Efficiency Estimation of the Spacecraft Propulsion Using Sublimation of Hydrogels

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Дата публикации:
24 января 2022, 18:56
Секция 02. Летательные аппараты. Проектирование и конструкция
The mass production of hydrogels in recent decades makes it possible to consider their use as water carriers for sublimation space thrusters. The hydrogel bound water does not require the hydraulic fluid circuitry required for liquid rocket engines, making it possible to create a low pressure sublimation thruster with reduced feed system requirements. The calculated specific impulse of water vapor in such a system reaches 1000 m/s, which potentially makes it possible to create a propulsion system with a mass energy output of 900 m/s, which surpasses most of the existing solutions in terms of a set of indicators.
Ключевые слова:
hydrogel, space, rocket engine, low thrust
Основной текст труда

At present, compressed gases or reaction products of one- and two-component liquid propellants are mainly used as a working medium in spacecraft (SC) for rocket engines and gas generators. At the same time, the volume and mass of balloons for compressed gas are very high, and in addition, it is necessary to reduce the pressure of the working fluid from the storage pressure and to clean it from mechanical impurities. Tanks for liquid fuel are structurally complex, requiring diaphragm, installation of filling necks, etc., as well as the use of a displacement system, which is an essentially compressed gas cylinder with a corresponding pneumatic circuit [1]. This prevents the creation of storage and supply systems for the working fluid with low mass, especially miniaturized ones, and a large number of component parts delays and increases the cost of development and production.

In this situation, attention is drawn to the sublimation-based sources of the working fluid, which have a simple design, constant readiness for launch, high reliability and safety, ease of operation and high technical and economic parameters. The search for ways to use water as a working medium for a sublimation gas generator at higher temperatures, at which it is a liquid at atmospheric pressure, leads to polymer hydrogels capable of absorbing up to 2 kg of liquid per 1 g of dry polymer [2–4].

The measured density of the dry gel is about 1400 kg/m3, in a saturated state it practically does not differ from the density of water. At temperatures above 60 °C, the mechanical properties of the gel granule are lost and its partial dissolution begins; at temperatures below 0 °C, the granule solidifies, forming ice with a viscosity slightly higher than that of pure ice. After thawing, the strength of the granule decreases slightly. The filled granule has the consistency of jelly, can be easily cut with a knife and pinched off with your fingers.

No loss of water by the hydrogel was observed under mechanical action. During evacuation, a rapid (less than 10 minutes) loss of up to a third of the volume of the granules was noted, after which the granules froze and the weight loss practically ceased. Heating (by mechanical pressing against the metal of a large mass or a heater) led to the complete drying of the granules in the same time. During thermal cycling, a sharp drop in temperature in a closed volume with hydrogel granules led to dew loss, and the dew falling on the granules was quickly absorbed.

The described properties of the hydrogel potentially allow it to be used as a water absorber in low-pressure gas generators for space purposes. Storing water in the absorber prevents splashing, stabilizes the center of mass of the gas generator and ensures the supply of the working fluid to the engine in the form of steam. The pressure of the steam supplied to the outlet of the gas generator will be determined by the flow rate and the capabilities of the hydrogel to replenish it.

When filled with a 1U CubeSat by hydrogel, an orbital average heat flux of 0.5 W arriving at its surface allows water evaporation at a rate of 0.2 mg/s. In this case, the saturated vapor remains a continuous medium at positive temperatures of the hydrogel, and the losses due to the increased viscosity effect due to the scale factor does not exceed 25%. The realizable specific impulse, taking into account these losses, can be about 1000 m/s at an engine temperature of 60 °C. Evaluation of the propulsion system's passive mass shows that its mass energy output (the ratio of the total impulse to the charged mass) can reach 900 m/s, which is on a par with most propulsion systems with chemical engines and exceeds the performance of gas-jet engines.

The steam coming out of such a sublimation engine can be additionally heated in an electric heater or used as a working fluid for a plasma engine, which will increase the specific impulse, but reduce the thrust. If, on the contrary, it is necessary to increase the thrust, electrical or chemical energy can be used to heat the hydrogel.

Thus, on the basis of a hydrogel steam generator, propulsion systems can potentially be created that are capable of solving most of the problems arising during the operation of spacecraft. Near the Earth, such systems can be used to form and maintain constellations of micro- and nanosatellites, to maintain spacecraft in low orbits, as part of facilities for servicing low-orbit satellites.

Also, the potentially high reliability and safety provided by such sources of the working fluid makes it possible to use them for transport servicing manned space stations —  from means of transportation for cosmonauts to controlling the movement of free-flying modules and launching spacecraft from the station to other orbits with a reusable refuelable tug. The latter application will be especially important for a promising Russian high-latitude station.

  1. Belyaev N.M., Velik N.P., Uvarov E.I. Reaktivnye sistemy upravleniya kosmicheskikh letatel'nykh apparatov [Jet control systems of spacecraft]. Moscow, Mashinostroenie Publ., 1979, 234 p. (In Russ.).
  2. Khokhlov A.R., Dormidontova E.E. Samoorganizatsiya v ion-soderzhashchikh polimernykh sistemakh [Self-organization in ion-containing polymer systems]. Uspekhi fiz. nauk [UFN], 1997, vol. 167, no. 2, pp.113–128. (In Russ.). DOI: https://doi.org/10.3367/UFNr.0167.199702a.0113
  3. Ivanov Yu.V., Ivanov O.Yu. Uchebnye issledovaniya fizicheskikh svoystv gidrogelya [Educational studies of the physical properties of hydrogel]. Problemy uchebnogo fizicheskogo eksperimenta [Problems of educational physical experiment]: collection of scientific papers. Moscow, ISRO RAO Publ., 2017, iss. 27, pp. 75–76. (In Russ.).
  4. Vukalovich M.P. Teplofizicheskie svoystva vody i vodyanogo para [Thermophysical properties of water and water vapor]. Moscow, Mashinostroenie Publ., 1967, 159 p. (In Russ.).
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