Uncrewed Solar Aircraft Passes Key Ground Tests

September 1, 2025

German Aerospace Center (DLR), Cologne, Germany

The high-altitude uncrewed platform HAP-alpha being developed by DLR will be capable of ascending into the lower stratosphere – at approximately 20 kilometers (12 miles) in altitude. (Image: DLR)

High-altitude uncrewed aircraft can remain in the lower stratosphere for extended periods, performing a wide range of Earth observation and communications tasks – from monitoring shipping lanes and supporting disaster response to providing internet access. The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) has now taken an important step in the development of its own high-flying solar aircraft by successfully completing a Ground Vibration Test (GVT) on its innovative HAP-alpha high-altitude platform. Extensive ground trials took place at DLR’s National Experimental Test Center for Unmanned Aircraft Systems in Cochstedt, Germany. Further tests will follow and the first low-altitude flight trial is planned for 2026, subject to ideal weather conditions.

“With HAP-alpha, DLR is demonstrating its comprehensive systems expertise in the complete design, development and operation of a new and innovative aircraft, incorporating all disciplines,” explains DLR Executive Board Member for Aeronautics, Markus Fischer. “This illustrates our engagement in an important field of innovation, to strengthen Germany as a location for technology and business and to open up new perspectives for our public stakeholders in collaboration and knowledge exchange.”

HAP-alpha is a highly elastic, solar-powered flight platform with an extremely lightweight design, manufactured at DLR’s site in Braunschweig with a total mass of just 138 kilograms and a wingspan of 27 meters. The goal of this DLR project is to develop, build and operate a high-flying uncrewed solar aircraft. It will serve as a test platform, flying into the lower stratosphere at an altitude of approximately 20 kilometers. There, sensor systems and technologies for long-term use on a high-flying platform will be tested.

Key Milestone in Flight Readiness Achieved

A particular challenge of the Ground Vibration Test (GVT) on the high-altitude platform – HAP-alpha – was its extremely lightweight and therefore highly flexible design. The aircraft’s construction enables a wingspan of 27 meters with a total mass of just 138 kilograms (304 lbs.). HAP-alpha’s wings are not yet covered for the tests, so the supporting structures are visible in this image. (Image: DLR)

The now completed GVT is an important milestone for investigating the vibration behavior of an aircraft. The goal is to identify critical vibrations that can occur during flight, takeoff and landing. This determines whether an aircraft is safe and thus airworthy. For HAP-alpha, the successful GVT marks a key step on the way to the first flight tests next year, which will involve basic flight maneuvers at low altitude.

“The successful Ground Vibration Test is a significant step in the development of our high-altitude platform,” said Julian Sinske of the DLR Institute of Aeroelasticity, which conducted the GVT. “It shows that we are on the right track to overcoming complex aeroelastic challenges and preparing the platform for flight.”

During the GVT, numerous sensors were used to record the dynamic properties of the platform, which was fitted with electromechanical vibrators so that its vibration characteristics could be identified. A particular challenge of the GVT on HAP-alpha was its very lightweight and therefore very flexible design.

During the Ground Vibration Test, numerous sensors were used to record the dynamic properties of the platform – including acceleration sensors mounted on its electric motors. The electric motors, pictured here, are equipped with acceleration sensors. (Image: DLR)

The acquired data will now be used to update simulation models and predict flight behavior even more precisely – especially during maneuvers, turbulence and gusts of wind. “This allows the project team to focus on the upcoming complete system tests, which will mark the completion of ground testing and enable subsequent flight testing,” adds HAP project leader Florian Nikodem from the DLR Institute of Flight Systems.

Testing Sensor Systems and Technologies

With HAP-alpha, DLR aims to build comprehensive expertise for the development of high-performance, high-altitude solar-powered aircraft. HAP-alpha will also serve as a carrier platform for testing sensor systems and system technologies under realistic operating conditions in the lower stratosphere. In addition to the platform itself, DLR is developing two sensor systems for use on HAP-alpha, as well as optimized evaluation methods.

The sensor systems are a high-resolution camera system (MACS-HAP – Modular Aerial Camera System High Altitude Platform) and a synthetic aperture radar (HAPSAR – High Altitude Platform Synthetic Aperture Radar). Through this work, DLR is developing sustainable, long-life high-altitude platforms that will enable a wide range of future applications in Earth observation, environmental monitoring and communications.

This article was written by the German Aerospace Center (DLR). For more information, contact Falk Dambowsky, at This email address is being protected from spambots. You need JavaScript enabled to view it., or visit here .