REIMAGINING INTERSTELLAR'S SHUTTLES: Single Stage to Orbit via Nuclear Lightbulb Propulsion
Abstract
It is common in modern space films for characters to be able to travel between a planet’s surface and orbital space with ease via a small shuttle. Despite the many difficulties inherent in reaching orbit, these shuttles are almost always compact, single-stage, and can operate in a wide variety of environments without dedicated retrofitting. Christopher Nolan’s film Interstellar is no exception to this trend, featuring two craft that match this description. However, the Ranger and Lander do not follow modern aerospace conventions, as their designs appear to emphasize aesthetics over functionality. Modern single-stage-to-orbit (SSTO) designs, while realistic, fall far short of the Ranger and Lander’s capabilities as they were designed for maximum efficiency in a specific set of environments.
The question then arises as to whether a realistically designed vehicle with the Ranger and Lander’s capabilities is physically possible at all, using only currently understood physics and technology. The author presents a Nuclear Cargo Vehicle (NCV) concept spacecraft with performance capabilities comparable to those demonstrated by Interstellar’s shuttles. Analyses of major subsystems such as the nuclear reactor, radiation shielding, thermal management system, payload bay, and others are given. Sample mission profiles for various Solar System destinations are provided to demonstrate the extent of the craft’s abilities. Finally, select scenes from Interstellar are reimagined using the NCV in place of the film’s shuttles, demonstrating that the design can work within the constraints of the story.
The Multi-Purpose Orbital Shuttle
One of the most difficult phases of any space mission is the initial launch to orbit. Fighting against Earth’s gravity and atmospheric drag, while attempting to accelerate to orbital velocities, requires tremendous energy expenditure to deliver even relatively small payloads into low Earth orbit (LEO). However, in fictional works set in space, launching to orbit and atmospheric entry are often largely ignored, and are treated as just another way to move characters from point A to point B. Instead of requiring large rockets, parachutes, or heat shields, fictional shuttles are often no larger than a fighter jet and can reach orbit without discarding any hardware. This type of craft has appeared in countless science fiction franchises under many different names; for our purposes, it will be classified as a Multi-Purpose Orbital Shuttle (MPOS).
General Description
Despite its many different incarnations, all MPOS variants share a select few defining features that distinguish them from other classes of fictional spacecraft.
First, a MPOS must be capable of reaching orbit under its own power without discarding any hardware – a SSTO. While this feature is common among fictional spacecraft, it is more notable in the MPOS because there is often little to no room reserved for fuel tanks despite the incredible energy expenditure required to reach orbit.
Second, a MPOS must be compact in size. Modern launch vehicles are as tall as several-story buildings, requiring large gantries and launch towers to move them into position. On the other hand the MPOS is often much smaller than the Space Shuttle Orbiter, and can sometimes be as small as an average fighter jet.
Third, and perhaps most importantly, a MPOS must be able to operate reliably in a wide range of environments without being specifically adapted to each one. Modern space vehicles are custom built for the environments they will encounter on their mission. For example, the ESA’s Huygens probe would not have fared too well if it had been sent to Venus instead of Titan. This is not the case for the MPOS, as it is often shown as being able to fly down to virtually any alien world with little to no concern over the specific planetary and atmospheric conditions it will encounter. As such, the vehicle cannot rely on engines that require a specific atmospheric composition, like turbojets or ramjets.
Fourth, a MPOS must be durable and require only minimal maintenance. The few partially reusable spacecraft that have been built to date, such as the Shuttle, require extensive maintenance and retrofitting after each flight. In the extreme case of the Shuttle, its main engines had to be completely disassembled and rebuilt in order to be used again. This is never the case for the MPOS, which is often ready to fly at a moment’s notice. Maintaining a MPOS is more similar to maintaining a modern jet airliner than a modern spacecraft, as refueling and basic system checks are usually all that is required in between flights.
Overview of Interstellar’s Shuttles
Out of the many spacecraft featured in the film, two of them – the Ranger and Lander shuttles – can be classified as MPOSs according to the preceding description.
Ranger
The more prominent of the two shuttle varieties carried by Endurance, the Ranger is a small vertical takeoff vertical landing (VTVL) SSTO capable of transporting four crewmembers and one robot to and from the surfaces of alien worlds. Roughly ~19 m long and ~2.5 m tall, the shuttle is quite sleek, resembling experimental hypersonic aircraft such as NASA’s X-43 or DARPA’s HTV series. A combination of chemical and plasma rockets power the craft, providing either high thrust or high efficiency when needed. Maneuvering in vacuum is accomplished through hydrazine-fueled RCS ports. The Ranger is coated with a layer of heat-resistant tiling on its lower hull in order to survive planetary entry even from interplanetary speeds. The upper hull is dotted with numerous high-strength windows, giving the crew an ample field of view of their surroundings. For long-duration missions, it also carries four cryogenic stasis units to reduce life support and power expenditure.
Lander
The larger of the two shuttle varieties (~20 m long and ~5 m tall), the Lander is described by its designers as acting “like a heavy lift helicopter,” as it is tasked with transporting supplies from Endurance to potentially habitable worlds. Unlike the Ranger, the Lander is not at all sleek and aerodynamic, instead adopting a much bulkier appearance. It is possible this was inspired by the Apollo Lunar Module’s ascent stage, with box-like protrusions made of triangular and rectangular plates. Due to its shape, the craft has its heat tiling on the upper hull instead of the lower hull, requiring it to perform planetary entry upside down. The passenger seats are mounted on swivels to allow them to flip accordingly. The rest of the Lander’s capabilities mirror the Ranger quite closely – it has chemical and plasma engines, hydrazine RCS, numerous windows, a crew capacity of four people and one robot, and VTVL SSTO capability much like its sleeker sister ship.
Realism Assessment
The Ranger and Lander are something of a mixed bag in terms of realism. According to behind-the-scenes material for Interstellar, Nolan and his team pored over several existing spacecraft designs when looking for inspiration for the shuttles. In particular they focused on the Space Shuttle Orbiter, as it was the most similar the craft they were designing. Their intent was to ensure that the film’s shuttles evoked a “NASA aesthetic,” as if the craft were designed with functionality in mind.
In terms of aesthetics, Nolan’s team succeeded. Several components of the Ranger and Lander designs, such as white/black thermal tiles, small & thick windows, and control panels covered with switches and readouts are very similar to their counterparts on the Shuttle. Unfortunately, the team’s assembly of these components into a complete design is lacking in basic realism in four main areas.
First, the shuttle interiors are abnormally spacious. Modern surface-to-orbit spacecraft, in an attempt to conserve mass, often have very cramped cabins. For example, the Space Shuttle housed up to seven astronauts in only ~66 m3 of cabin space. The Ranger and Lander, on the other hand, each have enough room for Endurance’s entire crew to stand and walk around in, despite the fact that this extra space appears to serve no functional purpose.
Second, the shuttles appear to lack fuel and propellant tanks. After accounting for visible engines and the crew cabin, almost no internal volume is left to devote to fuel. This is nearly the opposite of modern launch vehicles, which can in some ways be thought of as large flying fuel tanks with some spare room left over for engines and a small payload. It is especially problematic considering that the Ranger and Lander are supposed to be SSTOs, a task which requires a considerable mass ratio for any reasonable specific impulse.
Third, despite being VTVL-capable, neither craft appears to have any ventral thrust outlets. Any craft capable of switching from horizontal flight to vertical flight needs some way of directing the propellant flow downward, but the shuttles’ main engines are essentially locked in place. The one time in the film where the Ranger’s underside is visible, it appears to only feature thermal insulation tiles, and concept art for the underside of the Lander also does not show any vertical engines. How the shuttles are able to hover above the ground remains a mystery.
Fourth, neither craft’s hull is aerodynamically sound given the various regimes each is expected to fly through. The Lander is not streamlined in any way, as its design was apparently based off of a craft intended for use in a vacuum. As for the Ranger, its main hull is streamlined, but protruding from either side is a long, pointed prong-like structure that serves no apparent purpose. Such a structure could in fact endanger the craft at supersonic and hypersonic speeds, as interfering shockwaves could produce aerodynamic forces sufficient to tear the prongs off the shuttle and severely damage it.
It is clear from these four main oversights that while the Ranger and Lander excel in form, they lack in function and cannot serve as a realistic MPOS design. For that, we must look elsewhere.
Existing Proposals
SSTO designs promise many advantages over current rocket technology, such as reusability, low costs, and relatively quick turnaround times. Because of these, many design proposals have been put forth for SSTOs, although none have yet been built. This report summarizes three such proposals:
National Aero-Space Plane
The Rockwell X-30, or National Aero-Space Plane (NASP), was an attempt in the late 1980s and early 1990s to create a viable SSTO based on scramjet technology. The end goal was to create a craft that could take off from Washington, DC and reach Tokyo in two hours or less by taking a suborbital arc around the planet. Alternatively, a short rocket burn at the highest point in its trajectory could boost the craft into orbit. While in atmosphere, the NASP would be able to accelerate to hypersonic speeds, even above Mach 20, using an integrated ventral scramjet engine. The program was cancelled before any prototype craft were constructed, but considerable progress was made in developing high-strength materials to withstand the extreme heat on the vehicle’s leading edges caused by travelling at such high speeds.
VentureStar
The VentureStar, as well as its smaller technology demonstrator, the Lockheed Martin X-33, was an attempt to replace the Space Shuttle in the late 1990s using high-efficiency linear aerospike engines. VentureStar would have launched vertically off of a launch pad, delivered a roughly 20 metric ton payload to LEO, and then returned to land on a runway like a conventional aircraft. The use of a single simpler engine system, as well as a new high-strength metallic heat shield, would have made the craft fully reusable and much less maintenance-heavy than the Shuttle. Unfortunately, problems with constructing the X-33’s double-lobed forward fuel tank eventually led to its cancellation in 2001.
Skylon
Of the three SSTO proposals in this report, the Skylon is the only one still being considered for development in any reasonable capacity. The design is based on the Synergistic Air-Breathing Rocket Engine (SABRE) by Reaction Engines Limited, a novel fusion of a turbojet and rocket engine. SABRE contains a lightweight precooler that can drastically lower the temperature of incoming air, greatly increasing engine efficiency in a wider range of speeds and altitudes. Once Skylon has reached a certain height and speed in the atmosphere, SABRE can then switch modes and turn into a rocket engine, sealing off external air intakes and switching to onboard liquid oxygen (LOX) to lift the craft to orbit. If successful, Skylon could be lifting 15 metric tons to orbit multiple times a week within a decade.
MPOS Viability
While all three of these design concepts – NASP, VentureStar, and Skylon – are viable and realistic SSTOs, they ultimately fall short of the requirements for the MPOS and cannot serve as substitutes for the Ranger or Lander.
First, the craft are all far too large. Due to the relative inefficiency of chemical rockets and the resulting high fuel fractions, each design requires a vehicle comparable in size to a passenger airliner. Second, the craft all require existing ground-based infrastructure to operate. All three require a large runway for landing, meaning they would be unable to carry payloads to the surface of an unexplored world. Third, the craft are optimized to perform in a single environment, namely, Earth’s surface to LEO and back. Various planetary conditions, such as atmospheres of different thicknesses and compositions, could easily render the vehicles completely unusable.
Because of these limitations, a new form of spacecraft, capable of independently operating in a wide variety of ambient conditions, will be required to adequately replace the Ranger and Lander. This is the role that the NCV concept spacecraft attempts to fill.
