The dangers SpaceX’s Crew Dragon will face during its return to Earth
Reentry danger points
The extreme velocities and temperatures the vehicle must endure present a major challenge to engineers and makes reentry the most perilous part of a mission.
The danger starts withfinding the right angleof the trajectory as the spacecraft enters the upper atmosphere. If it is too steep, the astronauts will experience potentially fatal g-forces, and the friction of the air drag could cause the spacecraft to explode. If it is too shallow, the capsule will instead catastrophically skip off the atmosphere and back into Earth orbit.
The spacecraft will enter the upper atmosphere at 27,000km/hour. That is 7.5km/second, or more than 20 times the speed of sound. In whichever units you prefer – this is fast. At these velocities, a very strong shock wave forms around the front of the vehicle, compressing and superheating the air. Managing the immense thermal load is a huge reentry engineering challenge.
At the most extreme stage, the temperature of the air in the shock layerexceeds 7,000°C. By comparison, the temperature at the surface of the Sun is around 5,500°C. This makes the vehicle’s heat shield so hot that it starts to glow — a process called incandescence. SpaceX’s new and advancedPICA-X material heat shieldhas managed to protect the capsule in test flights, later being recovered in a verycharred state.
The air molecules around the vehicle also break down into positively charged atoms and free electrons –a so-called plasma. When some of the molecules recombine, excess energy is released as photons (light particles) – giving the air around the vehicle an amber glow.
This plasma layer may be beautiful, but it can cause radio blackouts. When an electron travels along a conductive wire, we have electricity. Similarly, when free electrons move through the plasma around the vehicle, we have an electric field. If the electric field becomes too strong, it can reflect and attenuate the radiowaves trying to reach the spacecraft.
Blackout not only leads to a loss of connection to on-board crew and flight data, it can also make remote control and guidance impossible. TheApollo missions, theMars Pathfinderand the recent, failed2018 Soyuz rocket launchall incurred communications blackout on the order of minutes. NASA mission control are anticipating a nervoussix minutes of blackoutduring the peak heating phase of Crew Dragon’s return – if anything goes wrong during this time, it’s in the hands of the astronauts.
Another risky stage is the parachute-assisted landing. The Crew Dragon will deploy four parachutes upon the final stage of reentry, as the vehicle descends toward a gentle splashdown in the Atlantic Ocean off the coast of Florida. This maneuver has been tested by SpaceX27 timesprior to next week’s crewed landing, so it should work.
Future goals
A successful landing will have huge implications – lowering the cost of space exploration through the use ofreusable rocketsand enabling private space exploration. While SpaceX engineered the Crew Dragon vehicle under contract to NASA, the company is free to use the spacecraft for commercial flights without NASA involvement after operational certification.
SpaceX has a partnership with commercial aerospace companyAxiom Space, which has the ultimate goal of building the world’s first commercial space station. The proposed commercial activities for the station are broad: from in-space research and manufacturing to space exploration support.
Then there is space tourism. Private citizens are already queuing for their ticket to space, and with a successful Crew Dragon splashdown, they won’t be waiting long. American space tourism company,Space Adventures(partnered with SpaceX), are planning to offer zero-gravity atmospheric flights, orbital flights with a spacewalk option and laps of the Moon by late 2021.
Whether the costs, environmental impact, and dangers of spaceflight are justified for space tourismis debatable. As this articles shows, the required safety briefing for Space Adventure ticket holders will be much more comprehensive than your regular “please take a moment to read the safety card in the seat pocket in front of you.”
This article is republished fromThe ConversationbyHeather Muir, PhD in Computational Physics,University of Cambridgeunder a Creative Commons license. Read theoriginal article.
Story byThe Conversation
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