In this article, we will explore why it is so difficult to send objects into space and the various propulsion methods we can use to control them once they are there.
In the previous article, we have seen the working principle of the different types of rocket engines used in modern orbital launch vehicles to get payloads into LEOs and beyond. But have you ever wondered why these rockets are so enormously huge?
Due to Earth’s gravitational well, the hardest part of space travel is the very act of getting into space – conventionally starting at the Kármán line which is situated at an altitude of 100km above sea level [1, p.84] itself. This is because a lot of energy needs to be imparted on the vehicle in order for it to reach a mean minimal orbital velocity of 7.8 km/s and achieve a stable LEO .
Figure 1: The specific energy distribution as a function of distance zO from
the earth’s surface to a spacecraft in a circular orbit (e = 0). [3, p.86]
However, once the payload starts to gain altitude, the gravitational field experienced by it gets weaker and thus fewer energy is needed in order to maintain the speed of the vehicle. In fact, the higher the spacecraft’s altitude, the larger the proportion of potential energy is stored when compared to its kinetic energy (Fig. 1). As such, once a spacecraft has reached the vacuum of space where there is no longer any air friction, its orbital speed would remain roughly constant even with zero energy expenditure.
Nonetheless, there still needs to be some form of propulsion once a spacecraft is in its desired orbit in order for ground controllers to perform attitude control. Depending on the function of the spacecraft, they would need to be correctly orientated at their subject of interest in order for them to get and transfer any useful data. Additionally, spacecrafts at lower orbits will still be subjected to drag from the thin atmosphere and thus orbital station-keeping (small, periodic burns to correct for orbital decay) is also required . This is the reason why all orbiting satellites have a useful lifespan – as there is only a limited amount of fuel a satellite can carry; the expenditure of those fuel sources would usually render the satellite useless as it can no longer be controlled.
Knowing this, multiple spacecraft propulsion systems specifically designed for the vacuum of space were developed and they should not be confused with the much larger rocket engines used by launch vehicles. The remainder of this sub-section would therefore be used to briefly touch upon these methods.
Firstly, most satellites would either use monopropellant rockets or some form of electrical propulsion (EP). A monopropellant rocket differs from a bipropellant one as it only uses a single chemical – commonly hydrazine. This is a toxic and carcinogenic compound and as such newer satellites are switching to EP while greener systems are being investigated .
Just like the bipropellant rocket engine, there are also many different types of EP-systems, but only two – the electrostatic ion and Hall-effect thruster being successfully flight proven. Rather than accelerating the reaction mass to high speeds by relying on fluid dynamics and high temperatures, these methods uses electromagnetic or electrostatic forces instead. Over the years, the proven reliability and higher specific impulse of these thrusters have led to a gradual shift in the number of spacecrafts equipped with EP technology (Fig. 2).
Figure 2: Number of EP-based spacecraft launched in the years 1981–2018, divided into mission type. [6, p.12]
As newer and smaller EP-systems are being rapidly developed, the small satellite sector is seeing a strong growth with more propulsion-requiring CubeSat missions being proposed. This presents one of the opportunities of space commercialization as the relatively lower costs of these CubeSat missions paired with a longer operating period by having an EP-system on board would allow small satellite designers such as universities and small organization's to justify the initial spacecraft investment. The maturity of these EP-systems from concept to flight heritage could still be improved however as the limitations of the CubeSat platform itself has meant that less than 10 CubeSats have flown with propulsion systems in the years leading up to 2017 .
End of Part III
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 O'Leary, B. L. & Darrin, A. (2009). Handbook of Space Engineering, Archaeology, and Heritage. CRC Press.
 Swinerd, G. (2008). How Spacecraft Fly. Praxis Publishing.
 Pasquale, M. S. (2016). Chapter 5 - Orbital Mechanics. Manned Spacecraft Design Principles. Butterworth-Heinemann.
 Phillips, T. (2000). Solar S'Mores, NASA.
 Gohardani, A. S., Stanojev, J., Demairé, A., Anflo, K., Persson, M., Wingborg, N., & Nilssone, C. (2014). Green space propulsion: Opportunities and prospects. Progress in Aerospace Sciences, 71, 128-149.
 Lev, D., Myers, R. M., Lemmer, K. M., Kolbeck, J., Koizumi, H., & Polzin, K. (2019). The technological and commercial expansion of electric propulsion. Acta Astronautica, 159, 213-227.
 Lemmer, K. (2017). Propulsion for CubeSats. Acta Astronautica, 134, 231-243.