top of page
  • Writer's pictureMatt Russell

#209 - Bianca Cefalo - Rocket Fallacies

This week we chat to Rocket Scientist Bianca Cefalo about being a rocket scientist and much more, and George joins Matt for a chat about Rocket Fallacies.

When I use the phrase “Cosmic Facts,” the reader is asked not to assume too rigid a meaning for the word “facts”; what is considered factual today is tomorrow recognized as capable of further refinement.

Harlow Shapley

(Nashville November 2, 1885 – October 20, 1972) was an American astronomer, scientist, and political activist and head of the Harvard College Observatory (1921–1952),

  • American scientist, head of the Harvard College Observatory

  • Shapley was born on a farm in Nashville, Missouri,

  • dropped out of school with only the equivalent of a fifth-grade education

  • Shapley went to study journalism at the University of Missouri. When he learned that the opening of the School of Journalism had been postponed for a year

  • He decided to study the first subject he came across in the course directory. Rejecting Archaeology, which Shapley later claimed he couldn't pronounce, he chose the next subject, Astronomy

  • Shapley received a fellowship to Princeton University for graduate work, where he studied under Henry Norris Russell and used the period-luminosity relation for Cepheid variable stars (discovered by Henrietta Swan Leavitt) to determine distances to globular clusters

  • Shapley's 1914 PhD thesis on the orbits of 90 eclipsing binaries virtually created in one stroke a new branch of double-star astronomy.

  • used RR Lyrae stars to correctly estimate the size of the Milky Way Galaxy and the Sun's position within it by using parallax

  • He was part of the great debate; Shapley took the side that spiral nebulae (what are now called galaxies) are inside our Milky Way, while Curtis took the side that the spiral nebulae are 'island universes' far outside our own Milky Way

  • "Here is the letter that destroyed my universe." He said when receiving the evidence that he was wrong for criticising Hubble, whose findings went on to reshape fundamentally the scientific view of the universe

  • In 1953 he proposed his "liquid water belt" theory, now known as the concept of a habitable zone

  • He is also responsible for the addition of the "S" in UNESCO (United Nations Educational, Scientific and Cultural Organization)

  • Shapley married Martha Betz, one of his five children, Llyod won a Nobel Prize in Economics in 2012

  • The Shapley-Ames Catalog of Bright Galaxies is a catalogue of galaxies published in 1932 that includes observations of 1249 objects brighter than 13.2 magnitude

  • The Shapley Supercluster or Shapley Concentration (SCl 124) is the largest concentration of galaxies in our nearby universe that forms a gravitationally interacting unit, thereby pulling itself together instead of expanding with the universe


Rocket Fallacy Special

The Rocket Pendulum Fallacy

Robert H Goddard the American contemporary of Von Braun, Sergey Korolev, Vikram Sarabhai, Konstantin Tsiolkovsky. The guy that has a list of rocketry firsts longer than the Saturn V first stage, built his very first rocket with the Rocket Engine at the Top. His thinking seems pretty intuitive.

How does the rocket keep pointing upwards? Well, I suppose, in Goddard's head, the weight of gravity would pull down on the dangling rocket below and it would act as a pendulum, always correcting itself, It’s much easier to pull a broom handle to a higher position than control it while balancing it in the palm of your hand.

Like the escape Launch tower of the Apollo 11, it pulls or tractors the astronaunts away from danger, so he was right ? ….right ? ….wrong.

1926 Goddards first liquid propelled rocket was completely useless. "Nell", rose just 41 feet during a 2.5-second flight that ended 184 feet away in Aunt Effie’s cabbage field, a distance shorter than the apollo Launch vehicle that was a direct descendent of this very first launch 40 years later. But you may have noticed that the rocket engines are definitely at the bottom with Saturn V.

So what gives?

Essentially Gravity is a universal (if fictitious) force, acting on the whole rocket equally, just because to us it looks like a pendulum it tricks into thinking that’s how it will behave, and if you are still wondering why it doesn’t work, don’t worry, after all Goddard didn’t figure it out!

The broom handle analogy has a few clues, yes when you are pulling the broom your feet are on the ground so it’s easy to pull, but how easy would it be if you were pulling yourself up at the same time?

The pivot point of a pendulum, where you perceive the rocket engine, has to be applying a force upwards, it does this in a clock for example because it’s bottled in place and attached to the wall which is attached to the floor, and using Newton reactionary force is applying a force in the opposite direction of the force of gravity, well until it breaks.

But the rocket can’t do this because it attached to the fuel tank below so the force will be in the direction that the fuel tank is facing when it needs to be pointing always up to counteract the force of gravity.

But the first thing I thought of is why not have the fuel tank on a pivoting attachment, but that doesn’t work because you need a mechanism to keep the rocket engine pointing upwards all the time, which was kind of the point of the pendulum wasn’t it, and if you now build a way of doing that “making sure the rocket engine is always pointing up” thing you may as well stick it on the bottom of the fuel tank where it won’t be firing flames down onto your ...fuel tank, the thing full of fuel.

An amazingly common mistake, that has reared its head again with the drone pendulum fallacy, with people mounting their batteries below the craft. But it’s all about how far away from your centre of mass is away fro the pivot point.

Pointing towards the planet and firing your rockets Fallacy

Say are in orbit around a planet and you decide, I wanna go home now. I’m pretty certain that most people would think, you just burn the rocket engines and point your space ship at earth and blast home.

The most amazing thing about this is that you may end up going higher than you started and all you’ve really done is slightly offset your orbit and made it more elliptical. Of course you could burn to go back to earth if you burnt your engines continually and pointed yourself at the earth all the time, but it’s wildly inefficient. Why? Well, you want to get rid of your velocity but burning the rocket engine perpendicular to your motion is not efficient as it is a vector, and the way these vector magnitudes add up means you would be better off doing the burn in the opposite direction of travel, just do the maths.

So burn your engines in the opposite direction of travel (retrograde), this will drop your orbit, but crazily you velocity will actually go up? Which is wildly unintuitive.

You are doing negative work on the system, but the kinetic energy increases. But your potential energy is decreasing of course but not as much as the energy needed to orbit so your speed has to go up to keep it circular, The change in kinetic energy is proportional to the vehicle's velocity at the time of the burn. So to get the most kinetic energy from the burn, the burn must occur at the vehicle's maximum velocity, at periapsis.

Pointing at the moon (from earth orbit)

Same deal as going home, Don’t point at it, but instead burn at perigee to increase the apogee. (or have it so halfway through your burn time you hit perigee) This makes use of the Oberth effect, basically, when you are at perigee you are going your fastest and this means you have the most kinetic energy (your potential is at it’s lowest) and the rocket is more efficient at higher kinetic energies. Why. Well the fuel itself has higher kinetic energy and the rocket can steal some of this energy as well as the fuels chemical energy, to get more energy for its orbit. The overall system remains the same energy, we didn’t create energy because the ejected propellant ended up with less mechanical energy.

Going to the sun.

Same Deal. Just blasting toward the sun is a bad move, a more efficient way is to make your orbit more and more elliptical, by firing your rocket retrograde as before this makes the orbit more and more elliptical and the perihelion gets closer and closer to the sun. again you would be doing your burns to get the most out of the Oberth effect. But there is an even more efficient way, but it takes longer. If you burn Prograde you can make your orbit more elliptical and make the spacecraft get higher and higher from the earth, and then because the velocity is so small at apogee that you can give it a slight nudge to bring in the perihelion.

However, you don’t do either in reality. You use Gravity assists to make a more efficient and quicker orbital manoeuvre.

Gravity Assist it’s not the gravity of the planet that speed you up.

It’s its orbital motion.

Gravity assistance can be used to accelerate a spacecraft, that is, to increase or decrease its speed or redirect its path. The "assist" is provided by the motion of the gravitating body as it orbits the sun. The gravity of the planet speed the spacecraft up as it approaches, but that same gravity slows the spacecraft down by the same amount as it leaves, But in the meantime the spacecraft has sped up because it hitching a ride with the planet’s orbit around the sun, to keep up with the orbit, the massive planet has donated some of it’s orbital energy to the tiny spacecraft. The spacecraft has slightly altered the planets orbit, but the effect is tiny.

It’s very similar to an elastic collision between a tiny object and a much bigger one.

Imagine standing at the side of the road, and throwing a tennis ball at 20mph in front of a car going past at 70 mph The driver of the car sees the ball approaching at 50mph and then departing at 50 km/h after the ball bounces elastically off the front of the car. Because of the car's motion, however, that departure is at 120 mph relative to me at the side of the road; the ball has added twice the cars difference in velocity to its own.

gravitational slingshots can also be used to reduce the speed of a spacecraft. Like Mariner 10. The effect can be enhanced with a rocket burn at periapsis the Oberth Effect.

Although the orbital speed of an inner planet is greater than that of the Earth, a spacecraft travelling to an inner planet, even at the minimum speed needed to reach it, is still accelerated by the Sun's gravity to a speed notably greater than the orbital speed of that destination planet. If the spacecraft's purpose is only to fly by the inner planet, then there is typically no need to slow the spacecraft. However, if the spacecraft is to be inserted into orbit about that inner planet, then there must be some way to slow it down

Suborbital spacecraft don’t get into orbit. But yes they do.

They don’t get into an orbit around Earth's surface, but they get into a highly elliptical orbit with the tiny point that is the centre of mass of the planet, it’s just that the earth's surface is inconveniently in the way.

There is no gravity on the space station

There is almost as much gravity on the space station they are pretty close to the centre of the earth in comparison to people of the surface. The difference is they are falling, and when you are falling you are not experiencing the effect of gravity. Or the fictitious force of gravity as you are following a straight line and you are not being accelerated to keep you on curve line, the curved line of spacetime itself.

To move an object in space is just as hard as to move an object sideways on earth.

Things don't move slowly in low gravity.


143 views0 comments

Recent Posts

See All


bottom of page