noahdemar.com

Relearning Reentry

From Cold War origins to hot space startups

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3D re-creation of a Zenit film-return mission: this satellite could photograph 60x60km swaths the ground from orbit.

Foreword

64 years ago, the Soviets had already solved the problem a generation of startups are now racing to rediscover.

In 1962 The USSR recovered their first film-return capsule, and went on to recover nearly 700 more before most of today's founders were born.

Now Varda is producing pharmaceuticals in orbit, Inversion is selling orbital logistics as a service, and SpaceX is scaling reentry to the tonne. Every one of them is chasing the coat-tails of a cold-war capsule. They are relearning reentry, at venture speed, and with it the hard won lessons of Zenit's cold war's legacy.

01 / Zenit

Quantity Has a Quality All Its Own

Zenit-2 was the Soviet Union's first photo-reconnaissance satellite. While, not first Zenit was -by sheer quantity the most launched film return capsule satellite. Unlike the American CORONA program, which ejected just the film, Zenit used a climate controlled return capsule that returned both camera and photographs to Earth.

A typical vehicle carried four cameras and a radio dish for electronic eavesdropping. A typical mission lasted 8 to 15 days before the descent vehicle was deorbited and recovered on the Soviet steppe. 54

The very first launch, on 11 December 1961, was lost when the R-7 rocket's third stage failed and the satellite triggered its own self-destruct charge; the second vehicle flew successfully as Kosmos 4 in April 1962 and re-entered Earth's atmosphere three days later. Across the whole Zenit family, 688 satellites were launched between 1961 and 1994 and 21 were lost. 5

Zenit Why climate controlled?

The Zenit-2 capsule is a dual-use sibling of the Vostok manned space capsule, piloted by the first cosmonaut: Yuri Gagarin.

This lifesupport legacy continues in recent times, with parts of the Zenit satellite still used in the Foton M-4 mission. Launched in 2014 with a biology research payload Foton M-4 carried geckos, silkworm eggs, dried seeds, fruit flies, and mushrooms into low earth orbit.

Zenit was a workhorse of film-return reconnaissance: Across ten variants and nearly 700 launches the Zenit capsule enjoyed 30+ years of service. Click to replay.wiki

02 / Zenit-2

Precious Cargo

The spacecraft optics required that internal temperatures be kept within 1 degree Centigrade, and that temperature variations be no more than 0.1 degrees/hour. Since the spacecraft was normally oriented to the sun, an active thermal regulation system was needed. Furthermore, The requirement for 10-15 m camera resolution from 200-400 km altitude and a speed of 8,000 m/s meant that a motion compensation system was needed. Pointing requirements necessitated the use of gyroscopes and infrared horizon sensors; all commanded from the ground. In the event of malfunction autonomous systems could trigger a self-destruct sequence. 5

Shown here is an advanced Zenit design that has both optical and radio surveillance sensors. It also uses aerodynamics surfaces to maintain orientation as it slices through the upper atmosphere at perigee.5

Deorbit & recovery

Reentry and Return

Zenit was subjected to scalding surface temperatures of 2500 to 3500 deg C on re-entry, 8 to 9 G's maximum load, with a resulting heat shield mass of 1300 to 1500 kg

Landing accuracy ±100–170 km from the aim point astronautix

Note that even a small error in the retro-burn moment throws the landing point hundreds of kilometres downrange.

Try it yourself:

Even with perfect timing accuracy atmospheric turbulence, orientation, and differential drag can massively change the final landing point. Today's capsules can correct for some of these effects, and can leverage them for a precision landing away from their original trajectory.

03 / Startups

Film return, reinvented

Interest in recovering capsules from space has resurged in the 2020s. Much like the chemical reactions in a Cold War film reel, or biological experiments in more recent times; new space startups seek riches producing pharmaceutical drugs in space. While the business model is new, the challenges of the 1960s remain.

Varda Space Industries

Pharma in orbit, profit on Earth

Founded in January 2021, Varda is the busiest of the new reentry startups: six W-series capsules have flown, beginning with W-1 on SpaceX's Transporter-8 in June 2023. Each rides up as a rideshare payload, grows pharmaceutical crystals in microgravity, then separates and reenters on its own; landing in the Utah desert or, lately, the Koonibba range in South Australia. W-1 famously loitered ~8 extra months in orbit waiting on its FAA reentry licence before recovery in February 2024; by W-6 (reentered May 2026) the cadence had become routine. 513

Company platform
Inversion

Return-as-a-service, one flight in

Founded in 2021 by two Boston University students in a Los Angeles garage, Inversion sells reentry as an on-demand service rather than building its own factory. Its track record is still young: one flight, the 63 kg Ray demonstrator on SpaceX's Transporter-12 in January 2025; a mission that completed its orbital shakedown but not the planned reentry. In October 2025 the company unveiled its maneuverable design: Arc, designed to bring roughly 500 lb (~227 kg) of cargo home on demand. 614

Arc overview
SpaceX Starfall

The latecomer with the biggest puck

SpaceX, founded in 2002, is last to the party but brings the largest vehicle. Starfall is a pancake-shaped reentry "puck" at ~2,100 kg empty - it claims to be capable of ferrying 1,000 kg home from orbit. It is about to fly its first demonstration mission (June 2026), riding a Falcon 9 to low Earth orbit from Cape Canaveral before deorbiting for a Pacific splashdown hours later. No flights to date, but a return in scale to the Zenit design. 7

SpaceX updates

Relative size

Starfall
Zenit-2
Varda W
Inversion Ray
Human

Drawn to a relative scale Zenit dwarfs all but the Starfall reentry "puck"

Payload weight

VehicleCapsule massReturnable payloadNotes
Zenit-2 film-return capsule~2.4 t~340 kg pressurized
~1400 kg bulk		Two conditions, inferred from the Vostok SA mass budget.astronautix Pressurized: the ~340 kg is with a temp/pressure controlled payload volume (as flown on mission). Bulk: capsule (~2.4 t) minus heat shield (~0.84 t) and parachute (~0.15 t) ≈ 1.4 t of inert mass as an upper limit.
Varda W-series<90 kg~10 kgCapsule total (heat shield included) is <90 kg.wiki The Payload User Guide supports up to 10 kg of interior payload returned to the customer.PUG
Inversion Ray18 kgunknown (~>1kg est.)Ray is a reentry demonstrator with no customer payload.businesswire Proportional estimate see footnote.
SpaceX Starfall~2,100 kg empty1,000 kgAdvertised customer-cargo capacity; 3.1 m disk, ~3,100 kg fully loaded.
Possibly an "Elon Number."

Why "returnable payload" is so much smaller than capsule mass: a return capsule's mass is dominated by heat shield, structure, and avionics; only a fraction comes home as useful cargo.

04 / Optimization

Orbit Optimization

For a factory in space, we need to optimize for multiple objectives. Some key tradeoffs are:

  • Minimize wasted mass (heat shielding)
  • Maximize power availability
  • Maximize landing site overpass time
  • Minimize landing site latency

Heat Shield Optimization

The chart shows a simplified ballistic reentry profile from a 120 km entry altitude. Entry angle, velocity, and vehicle geometry change the downrange arc and the heat flux each shield material can survive. Stagnation heating follows the FAA Returning from Space: Re-entry model.12

q̇ ≈ 1.83×10⁻⁴ · V³ · √(ρ / r_nose)  [W/m²]

Each vehicle's drag coefficient and ballistic coefficient (BC = m / (CD·A)) come from Newton's method for its shape121314 — Zenit a sphere (CD = 2.0); Starfall and Ray blunt bodies (large nose radius, low heating); and Varda the outlier: a 45° cone flown apex-first (CD = 2·sin²δc). That sharp geometry gives Varda a small nose radius (like an ICBM reentry vehicle) and by far the highest peak heating of the four. This penalty is an intentional engineering tradeoff that I'll describe below...

The climbing cyan curve is the cumulative heat load, Q = ∫ q̇ dt. Zenit flew a steep ballistic entry, and counter-intuitively that minimizes total heat: a steeper angle raises the peak rate (q̇max ∝ √sin γ) but shortens the exposure, so the integral falls (Q ∝ 1/√sin γ). Steepen the angle and watch peak q̇ climb while total Q drops. It's better to just get it over with if your payload can handle the G forces.

TLDR; If you can't maneuver much you want a steep reentry angle. Elliptical orbits can achieve this but you incur high max heat over a short time.
All this said... ceramics have advanced a great deal since 1960s. Varda seems to enter cone side in. This is backwards to the conventional blunt-body (pointy end away from the atmosphere) design that many capsules use. So I'm guessing they made the tradeoff for simplify the control system. Inversion seems to have the opposite apporach their follow on spacecraft is a lifting body to maximize manuever and minimize g loading.

~0th-order model; heating & deceleration per FAA III.4.1.7.1210

Power vs. Landing-Zone Overflight

Heat shielding is only one consideration. An orbital factory also has to keep the lights on and deliver its product to the delivery zone. For any landing site outside of the artic circle, these two goals pull the orbit in opposite directions. I show three candidate orbits: a low-inclination orbit that overflies the site constantly but sits in eclipse frequently, a sun-synchronous orbit optimized for solar power but rarely over the site, and the weighted compromise between them.

Power vs. Coverage

The amber bar is the fraction of each orbit in sunlight — also drawn as the orbit ring, gold where sunlit and grey through the eclipse arc. The green bar is the site's view-period coverage you can think of this as the percentage of the life of the satellite its over the intended landing zone.

Visit sixthsensor.io if you want to design an optimal constellation for this purpose. *Hint* It likely wont be a Walker Delta configuration

Power: beta-angle w/ J2 gravity model. Coverage: in-view period ratio from the rapid ergodic-coverage method.11

05 / Conclusion

Final Answer

Reentry is hard. And considering the USSR isn't pumping out Zenits anymore, we are forced to relearn these lessons.

Almost every objective in this problem is in diametric opposition. I would love to study this problem more but I'd like to time this article with the Starfall launch. Part 2 to be continued.

If you worked on any of these spacecraft and want to see an update on this article, please reach out to me: noahdemar2021@u.northwestern.edu

05 / References

References

Information gathered from various sources, often with conflicting and/or uncorroborated figures. For that reason I link my sources here.

  1. Gunter Krebs. “Advanced Zenit.” Gunter's Space Page.
  2. Astronautix. “Zenit-2 satellite.” Mission, specifications, and flight record. Astronautix page.
  3. Hicks, Kerry D. Introduction to Astrodynamic Reentry. AFIT/EN/TR-09-03, 2009. Local PDF.
  4. Allen, H. Julian, and Eggers, Alfred J., Jr. “A Study of the Motion and Aerodynamic Heating of Ballistic Missiles Entering the Earth's Atmosphere at High Supersonic Speeds.” NACA Report 1381, 1958/1959.
  5. Varda Space Industries. “W-Series: Built for orbital material production and reentry.” Varda platform page.
  6. Inversion. “Arc” and company reentry vehicle descriptions. Arc overview.
  7. SpaceX. “Starfall” orbital cargo return service announcements and Starship recovery updates. SpaceX updates.
  8. Acta Astronautica. “Guidance, navigation, and control solutions for spacecraft re-entry point targeting using aerodynamic drag.” DOI: 10.1016/j.actaastro.2018.10.016. Local HTML snapshot.
  9. Yousefi, Mohammad, et al. “Computational Design of a CubeSat Compatible Re-entry Capsule.” 8th EUCASS, 2019. Local PDF.
  10. A Rapid Method for Orbital Coverage Statistics — view-period ratio (ρ) via ergodic theory for rapid ground-coverage assessment. AAS/AIAA, 2021 (paper 2102).
  11. FAA. “Returning from Space: Re-entry” (§III.4.1.7), Space Transportation reference. Stagnation heating rate q̇ = 1.83×10⁻⁴·V³·√(ρ/r_nose). FAA PDF.
  12. FAA. Final Environmental Assessment for Reentry, Landing, and Recovery Operations of a Varda Space Industries Capsule within Utah Test and Training Range (UTTR) South or Northern Dugway Proving Ground, Utah, February 2024. Capsule: ~3 ft (0.9 m) dia × 2.5 ft (0.76 m) tall, <200 lb (<90 kg), 45° aluminum sphere-cone. FAA EA PDF.
  13. FAA. Draft Environmental Assessment for Reentry, Splashdown, and Recovery Operations of an Inversion Space Company Capsule within the Pacific Ocean off the Coast of Central California, May 2024. Ray capsule: aluminum sphere-cone 20.5 in (52.1 cm) dia × 11.8 in (30.0 cm) tall, ~40 lb (18 kg); splashdown ~45 nm (83 km) offshore. FAA EA PDF.
  14. FAA. Dynamic Regulatory System (DRS) reentry-licensing record, 2026. FAA DRS document.