Germany’s POLARIS is testing a spaceplane approach that assigns different jobs to jet engines and rocket propulsion. Its MIRA II demonstrator uses four turbojets alongside a linear aerospike rocket engine. According to the company’s demonstrator record, the aircraft ignited that rocket engine in flight on October 29, 2024. The test brought an unusual propulsion arrangement into an actual aircraft operating environment.
The basic reasoning starts with oxygen. Jet engines take oxygen from the atmosphere, allowing an aircraft to use the surrounding air during suitable portions of flight. Rocket engines carry their oxidizer onboard and can operate when atmospheric oxygen is insufficient for conventional jet propulsion. Combining the systems allows a proposed vehicle to use aircraft propulsion around an airport and rocket power for higher-performance portions of its mission.
POLARIS is pursuing that combination through successive demonstrators. MIRA II is approximately 5 meters long, with a design takeoff mass of 240 kilograms and a 1-kilonewton AS-1F rocket engine. It uses kerosene and liquid oxygen for rocket propulsion. These figures describe a test aircraft, giving engineers a manageable platform for investigating propulsion integration, controls, and flight procedures before applying the approach to larger vehicles.
The aerospike changes how rocket exhaust expands. In a conventional bell nozzle, the surrounding nozzle wall guides the hot gases. An aerospike directs exhaust along an external shaped surface, allowing ambient atmospheric pressure to influence the plume boundary. As pressure changes with altitude, the exhaust can adjust its expansion. This property is called altitude compensation, and it is one reason engineers have studied aerospikes for vehicles flying through a large altitude range.
A NASA technical report on the X-33 linear aerospike explains another attraction: integration with the vehicle’s structure. It describes potential performance and weight benefits, along with control through differential throttling of combustion elements. Adjusting the output of separate elements can change the direction of the resulting force. That historical design illustrates the broader possibilities of the architecture, without establishing that POLARIS uses the same control implementation.
For POLARIS, the engine’s shape is part of the argument. Its propulsion description identifies the compatibility of a linear aerospike with a relatively flat flying-wing configuration. A linear engine arranges its exhaust-producing elements along a line, rather than around a circular bell. The aircraft and propulsion system can consequently be designed together, making engine placement and airframe geometry closely related decisions.
New Space Economy’s aerospike background article places the idea within decades of rocket-engine development. The present interest is the opportunity to test it repeatedly in flight. A ground firing establishes engine behavior under controlled conditions. An airborne test adds changing pressure, airflow around the vehicle, movement, and interaction with onboard systems, providing another category of evidence for the engineering team.
The supporting research also explains why those interactions deserve attention. In a 2025 aerospike simulation study, Zachary Pyle and Gustaaf B. Jacobs developed methods for resolving unsteady exhaust flow. Their work examined rapidly changing flow structures under supersonic and hypersonic conditions. Although the paper is independent of the POLARIS aircraft, it shows that evaluating an aerospike involves time-dependent gas behavior as well as an average thrust value.
For a nontechnical audience, the practical point is that an engine experiences a changing environment throughout a flight. Its exhaust interacts with surrounding air, and the aircraft must remain controllable as thrust and aerodynamic forces change. Repeated testing allows engineers to compare predictions with measurements under progressively different conditions. A flight demonstrator is useful because the propulsion system, sensors, control software, and airframe operate together.
POLARIS has also invested in the operating infrastructure needed for that work. Its company news record documents the opening of a second location at Peenemünde Airport in January 2025 and the completion of 250 flights across its vehicle fleet by November 2025. The total covers the demonstrator program, rather than 250 aerospike-powered flights. A permanent airport location gives the team a base for preparation, maintenance, and successive flight campaigns.
Government procurement has added a larger development objective. In January 2026, POLARIS announced a German defense procurement contract to manufacture and operate a two-stage, horizontally launched, reusable hypersonic research vehicle. The Aviationist’s reporting described a requirement for flight readiness by the end of 2027 and connected the award to earlier engine and design work. The planned system would serve as a research platform with civilian and defense applications.
The company’s VEGA description helps explain the distinction between research and satellite launch. The hypersonic test configuration is described as a reusable two-stage system. A satellite-launch application would use an expendable upper stage. These are separate mission arrangements, and the hardware returned after flight depends on which arrangement is used. The research vehicle’s reuse should consequently be described in relation to its specific mission.
POLARIS also presents a longer-term AURORA spaceplane concept derived from German Aerospace Center work. This is a different program from Dawn Aerospace’s similarly named Aurora. POLARIS proposes applications including suborbital experiments and satellite delivery, with a separate upper stage for orbital missions. The common runway concept links these ambitions, but the smaller demonstrators provide the current flight evidence.
The sequence also creates useful intermediate products. An organization studying high-speed sensors or materials may need a recoverable test flight well before it needs satellite delivery. The proposed hypersonic platform addresses that research requirement directly. From a commercial perspective, this gives flight testing a customer-facing purpose: collecting experimental data under specified conditions and returning the equipment so that a research team can inspect and improve it.
Combining jets and rockets is ultimately a way to match propulsion to different parts of a mission. POLARIS’s aerospike adds a nozzle architecture intended to accommodate changing altitude and fit its aircraft shape. The MIRA tests, airport facilities, and larger research-vehicle contract turn that engineering proposition into a sequence of measurable activities. The resulting knowledge will concern the complete operating vehicle: how it takes off, accelerates, conducts a test, returns, and prepares for another flight.
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