Atmospheric entry

Most objects enter at hypersonic speeds due to their sub-orbital (e.g., intercontinental ballistic missile reentry vehicles), orbital (e.g., the Soyuz), or unbounded (e.g., meteors) trajectories.

In the United States, this technology was pioneered by H. Julian Allen and A. J. Eggers Jr. of the National Advisory Committee for Aeronautics (NACA) at Ames Research Center.

With a sufficiently small half-angle and properly placed center of mass, a sphere-cone can provide aerodynamic stability from Keplerian entry to surface impact.

The Mk-2 had significant defects as a weapon delivery system, i.e., it loitered too long in the upper atmosphere due to its lower ballistic coefficient and also trailed a stream of vaporized metal making it very visible to radar.

Subsequent advances in nuclear weapon and ablative TPS design allowed RVs to become significantly smaller with a further reduced bluntness ratio compared to the Mk-6.

On Columbia's maiden flight (STS-1), astronauts John Young and Robert Crippen had some anxious moments during reentry when there was concern about losing control of the vehicle.

Under a perfect gas model there is an elegant set of equations for determining thermodynamic state along a constant entropy stream line called the isentropic chain.

[21][22] The Lighthill-Freeman model initially assumes a gas made up of a single diatomic species susceptible to only one chemical formula and its reverse; e.g., N2 = N + N and N + N = N2 (dissociation and recombination).

The experimental measurement of radiative heat flux (typically done with shock tubes) along with theoretical calculation through the unsteady Schrödinger equation are among the more esoteric aspects of aerospace engineering.

Non-equilibrium air in the shock layer is then transported past the entry vehicle's leading side into a region of rapidly expanding flow that causes freezing.

Consequently, for entry trajectories causing lower heat flux, carbon phenolic is sometimes inappropriate and lower-density TPS materials such as the following examples can be better design choices: SLA in SLA-561V stands for super light-weight ablator.

For example, an ablative heat shield loses most of its thermal protection effectiveness when the outer wall temperature drops below the minimum necessary for pyrolysis.

[51][52] Various advanced reusable spacecraft and hypersonic aircraft designs have been proposed to employ heat shields made from temperature-resistant metal alloys that incorporate a refrigerant or cryogenic fuel circulating through them.

[citation needed] In 2005 and 2012, two unmanned lifting body craft with actively cooled hulls were launched as a part of the German Sharp Edge Flight Experiment (SHEFEX).

[55][56] The Stoke Space Nova second stage, announced in October 2023 and not yet flying, uses a regeneratively cooled (by liquid hydrogen) heat shield.

[57] In the early 1960s various TPS systems were proposed to use water or other cooling liquid sprayed into the shock layer, or passed through channels in the heat shield.

The current design is shaped like a shallow cone, with the structure built up as a stack of circular inflated tubes of gradually increasing major diameter.

The forward (convex) face of the cone is covered with a flexible thermal protection system robust enough to withstand the stresses of atmospheric entry (or reentry).

There are four critical parameters considered when designing a vehicle for atmospheric entry:[citation needed] Peak heat flux and dynamic pressure selects the TPS material.

After shock wave detachment, an entry vehicle must carry significantly more shocklayer gas around the leading edge stagnation point (the subsonic cap).

[76] The reentry was a major media event largely due to the Cosmos 954 incident, but not viewed as much as a potential disaster since it did not carry toxic nuclear or hydrazine fuel.

[79][49][80] The station had been boosted to a higher orbit in August 1986 in an attempt to keep it up until 1994, but in a scenario similar to Skylab, the planned Buran shuttle was cancelled and high solar activity caused it to come down sooner than expected.

On September 7, 2011, NASA announced the impending uncontrolled reentry of the Upper Atmosphere Research Satellite (6,540 kilograms [14,420 lb]) and noted that there was a small risk to the public.

[81] The decommissioned satellite reentered the atmosphere on September 24, 2011, and some pieces are presumed to have crashed into the South Pacific Ocean over a debris field 500 miles (800 km) long.

[82] On April 1, 2018, the Chinese Tiangong-1 space station (8,510 kilograms [18,760 lb]) reentered over the Pacific Ocean, halfway between Australia and South America.

[84] On May 11, 2020, the core stage of Chinese Long March 5B (COSPAR ID 2020-027C) weighing roughly 20,000 kilograms [44,000 lb]) made an uncontrolled reentry over the Atlantic Ocean, near West African coast.

[87][88] On May 8, 2021, the core stage of Chinese Long March 5B (COSPAR ID 2021-0035B) weighing 23,000 kilograms [51,000 lb]) made an uncontrolled reentry, just west of the Maldives in the Indian Ocean (approximately 72.47°E longitude and 2.65°N latitude).

On February 21, 2008, a disabled U.S. spy satellite, USA-193, was hit at an altitude of approximately 246 kilometers (153 mi) with an SM-3 missile fired from the U.S. Navy cruiser Lake Erie off the coast of Hawaii.

U.S. Department of Defense expressed concern that the 1,000-pound (450 kg) fuel tank containing highly toxic hydrazine might survive reentry to reach the Earth's surface intact.

[92] Considering space sustainability in regard to atmospheric impact of re-entry is by 2022 just developing[94] and has been identified in 2024 as suffering from "atmosphere-blindness", causing global environmental injustice.