Essay: Propellants used in space programs

1. Introduction
The propellants used in space programs pose specifically three environmental concerns: Ground-based Impacts which range from groundwater contamination to explosions caused by improper handling of propellants. Atmospheric Impacts generally caused by interaction of propellant exhaust with the atmosphere. Biological Impacts encompass toxicity and corrosiveness of propellants.
These impacts have sought to be alleviated by space system developers as doing so could potentially reduce both cost and risk- especially cost and risk related to propellant transportation and storage, clean-up of harmful releases, human exposure to toxic compounds, setup requirements for handling hazardous propellants, and orbital debris. The perpetuated utilization of highly toxic propellants that engender environmental pollutants keeps program costs high—but the cost of developing and qualifying green alternatives withal inclines to be high. This has traditionally slowed development even when a green propellant provides potential performance benefits. Moreover, the term “Green Propellant” is often misunderstood as totally environment-friendly. All propellants affect the environment in some way or the other. For example, all launch vehicles produce exhaust which can comprise carbon dioxide, soot, water vapour, sulphates, oxides of nitrogen, and inorganic chlorine. All of these compounds have an environmental impact in one way or other. Given these facts, a green propellant is more acceptably viewed as one that seeks to minimize or eliminate an acute environmental impact in one or more of the three areas. A green propellant is liable to have its own environmental impacts, which may be identically tantamount to the current technology in certain areas. For example, many green propellants seek to eliminate hydrazine because of its biologic impact, but they still present atmospheric or space-predicated effects.
2. The Need for Green Propellant
To reduce air pollution during rocket launches much research has been done to develop space propellants that are environmental friendly (“green”) and contribute towards non-toxic propellants. These propellants are generally easier and safer to handle than the conventional ones, and are expected to bring down the costs associated with propellant transport and storage, and in spacecraft development and on-ground operations. In recent years low toxicity (or “green”) liquid rocket propellants have become captivating as possible substitutes for hydrazines and nitrogen oxides in low or medium thrust engines because of the reduced environmental impact and, more so, the cost savings associated with the drastic simplification of the required health and safety precautions. High-energy green propellants (like ADN, HAN and HNF) are predicated on organic molecules and compensate the high molecular weight of their decomposition products with proportionally higher operational temperatures, which still pose paramount challenges to the entelechy of durable catalytic reactors and radiatively cooled thrust chambers. Hydrogen peroxide (H2O2) does not suffer from these disadvantages and is now being reconsidered as a promising green monopropellant and bipropellant (in conjunction with hydrocarbons) for low and medium thrust applications.
3. Development in Various Compositions
Fresh class of high energy propellant compositions, often in the literature verbally expressed as High Performance Green Propulsion (HPGP), include in their compositions such ingredients as Ammonium Di-Nitramide (ADN, e.g. Swedish propellant LMP-103S). Its specific impulse is around 235 seconds. The development of HPGP was initiated with the goal of meeting the requirements for future satellite missions. After more than 10 years of R&D and a successful in-orbit demonstration during the PRISMA mission, the HPGP technology has proven to provide improved performance, enhanced volumetric efficiency, reduction of propellant handling hazards and safer launch operations.
Another important alternative to be used as liquid storable propellant for small spacecraft and satellite ACS or RCS, especially taking into account the health and environmental concerns, is 98% hydrogen peroxide of HTP (High Test Peroxide) class. The substance is essentially non-toxic with exceptional environmental compatibility and no risk connected to use of high explosives. AND is high explosive compound that hypothetically may be separated from its solution or even may crystallize spontaneously under adverse conditions such as, vacuum. What more, the 98% HTP may certainly be applied in lieu of hydrazine monopropellant and it additionally might accommodate as an efficacious oxidizer in nontoxic bipropellant cumulations to develop and operate a satellite propulsion system for a fraction of the cost of traditional systems. With the correct materials of construction, felicitously prepared in the laboratory and conditioned, the chemical is very storable and stable — customarily decomposing at profoundly low rates (obtainable rates of self-decomposition with current HTP technology may be far below 0,1% per year). Certainly, the primary advantage of using much safer liquid propellant in the form of 98% HTP for many space propulsion applications such as, satellites, will be the reduction of costs, largely by eliminating the requirement for SCAPE suits required for traditional toxic propellants; no requirement of extensive propellant safety precautions and isolations of the space vehicle from ongoing activities during propellant loading operations; high density of 98% HTP would be the key feature for the reduction of mass of the entire satellite boards and quite lower cost compared to other thruster propellants.
Other oxidizer that can be used as a substitute for both hydrazine and ammonium perchlorate (AP, in solid rocket propellant) is Hydrazinium Nitroformate (HNF). Though substantial advances in recent years, there are numerous issues that argue against the use of HNF. There are unresolved problems concerning thermal stability and friction sensitivity of this compound, as well as various compatibility problems. HNF is also quite expensive to produce and due to its carcinogenic hydrazine base not strictly a green propellant.
Another chemical compound that is primarily considered as a possible hydrazine monopropellant replacement is hydroxyl ammonium nitrate (HAN), in the form of liquid solutions. However, it has not yet reached any practical applicability so far. Mostly due to the problems regarding lack of suitable ignition catalysts, complicated combustion mechanisms, relatively high sensitivity and material incompatibility.
The last but not least example includes the whole group of chemical compounds soi-disant ionic liquids. These are liquid chemical compounds (salts with very low melting points) that typically show substantial advantages over mundane fuels or solvents, such as high stability, low toxicity, good solvent characteristics, and virtually no vapour pressure. Unconventional propellants predicated on energetic ionic liquids have been proposed. Some of them have already been proved to be plenarily hypergolic with 98% HTP.
Yet there is need for further development of green chemical space propulsion technologies for their future implements in thrusters. New oxidizers, energetic materials and manufacturing materials are still a requisite. The development of these elements requires the use of many theoretical and experimental tools that in fact are already made available in modern chemistry, such as quantum chemical calculations, organic, inorganic or analytical chemistry and through powerful spectroscopic methods.
4. Solid Propellant Based On ADN
Solid propellants, in current era are widely used in booster in large boosters for launchers and, to minor extent, for in-space propulsion. Propellants for these applications are primarily based on the oxidizer ammonium perchlorate (AP), NH4ClO4, and aluminium powder embedded in a polymer binder matrix such as HTPB (Hydroxyl Terminated Polybutadiene) or PBAN (poly acrylonitrile-co-butadiene-co-acrylic acid). Although AP is an excellent oxidizer due to its relatively low hazardousness and the possibility to mould its airborne properties, it has negative impacts on the environment and on personal health. By replacing AP with ADN there will be no hydrochloric emission since ADN only contains hydrogen, nitrogen and oxygen. Calculations show that ADN-based solid propellant can achieve performance equal to or higher than that of the conventional AP-based propellants.
It is not feasible to employ newly developed propellant to a large sized vehicle such as, launcher boosters. Thus smaller and less cost-sensitive applications seem to be a better choice. So, ADN-based propellants are more likely to be employed for in-space propulsion applications, where liquid propulsion system is vastly used. Liquid rockets provide high performance and modifiable thrust, but they are costly and use toxic propellants such as hydrazine, mono-methyl hydrazine (MMH) and nitrogen tetroxide (NTO).
Solid propellants possess benefits such as simplicity, storability and compactness. Furthermore, no propellant distribution system is required which enables immensely colossal amelioration in reliability and cost. One disadvantage is however their relatively low specific impulse. Despite this, solid propellant rocket motors have been used to propel spacecraft in numerous missions since first used in the upper stage of the first U.S. Satellite Explorer I in 1958. More recently solid propellant rocket motors are considered to be used for the ascend module in the Mars sample return mission. Substituting the AP-based propellant with ADN will provide higher performance and reduced environmental impact. Future work concerning solid ADN-based propellants will focus on improving the mechanical properties and to characterize the sensitivity.
5. Liquid Monopropellant Based On ADN
One of the most promising alternatives to monopropellant hydrazine is blends predicated on an oxidizer salt dissolved in a fuel/dihydrogen monoxide coalescence. Hydroxyl ammonium nitrates (HAN) has been studied for this purport .Due to its high solubility, ADN can be utilized in the same way as HAN. The development of ADN-predicated monopropellants commenced at FOI in 1997 on a contract from the Swedish Space Corporation, SSC, and several different propellant formulations have been developed and tested. Future work concerning liquid ADN-based monopropellants will focus on ignition and thruster development.
6. Hydrozinium Nitroformate: A Novel Substitute for AP
Utilization of high performance oxidizers into solid propulsion system provide high specific impulse, reduced or low toxicity and have anticipated exhaust profile characteristics, when compared to other using traditional solid propellants. Solid propulsion system could provide very high specific impulse by utilizing high performance oxidizers such as Hydrozinium Nitroformate (HNF). HNF is much anticipated oxidizer to use in solid propellant formulations due to its energetic nature which provides high performance.
Many oxidizers suffer from varying degrees of instability, such as photosensitivity, shock, friction and impact sensitivity, decomposition in presence of moisture, sensitivity to pH and incompatibility (such as hypergolic reactions) to other propellant materials. A typical example of incompatibility is the reaction between HNF and curing agents used in solid propellant binder grain such as HTPB and GAP. In order to advance compatibility of the propellant and to reduce the risks by friction sensitivity during mixing and casting operation, Cesaroni et al. 2002) taught an oxidizer package comprising a solid oxidizer in the form of discrete pellets from a predetermined geometric shape, the pellets were arranged in an array with spaces amongst the pellets and a holder for maintaining them in the array to receive a binder introduced to spaces amongst the array of pellets. The binder introduced provides a support matrix to give complementary burn rates for the pellets and the support binder matrix. The pellets were produced with HNF or ADN and the composition can present yet ballistic modifiers, other additives and, additionally, ultrafine aluminium.
A monopropellant used in the conventional manner for spacecraft propulsion in existing systems, whereby it is to be noted that due to the properties of the system, less rigorous requirements concerning storage, transport, and handling are possible, was proposed by van den Berg et al. (2004). Their research showed that solid high-energy oxidizers, such as HNF or ADN, when dissolved in water, render a liquid monopropellant system with a specific impulse that could be equal to the specific one of the conventional monopropellant.
7. The Consideration
Use of Ammonium perchlorate in composite propellant and that of hydrazine in liquid propulsion is extensive. Yet they both shows high risk of adverse impacts on environment as well as human health. ADN and HNF are emerging as possible eco-friendly substitutes to the AP and hydrazine. Despite the ADN hygroscopicity and HNF sensitivity, they have considerably higher specific impulse than AP based propellant systems, reduced toxicity and desirable exhaust gas profile characteristics, when compared to traditional solid oxidizer (AP). Moreover they do not comprise chlorine, thus eliminating the generation of harmful chloric acids. These are the reasons why there is elevated interest in utilization of these propellants.
8. Applications
Probable application is the main driving force for the selection of a green propellant as exemplified by three examples following:
 Boosters
Since the volume of propellant contained in launcher boosters is huge, propellant cost plays a vital role in selection. The explosion risk is also an important factor. As oxidizers or fuels having potential monopropellant behaviour are not very good contenders from safety point of view. All these constrictions are fulfilled by LOX-hydrocarbons combinations such as kerosene. Many US and Soviet launchers are using special kerosene amalgams to reduce adversities like coking and combustion instabilities. The green propellant that can be used in place of kerosene is methane. The biggest advantage of methane over kerosene is the possibility to use a fuel rich gas generator without soot formation and noble cooling efficiency of methane. Moreover, methane is injected in gaseous state lowering the risk of combustion instabilities. Besides the conventional LOX-kerosene and LOX-methane recipes, some light hydrocarbons and ethers offer striking properties such as greater Isp, higher density and regenerative cooling followed by gaseous injection.
 Manned Capsule RCS And Landing Retrorockets
Till this date the reference propellants are MMH / N2O4. The substitution by non-toxic propellants would offer a significant improvement for the crew safety and for post recovery operations. Possible solution emerge viz. new monopropellants such as organic nitrate salts and mixtures or safe combinations like N2O and organic liquids. Certainly, they offer lower Isp than MMH-N2O4 but they are much nontoxic. A critical point would be the ignition reliability which is unconditionally essential for the crew safety. For monopropellants the critical point would be catalytic or non-catalytic ignition and for N2O / hydrocarbons, the catalytic (N2O decomposition) or electric ignition.
 AIM (Automatic Interplanetary Missions)
Currently MMH-N2O4 or hydrogen are used extensively in AIMs. For manned missions LOX-LH2 is perhaps the finest choice but may require considerable developments for the landing phase. For less severe Delta V requirements, N2O / hydrocarbons or new monopropellants (ADN or HAN) are preferable solutions.
9. Toxicity and Control
Many green propellants are certainly quite toxic. Toxicity level is defined as {Threshold Limit Value (TLV) — Time Weighted Average (TWA); 8 hours per day upon weekly exposure}. Many green propellants have toxicity level in the 1 to 100 ppm range. But there are some which represents astonishingly low limits viz. HTP, toxicity level is 1 ppm (like NTO) and it is 25 ppm for ammonia. Nonetheless the effective exposure risk is lower for HTP than for NTO, which is even worse in the case of ammonia. On the other hand, light hydrocarbons and N2O are non-toxic, they only have narcotic effect at high concentrations.
This toxicity can be alleviated in some cases by sub cooling. The explosion hazards should also be analysed carefully if large propellant quantities are used. This is a special concern for monopropellants, but N2O and HAN / ADN seem reasonably safe from this point of view.
Many propellants exhibit an astronomically immense density variation versus temperature. This is especially true for N2O (relative density 1.2 near boiling point and 0.7 near 30°C). From the system perspective, it is very efficient to increment the propellant density. This betokens that most light hydrocarbons and N2O should be cooled afore tank filling for astronomically immense quantities (boosters). The integrated advantage of cooling is the reduced vapour pressure (preferably below atmospheric pressure, except for N2O) resulting in low pressure operation for all ground equipment. In additament, the cooling requisites are much more lax than for cryogenic liquids. A classical industrial refrigerator, like those utilized in deep freeze industry, is sufficient to cool the propellant, vapours can be facilely recondensed. The integrated advantage of cooling is the lowering of vapour pressure for toxic or very flammable products. This will reduce the peril of toxic fumes (e. g. N2O4) or explosion (air / light HC vapour amalgamation) in case of spillage. All these points are taken into account for the trade-off.
10. Concluding Remarks
Finally, the providence of green propulsion will depend on its ability to satisfy the two primary requisites for its progress — higher performance and lower costs. U.S. space agencies have already begun to move towards green propellant with acceptance of LOX/hydrogen and LOX/kerosene launch vehicles and greater use of electric propulsion for spacecraft. Till this date environmental impact of launch vehicles might felt low, because launch rates are nominal. Green technology could make interplanetary missions more proficient and sample return missions from distant bodies more practicable. Green technology isn’t just a future prospect — it is already a part of space development, and further development seems highly prospective.