Back of the nose cap system with isolation and pressure measuring system (flush air data system, FADS)
X-38 was designed to be a technology demonstrator of NASA for the life boat CRV (crew Return Vehicle) for the international space station ISS. Placed on a booster rocket a later development into an independent Crew Transport Vehicle (CTV) should be possible. Within the TETRA program (technologies for future space transport systems) essential components were developed in the range of the hot structures as well as a sensor system for X-38 and supplied to NASA. Beside the German industry the DLR was integrated into the development of components as well.
The Institute for Structures and Design was responsible for the construction and the production of the ceramic nose cap of X-38. Over 10-years of experience in manufacturing carbon-based fiber-ceramic as well as the long-lasting competence in the range of system design of hot structures and thermal protection systems were converted into an innovative component concept. If the past work in this area had a rather experimental character (e.g. co-flight of ballistic capsule missions FOTON and CETEX on EXPRESS), the nose cap as a primary structure component had a mission-crucial task.
During re-entry into the earth’s atmosphere the nose cap of X-38 experiences the highest thermal load of the entire thermal protection system due to its exposed location within the stagnation point of the vehicle. During the approx. 20 minutes re-entry phase surface temperatures of up to 1750°C were expected with a stagnation pressure of up to 10 to 150 hPa.
The nose cap represents an absolute novelty regarding technological requirements. A component out of fiber-ceramic for thermal loads of up to approx. 1750°C had never been planned before with the claim of re-usability. The nose structure of the American Space shuttle is made of carbon/carbon (C/C) for example and the temperatures reach only scarcely 1500°C.
The shell structure is fabricated with the help of the liquid silicon infiltration process (LSI) of the DLR in Netshape technology as integral component by using the special in-situ joining technology. The flying unit of the nose cap is provided with an oxidation protection made of SiC.
The connection of the nose shell is made by 8 single fittings, which are made of fiber –ceramic as well respectively in the cooler range of a high temperature-steady (until approx. 1200°C) metal alloy (PM 1000 of Plansee, Austria). The special arrangement and design of the lever-like attachment system guarantees on the one hand a very good mechanical maximum load and on the other hand allows for an unrestricted thermal expansion of the structure, which can amount to 3mm with a medium diameter of the shell of 700 mm and the expected temperature level. In case of a rigid attachment the shell would be destroyed only by the thermal compulsive forces.
Qualification unit after integration
Another system integrated in the nose cap system is the pressure measuring system (flush air DATA system, FADS) whose data is used for the position control of the vehicle in the hypersonic flight. Pressure is taken from 9 crosswide-arranged bores in the nose shell and let to the interior of the vehicle to the actual pressure sensor.
A further component of the nose cap system is a so-called hard seal (Rigid seal), fastened to the edge of the nose cap and located under the adjoining side panels of the continuative thermal protection system.
By interacting with a flexible seal package this seal system prevents the penetration of hot gases under the nose shell at the joints of the individual components.
For thermal isolation a multilevel flexible felt isolation made of oxide fibre-ceramic such as alumina, is intended between the nose shell and the sub-structure. The isolation reduces the maximum temperature at the aluminium sub-structure to about 100 degrees Celsius over a thickness of approx. 45 mm. This isolation system was developed and manufactured by ASTRIUM. The entire nose system has a mass of approx. 13.2 kg, whereby the nose shell alone accounts for 7 kg.
For the qualification of the nose cap system an accurate copy of the later used flying unit was manufactured. Beside the nose cap itself the aluminium sub-structure (MT-Aerospace), the ceramic side panels (ASTRIUM), the so-called chin panel (MT-Aerospace) and ceramic tiles (NASA) were integrated into the qualification unit.
For the first time ever install ability and accuracy of fit as well as the functional interaction of all components of the nose section by X-38 could be proved. The qualification campaign at IABG was divided into three sequences. First, the qualification unit was submitted to a vibration test, where the launch loads inside the Space Shuttle payload bay were simulated.
Afterwards three thermal tests took place where the temporal as well as the local temperature distribution where simulated during re-entry. Temperatures of up to 1750°C had to be applied to the nose cap. During the last sequence the vibration test of the first sequence was repeated twice. The re-launch-capacity was therefore proved. In order to check the mechanical integrity the nose cap was treated with the 1,5 times of the maximum pressure load expected during the flight. After the test no damages were determined and the results of the computational forecast such as temperature and deformation were in good agreement with the measured data.
Nose cap system after integration to X-38 in Houston, October 2001
At the end of January 2001 the flying unit of the nose cap system of the X-38 was delivered to the Johnson Space Center of NASA in Houston and in October of the same year installed to the X-38. Large care was necessary with the positioning of the ceramic cap, since the entire nose section of X-38 is geared to the pressure measuring bores in the nose cap.
The intention was to bring the X-38 (weight12 tons, lenght10 m) with the help of the Space Shuttle Columbia into the orbit at the beginning of 2005 and accomplish from there a fully automatic pilotless re-entry into the earth’s atmosphere. The last part to the landing area should be reached via a controllable parachute. The landing distance on skids was limited to approx. 50 - 100 m. The project was stopped because of financing difficulties on the part of NASA 2002 and the X-38 that was finished 80% was stored in Houston.