Composites fuel tank technology for the NERVA launcher.

Tache, Florin ; Silivestru, Valentin ; Dobre, Tanase 等

1. INTRODUCTION

The NERVA is a typical Romanian project in the family of small orbital launch vehicles (Rugescu, 2008), largely considered today as an alternative to tenders on orbital launch capabilities from Russia. They answer the inability of reusable systems, like the space shuttle, to acquire a low cost capability of launching scientific and commercial 1payloads in low Earth orbit (LEO). Many other similar projects like the Spanish AQUARIUS air-launch project (Simon, 2006) are known. Some private companies with the capability of developing cheep space transporters are raising in this market, to only mention the international "Sea Launch" (Intl. Launch Services, 2005), the US "SpaceX" (SpaceX, 2006), "Orbital" & "t/Space" or the Russian "Air Launch Corporation" (Sarigul-Klijn et al., 2006).

The recent successes of University "Politehnica" of Bucharest (UPB) into FP7 European Space priority projects ORPHEE and NANOPROP bolsters the development of the NERVA launcher for PUBSAT nanosatellite (Rugescu et al., 2008). NERVA will be a readily available, low cost rocket launcher for achieving half of the local orbital velocity at 100 miles altitude. Enhanced with a high efficiency, dual mode combustion third stage, it will be able to inject the three-axes controlled (3AC) PUBSAT nano-satellite into LEO. The rocket transporter is accessible to Romanian current aerospace technology. The foreground of NERVA is the military soil-air SA-2 Guideline weapon, now obsolete in Romania, subjected to a precise re-conversion to scientific application. The source weapon is a derivation of the famous Rheintochter missile, successfully developed at Peenemunde in the early 1945 by the team of Wernher von Braun. The new NERVA is a peaceful renovation of that brilliant work, transforming the soil-to-air missile into a high performance orbital vehicle, perhaps the first orbital project developed from such a low performance, conventional rocket system.

2. COMPOSITE TANKS TECHNOLOGY

A desired improvement is the composite tanks technology, aimed to add further confidence in this project. The upper stage with 5000 m/s ideal velocity performance is at the limit of the current Romanian rocket technology. All align into building a cheap, ground launch vehicle by minimally modifying the basic SA-2 rocket system. In order to accommodate the high thrust enhancement of the solid motor (SM) booster, thrust vectorization of the liquid motor (LM) and lightweight structure for the second stage, with highly extended propellant tanks and lightweight guidance, a drastic improvement of the structural efficiency of the SA-2 Guideline system is however required.

Very light structures are required for the launch platforms that mainly depend on the manufacturing technology of the fuel tanks, as more then 90% of the vehicle mass concerns the consumable propellant. The tank structure must be as light as possible to allow a convenient payload to be added for injection into the LEO. Estimating the vehicle performance that a good tank technology induces, in terms of mass ratio, only proves the efficiency of a given manufacturing technology,

[[mu].sub.0] = [M.sub.0]/ [M.sub.0] - [M.sub.f] = [M.sub.0/[M.sub.f] (1)

Here, the total launching mass of the orbital rocket [M.sub.0] is the sum of the structure and payload mass [M.sub.f] and the propellant content [M.sub.p.] The same mass ratio is also defined for the liquid propellant tanks alone and is consequently called the structural factor o of the tanks,

[epsilon] = [M.sub.t] + [M.sub.p]/[M.sub.t] (2)

It is suggestive to compare the technological figures of the structural factor with some natural or well-known liquid containers (Table 1), from where the limits of the present technology are clearly emphasized.

It is really impressive that the structural factor 28.7 of the Shuttle ET lies very close to the limit of a simple Coke bottle. These antipodal structures make profit of the inner pressure stiffening effect, or the balloon effect. The Shuttle ET must also withstand high concentrated loads from the two solid boosters, from the main propulsion system of the orbiter and the hydrostatic pressure from the large mass of liquid propellant during the boost. The solution was found to distribute the loads, which are incoming through the dual strut joints (or bipod fitting) on the rear of the tank, along the thin structure itself (Fig. 1). The two bolt catchers fixed to the forward, or top area of the External Tank at the Solid Rocket Booster/External Tank forward attach point are only provisioned to prop laterally the SRB-s, not transmitting any axial thrust (Nemeth et al., 1996).

[FIGURE 1 OMITTED]

For the NERVA vehicle no such problems of uneven loads distribution into the tank walls are present, because almost entire loading goes into the longitudinal direction. Only small dynamic loads due to pitch and conning motions of the rocket in response to commands from the autopilot, inducing some sloshing in the propellant, create unimportant asymmetry in the axial loading, far below their level into the ET structure however. The knowledge and management of the bending moments and transverse forces during the NERVA ascent are still a main part of the design study to cover all unexpected occurrences.

3. COMPOSITES TECHNOLOGY ASSESSMENT

Extensive use of magnesium alloys matrix is envisaged, in combination with either boron fibers or carbon fibers fabric (Volkova, 2006). The following criteria were used to assess the technology of propellant tanks:

* preserve fairly unchanged the basic second stage diameter;

* preserve the second stage liquid engine construction equally unchanged;

* accept a sensible extension in combustion time of the second stage liquid motor;

* allow for a sharp lengthening of propellant tanks of the second stage;

* remove all military equipment and replace it with extra tankage, engine gimbals bearing and a much smaller navigation hardware;

* relocate the pressure gas vessel in front of the propellant tank.

One possible candidate solution for the NERVA launcher fuel tanks is a new composite material being developed within UPB. Carbon Fibers Reinforced Aluminum (CFRA1) is a new type of composite material being developed by the Chemical Engineering Department in cooperation with the Faculty of Aerospace Engineering at University 'POLITEHNICA' of Bucharest, Romania. The research so far shows promising results, CFRA1 having foreseeable excellent behavior in applications requiring a reliable, yet lightweight, reinforcement material that can also withstand powerful thermal shocks without excessive load concentration.

Consequently the material would have to possess good mechanical properties regarding, but not limited to, its behaviour under internal pressure loads and cryogenic temperatures of the propellant components (liquid oxygen or LOX at -183[degrees]C and liquid methane or LME -162[degrees]C).

Internal pressure is equivalent, considering a thin wall classical problem, with tension loads acting isotropic in all directions into the wall. To experimentally proof this assumption, standard tensile tests have been performed, revealing that CFRA1 has a very good specific strength (ratio of tensile strength over specific weight), as compared to other homogeneous or composite materials (Tache et al, 2007). CFRA1 fibres are used for tank construction, with expected good cryogenic resistance. Extensive tests are still under development, with emphasis on load concentrators created at sharp profile changes, around openings in the structure and around other discontinuous geometries of the tank shell.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

As an extension, ballistic tests have proved that CFRA1 is bullet-proof with respect to a 7.65 mm calibre bullet shot from a 10-metre distance. It needs some improvement when faced with a 9 mm calibre bullet, shot from similar distance. For the latter one, new samples made up of 14 carbon fibre layers and arranged in such a way that the impact shock waves are better supported by the entire composite mass are prepared.

4. CONCLUSIONS

The development of reliable technologies for composite tanks for cryogenic propellants is still incipient, on the international scale, due to low temperature cracks and difficult manufacturing of fittings for plumbing. Recent advances in some new magnesium alloys have re-launched this technology for lightweight magnesium-matrix composites.

For the specific application into the NERVA small orbital vehicle, the prediction of the available performance is performed by computer simulations, proving the feasibility of the NERVA vehicle as an orbital system when lightweight propellant tanks are used. Investigations have shown up to now that for storable propellants the MMC formulation with magnesium matrix is highly promising. Nevertheless, a lot of development is required for achieving acceptable reliability and load capacity within the end joints of the tanks.

The target of the described research is to advance in improving of CFRA1 material up to potential magnesium or beryllium alloys, continuing the work of the first author's PhD study, started at UPB in the last two years.

5. REFERENCES

International Launch Services Press Release (2005), Tuesday, September 6

Nemeth, M. P., Britt, V. O., Collins, T. J. & Starnes, J. H., Jr. (1996), Nonlinear Analysis of the Space Shuttle Superlightweight External Fuel Tank, NASA TP-3616

Rugescu, R. D. (2008), NERVA Vehicles, Romania's Access to Space, Scientific Bulletin of U. P. B., Series D in Mechanics, 70, no. 3, p. 31-44

Rugescu, R. D., Predoiu, I. & Aldea, S. (2008), PUBSAT and NERVA Launcher Fuel Sloshing Dynamics, Proceedings of ICAI-2008 Conference, June 24-26, Bucharest, Romania, p. 200-203

Sarigul-Klijn, M., Sarigul-Klijn, N., Morgan, B., Tighe, J., Leon, A., Hudson, G., McKinney, B. & Gump, D., Flight Testing of a New Air Launch Method for Safely Launching Personnel and Cargo into LEO, AIAA-2006-1040

Simon, J. (2006), The Aquarius, a proposal for a nano-satellites launcher vehicle, INTA, Torrej'on de Ardoz, Paper IAC-06-B5.5.05, Proceedings of the 57th IAC of the IAF, IAA and IISL, Valencia, Spain, Oct. 02-06

SpaceX (2006), Excellent Engineers Wanted, Aviation Week and Space Technology, Sept. 25, p. 67

Tache, F., Stanciu, V., Chiciudean, T.G., Toma, A.C., Stoica, A., Dobre, T. & Fuiorea, I. (2006), IAC-06-C2.8.04, 57th IAC, Valencia, Spain

Volkova, E. F. (2006), Modern magnesium-base deformable alloys and composite materials (a review), Metal Science and Heat Treatment, Springer, New York, Vol. 48, No. 11-12/Nov, DOI 10.1007/s11041-006-0120-0, pg. 473-478.

Table 1. Natural and ordinary containers

Container [sigma]

Hen's egg 8.9
Railway oil car 13.0
Coke bottle, half gallon (2 liters) 33.5