engines, further reducing the barriers for space exploration.
RATS returned the data chip with details of the flight, showing that the sounding rocket has successfully switched to DES once the combustion of the onboard solid fuel had completed. It was a great success, and at the end of 2024, it was time to test the newest updates.
The DES2 onboard sounding rocket S-520-34 would switch from using gas propellent to liquid ethanol and nitrous oxide. This was a significant upgrade, as the higher density of liquid allows a greater quantity of propellent to be loaded into the rocket tank. However, injecting a liquid propellent into the combustion chamber is more challenging that with gas, as liquids do not expand and there is no gravitational force in space to provide a push. To combat this, high-pressure nitrogen gas would be used to drive the liquid propellent to the bottom of the tank, where the two liquid would be kept in liquid phase, before being injected into the combustion chamber to be mixed and ignited by the detonation wave.
“This was a challenging development,” explains Professor Kasahara Jiro from Nagoya University who leads the research on DES. “To overcome the difficulties in using liquid propellent, we had to significantly modify the design of the propellent supply system.“
Kasahara’s team also took the opportunity to replace the double cylinder of the combustion chamber with a single cylinder. The detonation wave then rolls around the single outer wall. This simplifies the design and avoids the difficulty involved in cooling an inner cylinder. The DES2 onboard S-520-34 would be the first attempt to try using a liquid propellent in space.
Sounding rocket S-520-34 launched on November 14, 2024 at 11:30 JST. The flight would 459 seconds, reach an altitude of 217 km, and test the operation of both DES2 and another deployable aeroshell, RATS2.
“Lifetimes of sounding rockets are short, but long enough for exciting missions,” notes Professor Habu Hiroto, Sounding Rocket Experiment Group Lead. “They usually demonstrate advanced technology in a space environment, or investigate atmospheric phenomena. This is the fastest way to approach a representative space environment such as that of Low-Earth Orbit for researchers.”
As with S-520-31, the sounding rocket was initially powered by solid fuel, before switching to the DES2. The liquid propellent successfully combusted, and a thrust of 438N was generated.
“We achieved the expected thrust with detonation combustion using liquid propellent!” Kasahara was pleased, but does say that there is more analysis to come.
An additional challenge with a liquid propellent is how completely the fuel can be used, as the rotation of the rocket pushes the liquid propellent to the walls of the chamber due to the centrifugal force. Examining this effect can only be done in space, so an in-depth analysis of the sounding rocket results is needed to learn more.
DES2 had been a success, but as the helicopter scanned the open ocean, it became clear that not everything had gone smoothly. Data from the DES2 demonstration had been transmitted directly to Earth, but additional information was also on a chip being carried on a 1.2m diameter RATS2. The aeroshell had successful separated from the rocket, and broadcasted positional information until just before splashdown. But when the team visited the location, they saw only sea. It seemed that one of the biggest challenges with the RATS system had found its mark: the inflatable ring had got a puncture.
”The most difficult technical issue with RATS is the reliability of the inflatable ring. That is, to ensure that it does not get punctured during operation,” explains Yamada. “It seems that in this test the aeroshell was damaged during the gas injection, and not enough gas was injected. RATS2 with an insufficient inflated aeroshell then separated from the rocket, but could not produce the sufficient aerodynamic drag force and flew faster than expected.”
When RATS2 reached the ocean, there was not enough buoyancy to stay afloat, and the aeroshell sank at sea. It was a disappointment, but the purpose of testing new technology on the sounding rockets is to understand all possible outcomes and weak points.
“RATS2 was not able to complete its mission in this test, unfortunately,” agrees Yamada. “We realised the difficulty in reliably achieving repeated success! However, we can learn a lot from a flight test that did not go well. Our plan is to identify the cause of the problem, take countermeasures, and move forward without slowing development towards our next demonstration test.”
Kasahara also has his eyes on the next development for the Rotating Detonation Engine System. Previously, the team had used propellent tanks that were available for purchase, but Kasahara believes that results could be improved by developing their own tank design. Additionally, while the circular motion of the detonation wave allows the wave to keep igniting fresh fuel, the stability of the wave is difficult to maintain for more than a few minutes.
“We want to work on making the engine run longer,” explains Kasahara. “To do this, we will develop engines that include thermal protection systems. We are also planning to try to cluster the engines. Our aim is to conduct the world’s first orbital satellite demonstration of the detonation engine!”
Yamada is also focussed on a reentry mission from orbit as the next step on the way to Mars.
“The deployable aeroshell is eventually expected to be applied to a Mars lander,” Yamada says. “We will conduct repeated experiments using sounding rockets to mature the technology, and then we are considering a re-entry mission from Low Earth Orbit as the next step of technological demonstration. If we succeed in this technological demonstration, we believe that we will be able to pave the way to Mars.”
The sounding rocket tests are amazing glimpses of space technology that is in the development pipeline, show casing some of our most innovative ideas. Our future interplanetary journeys may use rotating detonation engines for efficient space transport, and drift down to the planet surface onboard an inflatable RATS ring.
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