2020 Volume 61 Issue 1 Pages 68-71
A novel deployable rocket nozzle utilizing superelasticity was proposed in this study. Ti–4.5Al–3V–2Fe–2Mo alloy (SP-700) sheets were heat-treated to have appropriate α/β ratio so that the sheet shows superelasticity at room temperature. A miniature nozzle model was fabricated through thinning, cutting, and welding processes of the sheets. Folding-deployment tests of the model were conducted in addition to finite element analyses of its folding behavior. The feasibility of the new concept of superelastically-deployable sheet structure was successfully verified.
Ti–Ni alloys have excellent shape memory and superelastic properties and have been used as small, light-weight, and high-performance actuators in medical and electric appliances, motor vehicles, aircraft, etc.1,2) In the space engineering field also, many small shape memory alloy devices especially holding and release mechanisms for deployables have been used.3,4) Moreover, use of simple large structural components changing their shape by superelasticity or shape memory effect (e.g. expandable landing legs of planet landers) is expected to reduce heavy and complicated actuators such as electric motors. However, owing to poor workability of Ti–Ni alloys, large sheets are difficult to be processed and thus to be obtained especially in Japan.
Our research group has tried to give superelasticity and shape memory effect to a structural titanium alloy, Ti–4.5Al–3V–2Fe–2Mo (mass%), which is near-β (α + β) type titanium alloy known as SP-700, by controlling its microstructure through heat treatments.5–7) SP-700 has a good cold workability at room temperature and also exhibits superplasticity at around 1048 K,8) which is over 100 K lower compared to Ti–6Al–4V alloy. SP-700 alloy has been already used in space engineering as a structural material; spherical fuel tanks and high-pressure gas tank liners were fabricated by superplastic forming and welding of SP-700.9) Higher temperature annealing of SP-700 decreases α phase fraction and increases instability of β phase, increases β (bcc)-α′′ (orthorhombic) martensitic transformation temperatures to just below room temperature and thus brings about superelasticity at room temperature.6) This superelastic structural sheet material opens the door for several new applications of superelasticity.
In this paper, we propose a novel deployable nozzle of rocket motors by utilizing the superelastic SP-700 sheet. In an upper stage motor of a multistage rocket, an increase in the nozzle opening ratio is effective to increase specific impulse (Isp), while there is a limit of space containing the nozzle. Institute of Space and Astronautical Science (ISAS) developed extensible nozzle, which have two-separated nozzle parts cut at the middle in the axis direction and overlapped to save space at the third-stage motor (M-34) of M-V rocket.10,11) However, the mechanism of extension is complicated, heavy and high-cost, and also generates space debris. Therefore, the use of extensible nozzle has terminated at Flight No. 2 of Epsilon rocket.
We propose a brand-new concept of expandable nozzle utilizing superelasticity without complicated mechanism. Figure 1 shows a schematic of a superelastic deployable nozzle extension which enable to fold the nozzle inside of the allowable space in the rocket and to extend its size by superelastic deployment. The operation temperature of this deployable nozzle in space is around 300 K. The aim of this study is to verify the feasibility of the deployable nozzle through processing a 1/4-sized miniature model and conducting a folding-deployment test. In the fabrication, conditions of heat treatment, plate thickness control, and welding were considered. Folding (bending) and superelastic deployment properties and residual strains after deployment were examined by the folding-deployment test.
Deployable nozzle extension for rocket motors.
The as-received SP-700 material8) is a 1-mm-thickness sheet with a composition of Ti–4.39Al–2.95V–2.01Fe–2.01Mo–0.09O (mass%). The size of a miniature was set to be 1/4 of a practical nozzle.
In order to obtain superelasticity at room temperature, transformation temperature control through α/β ratio control by heat treatment is necessary. Figure 2(a) shows a typical stress-strain curve of superelastic SP-700 sheet (α/β = 36/64 (vol%)) in tension along the rolling direction (RD) after heat treatment at 1073 K for 3 h followed by quenching into water. The stress for inducing martensite (σSIM) is around 670 MPa and 0.8% strain is remained after 3% deformation. The texture of β phase in a superelastic heat-treated specimen is {001}β⟨110⟩β,7) which is known in β-type Ti-based superelastic alloys to show homogeneous shape recovery strain in the rolling plane.12)
(a) Typical superelastic behavior of 1-mm-thickness sheet at room temperature in tension and (b) typical SEM image of the large sheet after heat treatment (Dark: α, Bright: β).
Since ω phase formation due to aging in a thinning process of sheets decreases the martensitic transformation temperatures, an acceptable annealing temperature was determined between 1073 K and 1098 K for room temperature superelasticity after the aging. Two rectangular sheets (160 mm × 290 mm) were heat-treated in an electric furnace in air sandwiched by 5-mm-thickness stainless steel sheets. The heat-treated SP-700 sheets at 1123 K were quenched into water within 30 s corresponding to 25∼50 K temperature drop.
The microstructure of the heat-treated sheets was observed by a scanning electron microscope (SEM) to check the α/β ratio as shown in Fig. 2(b). The SEM image was taken under a back-scattered condition and round-shaped α phase is shown in dark gray. The phase fraction of α/β was determined to 28/72, which corresponds to the annealing at 1093 K.
2.2 Thinning of heat-treated sheetsNext, the heat-treated sheets were thinned by grinding down to 60 µm. The sheets were fixed during the process by hot-melt adhesive and removed by putting into boiling water. A liquid lubricant was used as a coolant for suppressing the temperature increase by grinding to avoid excess ω phase formation and related hardening of the sheets. The process of fixing and grinding were repeated several times to reach the final thickness.
The superelastic properties of the thinned sheets were evaluated by a bending test by using a jig which can apply 3% surface strain. Releasing from the jig, a superelastic recovery was observed and total residual strain of around 1% surface strain was confirmed. The amount of apparent residual strain is almost equal to the one obtained in a tensile sample of a superelastic SP-700 after a 3% loading-unloading cycle as shown in Fig. 2(a).
2.3 Welding to shape a nozzleTwo pieces of a sector form were cut from two 60-µm-thinned sheets by electro discharging and were butt-welded by micro-plasma welding at their edges to shape a miniature nozzle of truncated cone shape as shown in Fig. 3(a). During welding, metal vises were used to grip around the welding area and also to remove heat to avoid aging. The width of the welded area was around 0.5 mm and change in color due to oxidization was less observed. The inlet of the welded nozzle was fixed at a plate jig by adhesion.
Superelastic deployable nozzle (a) before folding, (b) after folding with banding bands, and (c) Mises stress distribution by FEA.
The nozzle model was easily folded by applying load at six points equally positioned around the nozzle so as the nozzle outer size becomes almost the same as that of the inlet as shown in Fig. 3(b). The position of a welded line was set at the middle of two loading points to avoid a large deformation on the welded zone. The folded nozzle was fixed using a banding band.
The folding behavior of the superelastic nozzle was also simulated by finite element analysis (FEA) utilizing Abaqus/CAE. The model consisted of 21600 shell elements. The material property was set by two different elastic regions from the stress-strain curve taken from the RD of a superelastic heat-treated specimen (Fig. 2(a)); the elastic and stress-induced martensitic transformation regions correspond to the linear elastic regions below and above 670 MPa, respectively.
Calculation was done by the implicit method and the result of Mises stress distribution was shown in Fig. 3(c). The simulated result clearly represented the folding of the miniature model, where the same valley and ridge lines were observed. Stress concentrations at severely curved surface regions are found close to the inlet of both simulated and miniature nozzles as shown by circles. A loading point itself also creates another stress concentration region.
3.2 Deployment testThe nozzle deployment test was done by cutting the band and its behavior was recorded using a high-speed camera with a flame rate of 1000 fps. Figure 4(a) shows picked-up images of the deployment behavior of the superelastic nozzle. The deployment was very fast and completed in around 0.01 s. The outlook of the deployed nozzle is shown in Fig. 4(b). Small residual strains were found at the most stress-concentrated points and at the loading points in the middle of the nozzle height (see Figs. 3(b) and 3(c)), while the whole circular shape especially at the nozzle outlet was barely changed.
(a) Sequence of superelastic deployment and (b) comparison of the nozzle shape after deployment.
The same folding-deployment test was conducted also for a 1/4 miniature nozzle made from SP-700 sheets in the as-received condition (without superelastic heat treatment) for comparison (Fig. 4(b)). It is seen that the nozzle made from as-received sheets shows the residual strain due to plastic deformation more clearly as shown by the circles compared to the superelastic nozzle. The nozzle outlet highlighted in color was slightly deformed to the hexagonal shape, unlike the heat-treated nozzle. This result indicates that superelasticity is necessary to maintain the original nozzle shape for a homogeneous combustion gas flow.
The superelastic deployment test also suggested that further improvement of dimensional stability could be achieved by relaxing the sharp bending curvature at the stress-concentrated areas by adjusting the folding procedure. Achieving superelasticity in the weld part is also necessary in addition to increase superelastic recovery strain by optimizing thermo-mechanical treatment of SP-700. This superelastic deployment technique will spread over new applications of large and light-weight superelastically-deployable structure, e.g. deployable landing legs, antenna, etc. of planet landers in space engineering field.
A novel deployable rocket nozzle was proposed utilizing superelastic SP-700 sheets. The process of deployable nozzle including heat treatment, thinning, and welding were considered, and a 1/4-sized miniature nozzle model was successfully produced. Folding-deployment tests were conducted for the miniature model and compared with FEA. The folded shape was similar to the calculation, and elastic deployment was executed revealing almost similar shape as initial. It was confirmed that superelasticity was effective to maintain the circular shape of the nozzle. The feasibility of a new concept of superelastic deployable structure was thus shown. A relaxation of the curvature at the stress-concentrated areas and the optimum thermo-mechanical treatment both for the bulk and weld parts were suggested for the further improvement of dimensional stability.
This work was partially supported by the Research and Development Grant, 2017, from the Amada Foundation and by the Grants-in-Aid for Fundamental Science Research (B), 2019, from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
