Within the Horizon Europe project SALTO two vehicle demonstrators are investigated and developed: the T1H and the T3 vehicle. This article provides an overview of the SALTO project demonstrators and the key flight phases of the T3 vehicle. While T1H focuses on vertical hop testing, the larger T3 demonstrator is designed to replicate the critical phases of a reusable launcher’s first stage – from lift-off to re-entry and landing. Through this, SALTO contributes to the maturation of technologies required for future reusable space transportation systems.
T1H
T1H is a hop test vehicle. This means it performs a vertical take-off and vertical landing at the same location, without significant horizontal displacement. The T stands for Themis, which is the vehicle’s name. On one hand, the 1 stands for single Prometheus® engine, and the H stands for Hop.
T3
On the other hand, T3, is a larger vehicle, which aims to reproduce more accurately the flight phases of a landing first stage of a launcher. The T again stands for Themis, while the 3 stands for its three Prometheus® engines. While T1H is going to fly in the frame of SALTO, T3 is a vehicle that was started to be investigated with the aim of assessing possibilities of building such demonstrators after the end of EU-funded project SALTO.
With the three Prometheus® engines T3 has about 360 tons of thrust, which is more than most micro-mini launchers have. The thrust class of this demonstrator is high enough to perform high-altitude trajectories. T3 is only a 1st stage demonstrator vehicle. Yet, it could reproduce accurately the re-entry and landing phases of a first stage of a launcher (a detailed analysis of the phenomena to be reproduced was shown in Scalability of aerodynamics from demonstrator to full flight scale of a vertical landing reusable launcher).
This is a conceptual and illustrative image of T3, not approved design.
T3 flight phases
In the video shown here you can observe T3 and its flight phases.
Taking off
First, the demonstrator takes off. As it is fully loaded with propellant it is still slow at the beginning, but quickly gains more speed as the fuel is burned and it gets lighter, while the engines still provide all of their thrust. The flames of the engines are barely visible and slightly blue as the Prometheus® engines use Methane, in combination with Oxygen as propellants.
Main Engine Cut-Off (MECO)
At higher altitudes, the surrounding pressure around the vehicle drops and therefore the plume of the engines widens. At an altitude of about 80 km the engines are shut-down: this is called the Main Engine Cut-Off (MECO). In this phase, the second stage would be separated for an operational launch vehicle.
Flip-over maneuver
The next phase of the T3 flight follows immediately, which is the flip-over maneuver. In this phase small thrusters (the Reaction Control System) are used to reorient the vehicle, to “flip it”, so that it will be oriented with the engines first. In this phase, the vehicle follows a ballistic trajectory and the aerodynamic forces are still very low, so that it is comparably easy to turn the vehicle around. The grid fins would be deployed during this flight phase, as the forces are still low. The grid fins are small “wings” on the side of the launcher used to steer the vehicle later on.
Re-entry burn
As the vehicle gained a lot of speed during ascent, while re-entering in the atmosphere, the vehicle would experience high heat loads (high temperatures) and the aerodynamic forces would be high. To reduce these loads, a braking maneuver is performed after the ballistic phase. The engines are fired against the flight direction of the vehicle to reduce its speed. This phase is called the re-entry burn. The number of engines used for this maneuver depends on several design decisions. In our case, we use two. Very specific highly fluctuating flow fields are formed in this phase.
Aerodynamic phase
An aerodynamic phase follows the braking maneuver. This is a phase where the grid fins on the vehicle are used to steer it to the landing pad. As the vehicle is already in lower altitudes the pressure is higher and so the aerodynamic forces are high. Therefore, the easiest way to get the vehicle to the landing pad is by steering it aerodynamically. Touch down.
Touch down
Finally, the vehicle landing is performed with one active engine. So that a soft touch down can be achieved, where the vehicle is oriented in a vertical manner. This maneuver is the so-called landing-burn. Shortly, before the touch down the landing legs are deployed.