TECH FOCUS AREAS: General Warfighting Requirements (GWR) TECHNOLOGY AREAS: Space Platform OBJECTIVE: Landing on unimproved, irregular surfaces will be required for Rocket Cargo and would enable many missions supporting Tactically Responsive Space Access. Little information is available on the interaction of a plume and different types of surfaces. Additionally, necessary data for anchoring and improving models is unavailable. The objective of this topic is to develop diagnostics capable of measuring velocity and/or structures involved in plume-ground interactions. Solutions will be demonstrated at the 1 klb thrust scale, and attention should be given to scale limitations. DESCRIPTION: Although vertically landing a rocket on an improved, flat surface has been achieved by multiple launch vehicle companies (Masten Space, SpaceX, Blue Origin) [for example, 1], landing a rocket vehicle on an irregular, unimproved surface has a number of challenges including, but not limited to the plume kicking up dust and creating an observable event and erosion of the surface leading to a crater and/or rocket instability. The terrain the rocket vehicle may land on is not known a priori and may vary widely. Plume-ground interactions must be sufficiently understood to make decisions on acceptable landing sites and to provide necessary mitigations to enable the use of initially unsuitable sites. Additionally, information on what happens to eddies and heat after the ground stagnation point are required to determine safe stand-off distances for equipment, ensuring they are not impacted by debris (e.g., rocks) nor experience unacceptable temperatures. This information on interactions will additionally aid rockets operated for commercial applications since many companies are focused on landing and reusing their rockets, and the data can be used to inform decisions on manufacturing landing pads. Some information on plume-ground interaction is available from landing on improved surfaces and from NASA investigations of extraplanetary landing [2]. Such data is of limited scope and fidelity, however. Small-scale studies of plume-ground (simulant) interactions are of interest due to the increased fidelity available. Both improved surface landings and small-scale experiments have demonstrated the ability to obtain temperature and heat flux profiles on the ground. However, structures within the plume, including their interaction with the ground and evolution thereafter, are not yet available. Similarly, velocities within the plume are unattainable. These metrics are crucial for assessing surface survivability, stand-off distances for personnel and critical ground equipment, and for developing accurate computational models. However, they are difficult to acquire due to the large luminosity of the plume, density gradients within the plume, and, to a lesser extent, general size scales involved. For example, traditional and even next-generation Particle Image Velocimetry can only provide near-surface data due to the luminosity of the plume overcoming that of embedded particles [3, 4], and introduction of particles, especially not eroding the chamber throat, can be problematic and difficult. NASA's HiDyRS-X project was able to overcome the luminosity challenges but had limitations with temporal resolution making evolution studies difficult or impossible [5]. This topic seeks solutions which enable visualization of large structures and their interactions with the ground and/or the quantification of velocities within the plume before and after interaction with the ground. Temporal resolution must be sufficient to understand evolution as the plume contacts the landing site. To meet necessary acceleration of technology development and the demands of high-fidelity CFD, the methods must provide two- or three-dimensional data. Methods will be demonstrated on a small-scale, 1 klb thrust, kerosene-oxygen rocket chamber during Phase II. AFRL/RQRC will provide one week of testing time, up to ten tests a day, and the rocket chamber and ground simulant to carry out such a demonstration. AFRL/RQRC will also provide up to two black-and-white, high-speed cameras (Vision Research Phantom) as necessary. Proposals should consider limitations for applications on larger rockets including a scale at which they would be untenable due to luminosity, size, or other complexity. Resolution and/or uncertainty estimates, as applicable to the measurement, should also be included. PHASE I: Selected companies will establish the method overcoming complexities related to plumes. Verification can be a combination of reduced-scale demonstration (e.g., within bunsen-burner flame) and analysis. This verification, however established, will be documented and delivered as part of the Phase I work and will provide confidence that the system can be used successfully to collect data from a small-scale plume. Efforts should also quantify resolution and/or uncertainty of method, which will be documented as a deliverable. Companies will interact with CFD model developers to ensure needs are met. Data collected as part of verification will be provided to model developers as well as sufficient information regarding experimental set-up to allow data use in testing models. PHASE II: If selected, companies will deliver necessary software and hardware prototype package to AFRL. Efforts will demonstrate the diagnostic technique with a 1 klbf, kerosene-oxygen engine plume impinging on a landing pad simulator. AFRL/RQRC will provide hot-fire testing with their 500-1000 lb thrust stand for such demonstrations, or an equivalent or larger system shall be used. Operation of the diagnostic will be shown across mixture ratios from 2.2 to 2.8 (at a minimum). Landing pad simulator will be located at a range of distances to be determined, but within the overall range of 18-72 inches. Air Force will be provided data package from demonstration to CFD modelers. PHASE III DUAL USE APPLICATIONS: Phase III efforts will scale the diagnostic to a 10 klbf thrust engine or larger and provide demonstration of efficacy and/or field prototype system for demonstration with medium or large rocket landing (to include dust and other environmental factors). This demonstration will necessarily involve commercial partners since the military does not manufacture nor purchase rockets. REFERENCES: Falcon 9 flight 20, https://en.wikipedia.org/wiki/Falcon_9_flight_20; Plume Surface Interaction (PSI), https://www.nasa.gov/directorates/spacetech/game_changing_development/projects/PSI Westerweel, J., Elsinga, G.E., and Adrian, R.J., Particle Image Velocimetry for Complex and Turbulent Flows, Annual Review of Fluid Mechanics, Vol. 45, pp 409-436, 2012.; Balakumar, B.J. and Adrian, R.J., Particle Image Velocimetry in the Exhaust of Small Solid Rocket Motors, American Physical Society, Division of Fluid Dynamics 55th Annual Meeting, 2002.; Revolutionary Camera Recording Propulsion Data Completes Groundbreaking Test, https://www.nasa.gov/feature/revolutionary-camera-recording-propulsion-data-completes-groundbreaking-test KEYWORDS: rockets; plumes; plume-ground interaction; plume-pad interaction; diagnostics
