Nexø II will launch this year.
The Nexø I and Nexø II rockets will serve to demonstrate several key technologies on our roadmap towards the much larger Spica rocket. Technologically the Nexø rockets are very similar to the Spica rocket, thus if we can fly Nexø we have the necesarry technology in place to fly Spica.
Nexø II is the second liquid bi-propellant rocket launched by CS. Until the Nexø class all rockets we have launched have been propelled by hybrid rocket engines running on either polyurethane and liquid oxygen or on polyurethane and liquid nitrous oxide. Nexø II is propelled by our own custom build BPM5 engine. You can read all the details over on the BPM5 engine page.
Nexø II is currently expected to launch between July 1st and September 1st 2017, keep up to date on the launch date and launch coverage on the Nexø II mission page.
Nexø II design
The Nexø II rocket is 300 mm in diameter, 6.7 meter tall and have a Gross Lift Off Weight (GLOW) of 205 kg. The propellant tanks and the main structure are made in aluminium to keep it as light as possible. The nose cone and parachute bucket are made in carbon fiber to reduce weight even further.
Nexø II is actively guided by our own custom build Guidance and Navigation Computer (GNC). A set of four jet vanes are used as the Thrust Vector Control system (TVC) commanded by the GNC. A similar system was succesfully used on the Sapphire launch in 2013.
Jet vane section
The graphite jet vanes and their servos are mounted on a flange which is fitted on the lower engine compartment flange. Thus the jet vane assemlby is a separate unit which can be mounted and unmounted when needed.
Each jet vane sit in a frame of stainless steel to prevent transfer of heat to the servo. Behind it is a 1.5 mm thick, rounded stainless plate shielding against radiant heat. On the other side of this there is a large cog in aluminum, this is engaged in the smaller cog which is connected to the servo. We use Futaba BLS172SV, with a torque of 37 kg-cm and a weight of 74 grams, it is one of the strongest torque / weight ratio we could find in this scale. The stainless house of the jet vane sits on an arm which in turn sits in two ball bearings. The bearings sit in a housing in which the servo is mounted.
Nexø I is propelled by our own BPM5 engine which has been modified slightly to fit within the Nexø airframe. Most noticably the LOX dome has been weight-optimized, this saves about 1 kg.
At the bottom of the engine compartment, sitting in between the jet vanes, are four modified quick couplings. These are inlets for filling LOX, helium inlets for tank pressurization and inlet for nitrogen purge of the engine. Many pipe joints are made with Tri-clamp fittings for easy assembly and servicing.
The LOX dome is mounted on four brackets that are mounted directly on the structural part of the airframe. The brackets are fitted with adjustment screws that allows accurate adjustment and alignment of the engine.
The main valves are placed above the engine. They are driven by two MAC servos from JVL, the exact same type as we use on the test stand for static engine tests.
The LOX and fuel tanks are completely identical. They are made form 4 mm aluminium, the top and bottom domes are however 5 mm. Inside sits an anti-vortex baffle which prevents the formation of a vortex which can pull down gas when the last liquid runs out. The tank terminates with a 2″ spigot on which is welded a Tri-clamp fitting. The flanges are made with pop-nuts so the tanks can be mounted several times without destroying the thread.
At the top of each tank is a combined sensor and valve mount unit. It serves several purposes. At the far left of the picture sits a solenoid valve which we can use to ventilate the tank. It is also part of the pressure relief system of the tank. The valve is automatically activated at a predefined pressure. Tucked behind this sits a burst disc. It is a 0.3 mm thick aluminum plate which is sandwiched between two flanges. The thin plate bursts at 27 bar and is the ultimate safety feature agains generating excess pressure in the tank. Pointing out to the right is the connection to the gas system, it is through here that we pressurize with helium. Sitting on top is two pressure sensors. Finally, in the center of it all is a thermocouple which measures the temperature in the gas phase in the tank.
The DPR system on Nexø II is very similar to the system we know from our test stand, the main difference is the pressure tank. On the test stand it consist of two common diving bottles totaling 24 liters. Such diving bottles are obviously too heavy, so for Nexø II we purchased a 20 liter composite tank, which is approved for 300 bar. The tank itself weighs 12 kg.
The regulation of the pressure is going to happen with the same type of valve that we use on the test stand, that is two piece of our faithful HYDAC PWK06020W sponsored by HYDAC. In addition, a main valve, located between the high pressure bottle and the control valves. The main valve is required since the two control valves are leaking a little bit at high pressure. That is not really an error on the valves, they are actually designed to control hydraulic oil, no pressure gas in a rocket. We are just using them for a purpose quite far from the intended use. But that is how we usually do stuff.
Read more about the DPR system here.
The section comprises a total of four electronics boxes. One for the Guidance and navigation computer (GNC), which job it is to control the rocket. One for a radio unit to both send and receive data. One for a GPS box which contains two different GPS devices and finally one for a video transmitter.
On top of the avionics section we find the parachute bucket and nose cone. The inner part of the parachute bucket is made of carbon fiber to save weight. In the bucket lies a 34.7-square-meter parachute which will land Nexø II at 8 m/s. In the side of the parachute bucket sits a camera that will film the unfolding of both the parachute and the ballute which we find in the upper section. Here is also an (inverted) carbon fiber bucket which houses the ballute. Additionally it contains two gas generators (air bags) each emitting 100 liters of gas in a fraction of a second. Obviously they are used to shoot of the nose cone. When the pressure in it becomes too large the many small “dog bones” on the outside will be torn and the ballutte will be in the open air. It is attached through a three-ring system to the main parachute, which can then be activated after a given time or a given height. The nosecone is also equipped with its own small parachute which is equipped with its own small transmitter, so that we can track the nose cone on the way down and find it again.