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Skybolt Sounding Rocket

Sounding (noun); measurement of depth of water: The term 'sounding' is taken from the maritime expression where soundings were taken onboard a ship to measure the depth of water below the keel as the ship moved through shallow waters; the term sounding now means "to take measurements".

Rocket (noun); a vehicle, missile or aircraft which obtains thrust by the reaction to the ejection of fast moving exhaust from within a rocket engine.

Working towards launching 3 people on a sub-orbital trip into space requires a pragmatic and disciplined approach to say the least. Over the past 3 years the THUNDERSTAR / STARCHASER 5 RLV has taken shape, this being defined in incremental steps as our research and development activities yield results. Following the Starchaser tradition of 'one step at a time', combined with the definition of our 5 year business plan, the need for an intermediate step between NOVA and THUNDERSTAR / STARCHASER 5 became apparent.
Over the last 18 months we have researched and developed our 5 year business plan to address all the areas required to grow into an organisation able to operate in the commercial space access market. One of the more traditional sectors within this market is that of launching scientific or experimental payloads on a sub-orbital trajectory onboard what are collectively known as 'sounding rockets'. It became apparent that, if we were to develop a rocket capable of sub-orbital space flight we could prove out and flight test the whole design, manufacture and assembly methodology we are proposing for STARCHASER 5, and hence SKYBOLT was born.

SKYBOLT is being designed to flight test a Starchaser rocket engine and prove the modular vehicle construction and secondly to achieve real space flight experience. The evolving design will be reusable, a trait that is almost unique compared to other existing sounding rockets. At around 800mm diameter and nearly 12m tall SKYBOLT will deliver a 20kg payload to a target altitude of over 130km (83 miles).

The development of SKYBOLT is a top priority. Current activity is focused on initial design studies to confirm rocket stability, overall sizes, performance and ease of manufacture. One of the first tasks is to conduct comprehensive Computational Fluid Dynamic (CFD) analysis to determine the complete flight profile, taking into account the different atmospheric conditions throughout the flight. As SKYBOLT will be super-sonic, with an estimated maximum velocity of MACH 5, the CFD modelling and analysis will be quite involved. The use of CFD will allow us to optimise aerodynamic features such as nose cone angle and fin shape and size. We have met with a world leading CFD company and reached an agreement for collaboration on the SKYBOLT project.

() Skybolt Aerodynamics

Skybolt Article taken from Ignition #5, December 2005

Electronic Control System
One of the new systems that will be employed in Skybolt is an electronically controlled regulator system that will precisely control the propellant tank pressures throughout the flight to maintain a constant thrust from the engine. We came across a company during the latter design stages of Thunderstar who builds these regulators. Up until then it was assumed that we would have to pre-set a pressure on the tank regulators manually and allow that pressure to bleed down during the flight as the pressurant gas tank levels drop. By adding electronic control, we can monitor the propellant tank pressure levels, and as they fall, send a feedback signal to the electronically controlled regulator to open up the regulator to allow more gas through the system, or vice versa. This is all done automatically by an electronic control unit the same company produces. It is programmable through a PC or laptop and then will run independently from the computer until a new program is input. It can be set to start regulating pressure through simple DC signals which can come from our PLC or system controller or ground test station. That way we can pre-programme the pressure levels we require in each tank and then walk away from the tank before anything is pressurised, thereby ensuring greater safety. The system can also be programmed to pressurise the propellant tanks slowly so instead of fully opening the system and shocking the tanks up to full pressure right away, the pressure can be ramped up slowly to the set-point pressure over a period of time which we can decide upon.

One of the other exciting results of using these regulators is that we may be able to adjust the pressures in the propellant tanks to account for pressure due to flight forces on the propellants. In Black Arrow for example, it was found that the propellant levels in the rocket would deplete slightly faster during flight than in ground testing. This is because the extra head pressure caused by the g-forces during the flight would force the propellants out of the tanks at a proportionally higher rate. If we use these electronic regulators, then we can automatically adjust for this by smart positioning of our pressure sensors that will be giving the feedback for the control unit. In the future we may be able to experiment with this technique for throttling the engines.

Airframe Design
Design of the rest of the rocket airframe has been proceeding as fast as possible. The majority of the airframe consists of the tank sections. Bonded to the outer ends of the tanks will be carbon fibre skirts that will slot together with each adjacent tank so that we can easily join them. This will form the main body of the rocket. These skirts have access hatches so we can work on the plumbing / electronics etc. On the lower skirt, below the kerosene tank will be a metalwork frame that will hold the engine in place and will be designed to transfer the thrust force of the engine evenly through the rocket. This section will have a removable outer skin as this will be the most accessed section of the rocket. All the fill ports for both tanks and the main propellant valves will be here and will require easy access. There will be a lower metal ring that has four mounting points at the base of the rocket. During static tests and launch these pads will house load cells that can be used to measure the rocket weight and also vertical thrust. These points will also house the launch hold-down clamping system which will only release once a sufficient thrust level has been achieved. These will be in the form of explosive bolts or shear pins.

On the other end of the rocket, at the top above the helium tank will be a similar metal work frame with removable covers that will house the recovery system and the majority of the electronics for the flight. Above that section will be the payload section and nosecone, including a separate recovery chute for the payload.

The carbon fibre contractor we use will be winding the carbon fibre onto the tank liners just after Christmas and then will be adding the tank skirt sections soon after. We are hoping to have the finished tanks ready by the time we return from New Mexico after completing the first series of Storms static test firings.

We are looking at the possibility of using a small commercially available guidance system that consists of a set of steerable guide fins located near the top of the rocket between the helium tank and the parachute system. These have been designed as a ready made system for sounding rockets to counteract any deflection off course by high altitude winds. If we can integrate such a system into Skybolt then we will do so, but it should be possible to fly using the passively stabilised design without too many problems.

Since the current Skybolt design is a basic rocket shape that is well known, the aerodynamics should be relatively simple, and we don't foresee any particular problems with the design, as long as we keep the centre of gravity two calibres forward of the centre of pressure the rocket will fly stably. But it is always good to optimise a design to get the most we can out of it, so we have been looking at varying aspects of the nosecone shape and fin design to try to minimise the total drag on the rocket during flight. This is being done using CFD software and is being modelled by a student from Manchester University. We are looking at varying the nosecone angle, nosecone tip features such as how blunt to make it, whether to include an aero spike to break up the air earlier to reduce drag etc. This study is going very well (see aerodynamics article) and we expect to have the full rocket design optimised by the New Year.

Launch Pad / Vertical Test Stand
We have also been working on the new launch pad design. The current idea is to design a pad that will allow us to vertically test the engine but also provide an adequate platform to launch the rocket from, so we are not building multiple test sites. We are looking at either using the side of a hill if there is a suitable site out in the SRS, or digging down and installing concrete blast deflection tubes so that the exhaust plume will not throw up debris. The same blast deflection pit will be used underneath the rocket when it is being launched. Next to the pad there will be a 36 metre tall tower that will be constructed of standard tower crane sections. From this we will hang launch rails. Over the blast deflection hole there will be a standard table to which we can bolt different adapters for the rocket system we want to test. The blast deflection hole has been sized to be about 2m in diameter so we can use the same pad eventually for a rocket as big as Thunderstar. We may also install an underground water pipe system should we find we require water for vibration suppression.

For the vertical test firings of Storm we will not need the tower. We will build the first two frames over the pit that will be used to launch the rocket, and the thrust mounts that will connect to the rocket. This will be hard connected to the four load cells that will be used to help determine if the rocket has achieved a safe thrust for launch, and we can use them to measure the 'pull' the engine produces rather than pushing into them as we have done previously. Without the weight of the rocket above the engine we will be measuring the thrust directly, rather than the difference between the rocket thrust and the rocket weight. The propellant tanks will be mounted on a separate 'table' or platform that will be removable for full tests of the rocket.

The same pad will be used for all calibration tests of the full rocket system and finally upgraded for launch by adding the full launch tower, launch rails and any additional equipment required.

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