## 3 Experimental System Description
### The Laval Nozzle  
The design of Laval nozzle [31] and real nozzle can be seen in Figure 9a,b. The nozzle critical section has a diameter of 3 mm. The nozzle is soldered with silver on a fitting which is screwed into the solenoid valve exhaust end. The valve diameter is 4 mm, less than the fitting inner diameter of 5 mm. The fitting is screwed into the solenoid valve structure right up to the level of the valve. This design ensures the smooth operation of the valve, with a minimal loss of N\({}_{2}\). The Laval nozzle is made out of 316 L stainless steel using a computer numerical control (CNC) milling machine. The upper structure of the hot gas propulsion system consisted of the propellant tank's supports and ring supports. The lower part of the supports was attached to the mirror, while on the upper part, a total of three supporting rings were fixed on each support. The supporting rings were laser-cut from 316 L stainless steel sheet. Between the propellant tank support and the mirror, polytetrafluoroethylene (PTFE) spacers 5 mm-thick were added to prevent the concentrated heat transfer on the mirror. The ring supports were fixed with the propellant tank supports using M5 screws, nuts and washers. The propellant tank was supported by the three supporting rings. The lower support system was attached to the underside of the mirror with the screws that passed through the upper support system, Teflon spacers and mirror. The lower support system consisted of a central \"C\" shaped support and two lower support rings for the main feed tank. The central C-shaped support had the mirror fitted on the upper part and its lower part was fitted on the balance arm end. The central support system was the only element which connected the mirror and its upper elements to the balance arm. The lower support rings were laser cut from a 316 L stainless steel sheet. The main propellant tank was inserted into the two rings and central support system. The support ring which fixes the rear of the main tank has a balance pan attached to it. It was used to put weights on it in order to cancel out the imbalance due to weight of the piping, pressure regulator, safety valve and electromagnetic valve mounted on the front part of the system, as shown in Figure 10.  
Figure 9: (**a**) The design of Laval nozzle; and (**b**) Laval nozzle. When the infrared lamps were activated, they radiated parallel rays of infrared light to the concave mirror which concentrated them on its focal line where the service tank was placed. In order to avoid force transducer damage during the experiment, the balance lever position was limited by a position limiter (Figure 4), which was fixed on the base plate. The position limiter does not allow the tilting of the balance lever beyond a certain value. The travel limit of the balance lever is controlled by a thin pitch screw which pushes on the balance arm on its upper side. The force transducer (Figure 4) was a high precision one, measuring a maximum force of 50 N with a precision of \(\pm\)0.01 N. The force transducer also had the option to record 10 readings and the PC live monitoring of measurements. The force transducer was mounted between the base plate and the balance lever (Figure 4). It featured a guide path machined into the base plate, which allowed for the transducer to slide across the length of the balance arm. The guide channel had graduations of 1 mm across its length. The center of the force transducer was marked, thus enabling for the easy read of the length of the arm at which the force was applied by the hot gas propulsion system. In Figure 10, one can see that the pyranometer is placed at a distance of 1 m from the radiation sources and at 200 mm above the focal axis of the mirror. It measures the temperature (\({}^{\circ}\)C) and irradiance (W) emitted by the radiation sources when the arm is being hinged into the path of the infrared light from the sources.