Category: Orbital Basics

Satellites Part 2: Lunar Orbit

by aubreygoodman, posted in Kerbalism, Orbital Basics

Vimeo: https://vimeo.com/307753956

YouTube: https://youtu.be/qCvHz1n0qxU

Welcome to Kerbalism. I’m your host, Aubrey Goodman. In this episode, we’ll review the deployment of satellites to lunar orbit.

First, let’s expand to consider the nearest moon. It requires more deltaV to get there, and more still to stabilize in a circular orbit. The good news is our Kerbin orbital satellite is over designed for its task. Its first stage does almost all the work, and we have plenty of fuel left over in the second stage for transfer orbit burns.

Once the craft is in planetary orbit, we need to perform two maneuvers to stabilize into orbit around Mun. If we do a really good job executing the maneuvers, we will settle into a circular orbit.

Orbital transfer between Kerbin and Mun can be done really at any time from a mostly equatorial orbit. This refers to the inclination of the orbital plane relative to the rotation of the body. Kerbin and Mun have very similar inclination, making it convenient to transfer between them. As we’ll find later, Kerbin’s other moon, Minmus, has a different inclination.

While a transfer can be made between Kerbin and Mun at any time, there are optimal points along the orbit where fuel use can be minimized, due to favorable alignment. Sometimes, we can save a huge amount of fuel simply by waiting for 20-30 mins.

Once we find a transfer orbit we like, with a destination periapsis at the desired altitude – that means the periapsis of the resulting orbit around Mun – once we find that periapsis, we can proceed with executing the maneuver at the appropriate time. Even perfect execution will result in slight misalignment with your designed objective. This is expected. If necessary, you can make corrections with RCS, but this is generally not required.

Now, after some time has passed your craft has traversed its path and is now approaching the periapsis of the destination orbit. You must burn retrograde until you slow down enough to stabilize into an elliptical orbit. Then, bring the apoapsis down to around the same altitude as the periapsis, resulting in a circular orbit.

Kerbin has a second moon, called Minmus. Its orbital inclination is about 6 degrees higher than Kerbin, so any craft headed there must also perform a maneuver to align its inclination. This is ideally done during orbital ascent, which reduces the inclination difference.

Our over designed satellite has enough fuel to enter stable orbits of both moons. But it also does very little. As we add capability to our satellite, the payload mass increases, and the first stage fuel requirements increase exponentially.

So that’s it for lunar satellites. In the next episode, we’ll focus on solar satellites; that is, satellites on an orbit similar to a planet. Those will lead us to a place where we will be able to establish a relay network of satellites that allows us to explore a wider part of the solar system. So stay tuned for that and much more!

And thanks for watching Kerbalism!

Satellites, Part 1: Deploying to Low Orbit

by aubreygoodman, posted in Kerbalism, Orbital Basics

Vimeo: https://vimeo.com/307509709

YouTube: https://youtu.be/ZcPy605hAiM

Welcome to Kerbalism! I’m your host, Aubrey Goodman. In this episode, we’ll review the basics of deploying a satellite to low orbit. Let’s get started!

Long before we think about sending people into orbit, we need to send smaller unmanned probes. These serve as opportunities to learn about the perilous environment of low orbit. As we’ll show, they also present a solution to the issue of communication within the solar system. Without relays to boost the signal back to the origin planet, probes must carry heavy radio hardware powerful enough to reach home. Heavy things cost more to send to orbit. Lowering the cost is often a matter of reducing payload mass or using fuel more efficiently. There are many things to consider, even when designing a simple satellite deployment mission.

First, let’s look at the simplest case, deploying to planetary orbit. Here, we use Kerbin as our planet. It is similar to Earth, so it works very well as a learning and planning tool.

The simplest form of an orbital launch vehicle is a single stage configuration. This means the entire vehicle is one single unit ready to fly to orbit, from takeoff through ascent to low orbit. This is both inefficient and expensive for a satellite. There’s really no reason for the heavy lift stage to remain in orbit once the satellite has reached its intended orbit.

Rather than hauling heavy engines and fuel tanks into orbit to live forever awkwardly attached to our satellite, we will consider a more popular two stage configuration. In this case, we use a solid rocket first stage and a liquid second stage for orbital stabilization and precision maneuvering.

Using a solid rocket first stage has some side effects. The most important is they burn until the fuel is gone. There is no in-flight throttle control. Throttle can be achieved by narrowing the nozzle geometry, but this is a design-time decision and significantly impacts the construction cost.

Using a liquid first stage, we would be able to cut throttle on the ascent, and throttle back up at the peak of ascent to circularize the orbit for payload deployment. Then, if we’ve done well, we can burn retrograde to return the first stage back to the surface.

Terms & Concepts

by aubreygoodman, posted in Kerbalism, Orbital Basics

Vimeo: https://vimeo.com/309674273

YouTube: https://youtu.be/pAcyaTRn_JM

Welcome to Kerbalism! I’m your host, Aubrey Goodman. Space is strange and foreign. There are lots of terms and concepts you probably haven’t heard before. We will use this terminology throughout the show, so let’s review the basics!

First, you blast off straight upward until your orbit reaches the desired altitude. The point with highest altitude along your orbit is called the apoapsis. Accelerating upward will raise your apoapsis. This is called a prograde burn; prograde means adding energy to the vehicle. When the craft reaches the apoapsis, we initiate a second maneuver (also a prograde burn) to accelerate sideways and increase our speed.

This raises the periapsis, or point with lowest altitude, up to the same altitude as the apoapsis, resulting in a circular orbit. In a circular orbit, the craft’s velocity doesn’t change very much. If we are orbiting a body outside of its atmosphere, there are no forces to accelerate the craft. This is referred to as a stable orbit.

If we wanted to land from a circular orbit, we would need to slow down, performing a retrograde burn. Retrograde means removing energy from the vehicle. Wherever we are along a circular orbit, burning retrograde will cause the apoapsis to settle at the craft’s location while the periapsis descends toward the planet’s surface on the opposite side of the orbit.

As the craft descends, its velocity increases. To be clear, we slow down now to speed up later at a lower altitude. If the periapsis is inside the atmosphere, we will begin to lose energy to drag forces. As we drop lower into the atmosphere, drag increases and vehicle speed decreases.

If instead we wanted to visit a moon orbiting the planet, we would need to burn prograde to speed up and raise the apoapsis to a higher altitude. This has the opposite effect as before. As the craft ascends, its velocity decreases. This time, we speed up now to slow down later at a higher altitude.

Each of these maneuvers is introducing a change in velocity, known as delta-v. As we compare different aspects of spacecraft design, we will be focused on optimizing delta-v. Many of the tools we use include automatic calculations for delta-v.

At this point, you might feel confused. In an elliptical orbit, the velocity changes along the path, so isn’t that the same as delta-v? Well, not exactly. As the craft moves around the ellipse, it speeds up toward the low point (or periapsis) and slows down toward the high point (or apoapsis). Each time the craft travels around the path, it returns to the periapsis with the same speed as it had the last time. Unless there is some other influence acting on the craft, it must obey the laws of physics and mathematics.

That’s all for this quick review. Stay tuned for satellite deployment!

And thanks for watching Kerbalism!