Star Struck

More Rocket Science

Posted on by nealsumerlin Posted in Rocket Science

Take a look at these two images, both the tail end of the first stage of a really big rocket. One is from almost sixty years ago. The other is contemporary. Both have a lot of rocket engines designed to work together. Can you tell which is which?

https://i.sstatic.net/45zNk.jpg

https://spaceflightnow.com/wp-content/uploads/2023/02/20230208raptors.jpg

OK, the graininess of that first image may have given it away. Those 30 engines are on the first stage of the N-1 rocket, the rocket that the Soviet Union hoped would take their cosmonauts to the moon. As we all know, the American moon rocket, the Saturn V, worked spectacularly. The second image is of the SpaceX Super Heavy booster, the first stage of the Starship launch vehicle. It has 33 rocket engines.

The N-1 rocket flew four times between 1969 and 1972, none successfully. The record for the Super Heavy is mixed, but it has scored some notable successes in itself, independent of the success of the Starship second stage.

Why did the N-1 fail and the Saturn V succeed?

There were two ways to build a rocket big enough to take men to the moon in the late 1960s, each with its own problem. You could build really big engines and cluster a few of them together in the first stage, the one with the hard job of lifting the entire vehicle from the launch pad. This was the approach taken with the successful Saturn V. Five enormous engines powered its first stage.

https://www.clarionledger.com/gcdn/-mm-/92a9acee110601543bc673fc67b15afad0432a11/c=0-62-1277-783/local/-/media/2016/06/17/JacksonMS/JacksonMS/636017701705752423-saturn-rocket-stage-2.jpg?width=660&height=373&fit=crop&format=pjpg&auto=webp

The problem with such huge engines is something called combustion instability. In such a large engine, with massive amounts of fuel and oxidizer mixing and igniting, turbulence and pressure variations could multiply and blow the engine apart. The solution lay in the design of the engine’s injector plate. A plate sat at the top of each F-1 engine. Holes across the plate’s surface forced the engine’s propellants (liquid oxygen and kerosene) into the combustion chamber. The instability was solved by the addition of baffles (dividers) across the injector plate’s surface.

http://heroicrelics.org/info/f-1/f-1-injector/f-1-engine-injector.jpg

Static testing (firing the engines on the ground) gave NASA the confidence to fly the rocket. The Saturn V flew 13 times between 1967 and 1973, all successfully.

The other approach is to use a smaller rocket engine and cluster a lot of them. That was the Soviet approach. In the rush to get to the moon first, the Soviets did not run static tests. The problems were not so much with the engine itself; in many ways it was a very efficient design. Without diving too deep into the details of each failure, we can say that the coordination and control of thirty engines was just too much to manage.

But Starship has 33 engines! Why does it work?

Well, it hasn’t always. But it hasn’t always failed, either, as seen in this image of all 33 engines firing.

https://cdn.mos.cms.futurecdn.net/ZW6aFHbMYgrgQ6fe4Emnfn-1200-80.jpg

The shortest answer to the posed question is 21st century computer control versus that available in the late 1960s.

The Saturn V was a marvel, but that first stage used brute force. SpaceX’s Super Heavy Booster, if it can ever get past the testing stage, really is an elegant and efficient rocket.

It’s Not Rocket Science

Posted on by nealsumerlin Posted in Uncategorized

The basics of rocket science really are pretty simple. Send a lot of hot gas out one end of a rocket, and it will go in the opposite direction. You know what’s hard? Orbital mechanics, maneuvering a spacecraft to rendezvous with something else. Or at the very least, it’s counterintuitive.

On June 3rd in 1965, the Gemini 4 mission was launched, with Jim McDivitt and Ed White as the passengers. The mission is best remembered for White’s spacewalk, when he opened the hatch and floated free, attached to his craft only by a tether.

https://www.nasa.gov/wp-content/uploads/2023/03/178429main_image_feature_838_ys_full-a.jpg

Before that however, McDivitt was instructed to rendezvous with the spent second stage of his Titan booster, trailing behind him in orbit. Being able to find and meet up with another object in space was crucial to the success of the moon missions, where the two astronauts in the lunar module would have to catch the command module for their ride home. McDivitt did what seemed quite natural to an experienced pilot. He turned his capsule around, pointed it at the rocket stage, and fired his thrusters.

https://live.staticflickr.com/5814/22836085892_9af3aec7eb_b.jpg

What happened? Not what he expected! He found himself moving away and downward.

NASA had not yet figured out quite how things worked in orbit. Counterintuitively, you have to slow down in order to speed up. And doing so lowers your altitude.
Something orbiting close to the Earth, where its gravity is stronger, has to move fast to stay in orbit. The International Space Station orbits at 17,160 miles per hour, 250 miles high. Communication satellites that appear stationary from the ground orbit 22,236 miles high, at 6,867 mph. And when the moon is at its average distance of 239,000 miles, it is moving at the leisurely pace of 2,258 mph.

Of course this problem was solved, and any time astronauts launch to the ISS, they apply the lessons learned. What would have worked for McDivitt?
With an object behind him, he would need to increase his speed by firing his thrusters. This would move him into a higher orbit, where he would move more slowly. This would allow that Titan second stage, moving faster in a lower orbit, to catch up. Eventually the Gemini spacecraft would be behind the rocket stage. A series of maneuvers that allowed him to match orbits would let him rendezvous.

This excellent animation will tell you all about it!

Location, Location, Location

Posted on by nealsumerlin Posted in Rocket Science, Spacecraft

Why does SpaceX launch its gigantic Starship from as far south as you can get in Texas?

Google Maps Image

Is it because Elon Musk likes lower taxes and a looser regulatory environment than he might find in other states? Probably. But there are very good technical reasons why orbital launch facilities are generally situated close to the equator. American rockets lift off from Cape Canaveral in Florida, at 28.6 degrees north latitude, while the SpaceX facility is just south of 26 degrees north. The European Space Agency launches from French Guiana, about 5 degrees north of the equator.

Google Earth Image

All of these face bodies of water to their east, the Atlantic Ocean for Florida and French Guiana, and the Gulf of Mexico for Texas.

All of these locations take advantage of the Earth’s rotation to give them a little more velocity than they would have from a stationary launch pad. And the closer you are to the equator, the faster that speed.

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjUMkxdyym4NkDDYw3LGn0vYJCSjqVDD4-OAoxbeA4GRBwwqQzt8n5Tu5XX-mL0Au0NZiPzmC13tcKNl_tNyUMpZeh8gDtW4LQ0puwLxlvpjayG17XOyqhQslgpfBM6ZPqljOPg0neeJRiq/s1600/earth.jpg

Could you launch from farther north or south? Sure, you could. The Russian launch facility for everything from Sputnik to Yuri Gagarin to today’s trips to the International Space Station—they all start from the Baikonur Cosmodrome in Kazakhstan at 46 degrees north. But a look at the geography of the old Soviet Union tells you there really wasn’t a better option. It requires a bit more energy than from a more southerly location, but it’s not a huge problem.

Could you launch aiming west, against the Earth’s rotation? Again, yes—but why would you? Instead of that rotation adding to your speed, it would subtract from it.

A big enough rocket can launch you into any orbit desired. But the most energy-efficient low earth orbit moves from west to east, close to the equator.

Where Does Space Begin?

Posted on by nealsumerlin Posted in human spaceflight, Spacecraft

If you are old enough, like me, to remember when riding a rocket into space was something no one had ever done, you may be joining me in some mix of amusement, mild disdain, and (let’s face it) jealousy at all the millionaires and celebrities, and a few regular folks, who get to do so.

As of this writing, Jeff Bezos’ New Shepard rocket is scheduled to carry six persons on an 11-minute ride into space in a couple of days from now. They will reach a peak altitude of around 66 miles (106 kilometers) and experience “a few” (probably about three) minutes of weightlessness.

https://everydayastronaut.com/wp-content/uploads/2021/08/blue-origin-first-human-flight-l0-new-shepard-launch.jpg

The best thing about this rocket is its name. Alan Shepard was the first American in space, and his flight, like this one, was suborbital—up and down. Shepard’s peak altitude, however, was 116 miles, much higher than his namesake rocket.

https://upload.wikimedia.org/wikipedia/commons/b/b7/Alan_Shepard_pouso.jpg

So are these space tourists actually going to space? Is Katy Perry an astronaut?

There are several rather arbitrary definitions of how far above the surface you have to rise to be in space. There is no abrupt boundary between the Earth’s atmosphere and outer space; the air gradually gets thinner as you ascend. Even the International Space Station experiences some atmospheric drag at 400 kilometers (250 miles) up, and has to be periodically boosted higher.

The U.S. Armed Forces definition of an astronaut is a person who has flown higher than 50 miles (80 km) above mean sea level. Several X-15 rocket plane pilots flew above this height in the 1960s.

The Kármán line is probably the most widely accepted definition. It lies at 100 km (62 miles), and the New Shepard spacecraft takes you just barely above that.

As a practical matter, a satellite in orbit can’t maintain that orbit if it is too low, where the atmosphere will slow it and bring it back to Earth. That limit is roughly 150 kilometers (93 miles).

https://pbs.twimg.com/media/GHXkf4WWIAAeIYF?format=jpg&name=large (modified by Neal Sumerlin)

Have these suborbital space tourists been to space? We’ll grant them that. Are they astronauts? Nah.

 

Twins, Siblings, and Only Children

Posted on by nealsumerlin Posted in Stars

Many people have at least a passing interest in astronomy. It’s why, even though my graduate education is in chemistry, I’m always sure to tell new acquaintances that I taught astronomy for much of my career. Chemistry is probably more obviously relevant to them, but astronomy is more popular.

Something they may have heard somewhere along the way is that the majority of stars are binaries, two stars orbiting a common center.

https://www.iac.es/sites/default/files/styles/color/public/images/news/main-qimg-a231546b8a3369ba260078cce40d4f99-c.jpeg?itok=8XADbh6m

But this is a classic case of selection bias, where the samples chosen are not representative of the whole population. The stars that are easiest to see are ones like our Sun, or less numerous ones even larger. But the most numerous stars are smaller and dimmer than our home star. They are red dwarfs, or M-class stars. (Not to be confused with Star Trek Class M planets, Trekkers.)

https://static.wikia.nocookie.net/kerbol-starsystem/images/1/19/AStar_types.png/revision/latest/scale-to-width-down/1000?cb=20171221083246

Stars not too dissimilar from the G-class Sun—classes F, G, and K—comprise very roughly one fourth of all stars. The big and bright O, B, and A stars—hard to miss because they are so luminous—are rather rare. M-class stars are quite common, roughly three fourths of all stars. They are just more difficult to spot unless they are nearby.

But better instruments let us do that. And surveys show that about three fourths of the abundant red dwarfs observed are single stars.

So solitary stars like our Sun are not particularly unusual. As an only child, I find this reassuring!

Theories and Observations

Posted on by nealsumerlin Posted in Solar System

I’m old enough to remember when the solar system was described as a fairly well-ordered place. There was the Sun, the rocky inner planets, an asteroid belt of smaller rocky objects, four large gaseous/icy planets, the since demoted Pluto, and smaller icy objects beyond the most distant currently recognized planet Neptune, from which comets originated. All of these objects were presumed to have formed in place and not to have shifted over their 4.5 billion year history.

https://media.hswstatic.com/eyJidWNrZXQiOiJjb250ZW50Lmhzd3N0YXRpYy5jb20iLCJrZXkiOiJnaWZcL2t1aXBlci1iZWx0LTEuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjo4Mjh9fX0=

That ordering is still valid, but we’ve come to realize that planet formation is much more dynamic than we had imagined. Giant planets migrated from their birthplaces, scattering smaller objects and subjecting the inner planets to bombardment so intense as to give that epoch its name—Hadean. The current stable configuration of planetary orbits is not as it has always been.

Understanding how planets and smaller objects coalesce from a protoplanetary disk of gas and dust is an ongoing area of research. The observational side of this research relies on telescopic scrutiny of young stars whose planets are in the process of being born. The Atacama Large Millimeter/submillimeter Array (ALMA), the largest radio telescope in the world, obtained these images of several nearby stars and their surrounding disks.

https://www.nasa.gov/wp-content/uploads/2021/09/stsci-j-p2152b-f-1807×2256-1.jpg?resize=1640,2048

The theoretical side uses increasingly powerful supercomputer simulations.

There is a superb article in the June issue of Sky & Telescope magazine by one of my favorite authors, Emily Lakdawalla, about the icy objects beyond Neptune, collectively known as Trans-Neptunian Objects, TNOs. These objects are further subdivided into categories based on their orbital characteristics, and understanding how they came to be where they are has defied attempts to duplicate those positions in simulations.

Without diving too deep into the details (if interested, I highly recommend Lakdawalla’s article), it turns out that many of these objects will be binary: two objects of roughly equal mass orbiting a common center. This artist’s depiction of the near-Earth asteroid 2017 YE5 illustrates.

https://cdn.mos.cms.futurecdn.net/hNwyVz7fT2FsHPB8ovLoPo.gif

And as these objects orbit, they may spiral ever closer, perhaps even merging to form what is called a contact binary.

When the New Horizons spacecraft flew past Pluto in 2015, it continued on to a flyby in 2019 of a smaller object later named Arrokoth.

https://cdn.mos.cms.futurecdn.net/WQCGinFQmCvt6LjuQxTU45.jpg

These two lobes had to have come together very slowly to create this peanut-shaped object without shattering them both.

More recently, the  Lucy spacecraft obtained a close look at the asteroid Donaldjohanson  as it flew by on April 20, 2025, on its way to explore the Jupiter Trojan asteroids.

https://assets.science.nasa.gov/dynamicimage/assets/science/missions/lucy/final_0798443319_dec.png

Another contact binary! Theories are of course constrained by observations, but observations are just that. WHY do we observe what we do? One of the wonderful things about science is that we will never run out of questions to ask and mysteries to solve.

A New Moon Race?

Posted on by nealsumerlin Posted in human spaceflight, The Moon

When the Apollo astronauts landed on the moon between 1969 and 1972, they all landed fairly close to the lunar equator.

https://lightsinthedark.com/wp-content/uploads/2016/06/apollo-landing-sites-new.jpg

The largest excursion north or south was that of the Apollo 15 mission. It landed 26 degrees north of the equator in the Hadley–Apennine region, almost to the rim of Hadley Rille, hypothesized to be a collapsed lava tube.

https://upload.wikimedia.org/wikipedia/commons/7/78/AP_15_LS_1.png

The orbital mechanics of the Apollo missions made trips far from the lunar equator too expensive in terms of time and fuel. With even more powerful rockets than the Apollo-era Saturn V now available, there are more options.

The next humans to land on the moon, whether they be in American or in Chinese spacecraft, will land farther to the south than ever before, in the moon’s south polar region. In fact, NASA has already designated potential landing sites for its Artemis III mission. Two astronauts are slated to land on the moon sometime before the end of this decade. Maybe. Much depends not only on solving engineering issues, but on the uncertainties of political decisions made in Washington. China has also announced its intention to land two astronauts at the lunar south pole by 2030.

https://www.nasa.gov/wp-content/uploads/2024/10/artemis-iii-landing-region-candidates.jpg

Let’s set aside for now all the complications of this endeavor, and focus on why the lunar south pole is a target for exploration.

The moon is an airless, rocky, and dry place. Its day/night cycle is 29.53 days. It bakes at up to 121°C (250°F) for almost 15 days of sunlight, and plunges as low as -133°C (-208°F) in its 15 days of darkness.

These are temperatures at the moon’s equator. But its poles have deep craters that are permanently shadowed, that never see sunlight, and that are consequently much, much colder. The interior of Shackleton Crater at the south pole averages −183 °C (−298 °F).

https://svs.gsfc.nasa.gov/vis/a000000/a004700/a004716/shackleton_print.jpg

Did I say the moon was dry?

Those permanently shadowed areas do have water ice in them. A NASA instrument flying on an Indian lunar orbiter detected such, and produced this map of the lunar south pole (left) and north pole (right).

https://d2pn8kiwq2w21t.cloudfront.net/original_images/imagesmoon20180820elphic20180820-16.jpg

Blue represents the ice locations, plotted over an image of the lunar surface, where the gray scale corresponds to surface temperature (darker representing colder areas and lighter shades indicating warmer zones). The ice is concentrated at the darkest and coldest locations, in the shadows of craters. [From https://www.jpl.nasa.gov/news/ice-confirmed-at-the-moons-poles/]

Note that because of the different topography of the two poles, there is more ice at the south pole than the north, and it is less spread out.

The two nations with the ability to return to the moon in the next decade are the United States and China. Both have continuously inhabited space stations in orbit, assembled and currently crewed with rockets launched from their territory. The remnant of the former Soviet Union’s space program simply lacks both the will and the ability to go to the moon.

The Space Race of the 1960s took place for a lot of reasons, but it wasn’t to claim ownership of valuable and limited resources. Lunar water would be useful in making long-term habitation possible on the moon. A lunar real estate agent would remind you of the classic maxim: location, location, location! A new Moon Race could be more contentious than the USA/USSR completion of the past. There is plenty of room in low Earth orbit, and no one was claiming territory on the moon, despite the planting of the U.S. flag. This time it could be different.

Perihelion Day

Posted on by nealsumerlin Posted in Earth Science, Mars, Planets, Sky Phenomena, Solar System

On January 4th, 2025 at 8:28 a.m. EST, Earth will reach perihelion, the point in its orbit when it is closest to the sun. Then why is it so cold, at least in the northern hemisphere?

Winter—indeed all seasons—don’t depend on the distance from the sun. If they did, then Australia would celebrate the Christmas season with snow instead of visits to the beach. Seasons are due to the axial tilt of the earth relative to its orbit. Throughout the year, the sun appears lower or higher in the sky, and the hours of daylight are shorter or longer.

https://www.timeforkids.com/wp-content/uploads/2022/07/K1_220902_backpage_seasons.jpg?w=1024

At that moment on the 4th, Earth will be 91,405,993 miles away from the sun. Six months later on July 3rd at aphelion, it will be 94,502,939 miles away. The earth’s orbit only varies by plus or minus 1.7% from an average value. Yes, it’s elliptical and not circular, but not by much!

An interesting contrast arises on Mars, whose orbit is much more elliptical, varying by 9% from the average. Combined with Mars’s axial tilt (very similar to Earth’s), this makes seasons in Mars’s southern hemisphere rather extreme. When it is summer there, Mars is at perihelion, making it warmer. And when it is winter in the south of Mars, the planet is farthest from the sun at aphelion, making it even colder.

https://www.britannica.com/place/Mars-planet/Basic-astronomical-data#/media/1/366330/70791

We should note that “warm” on Mars means a maximum of 68°F (20°C) at noontime on the equator. With an extremely thin atmosphere, temperatures plummet to -100°F (-73°C) once the sun goes down.

We should be grateful for many aspects of the Earth’s orbit for our relatively mild diurnal and seasonal variations!

Rocket Science: Simple and Not So Simple

Posted on by nealsumerlin Posted in Rocket Science, Spacecraft

Rocket science at its most basic is surprisingly simple. If you throw enough stuff fast enough out the back of your rocket, it will make the rocket go forward. The more stuff you throw out and the faster you throw it, the faster your rocket will go.

If you’ve ever blown up a balloon and then let it go, you’ve seen this in action.

But this isn’t very efficient, is it? The balloon just flies around randomly. This is why rocket engines have nozzles at the rear to direct the flow of hot gases created in the engine.

https://www.spacevoyaging.com/wp-content/uploads/2023/06/3382447074_fe0645d5e4_k.jpg

So here is a puzzle for you. The SpaceX Starship uses the same Raptor engine for its first and second stages. But as this image shows, even though the engine hardware for the engines used on the first stage (on the left) is the same as for the engines used on the second stage (on the right), the nozzle is much larger for the second stage engines. Why is that? Fair warning: we are going from Rocket Science 101 to 201 in the next two paragraphs.

https://cdn.wccftech.com/wp-content/uploads/2020/09/SPACEX-RAPTOR-ENGINE-STARSHIP-NASA.jpeg

Take a close look at the diagram below, and focus on the dotted lines representing exhaust gases emerging from the nozzle. If those lines are not parallel to the desired direction of motion, they are not contributing 100% to that motion. Think of all the lines above the black arrow that will push the rocket down, and all the lines below that will push it up. That is wasted effort, and means the rocket is not maximally efficient. That maximum efficiency is reached only when ALL the exhaust gas is ejected in the same direction.

https://lh5.googleusercontent.com/ra83SQfdr-LtTbDOBsiUD6QqdmgfvC65wuNPsv6nEmSt178AlEDmSX29m2LJFYjGDJSQGLcttsTojoYThrb4hSYLCHAI3hDmZf1DzGgpEttvjOFMDw6MSGBQg52AHlsCQkZKU1w1

The velocity with which the gases exit the nozzle is a key measure of how much thrust they can impart. But the direction in which they exit is a key measure of how efficiently they move the rocket forward.

Now to Rocket Science 301.

This is the purpose of the nozzle—it allows the gases to expand before they exit. Ideally, the gases exit the nozzle at the same pressure as the surrounding atmosphere. At sea level, where the atmospheric pressure is greatest, the gases don’t need to expand much to reach that pressure, and the nozzle can be smaller for the first stage. For the second stage, which only ignites in the upper atmosphere where the atmospheric pressure is much lower, the nozzle must be larger to allow the gas to expand more to reach a lower exit pressure.

Of course, the engine will only reach its maximum efficiency at a specific pressure, and therefore at a specific altitude. The size of the nozzle is a compromise meant to operate well enough through a range of altitudes from sea level to about 40 miles high, where the first stage of Starship falls away and the second stage engines ignite. The atmospheric pressure this high up is close to zero, and the second stage engine nozzles are much larger to allow for more expansion and a lower exhaust gas exit pressure.

Screenshot from https://www.youtube.com/watch?v=yMrJl-lJrRI

For a rocket engine meant to operate only in the vacuum of space, the nozzle will be as large as possible given the constraints of weight and practicality. It can’t be infinitely large! Look at the nozzle on the Apollo spacecraft. This engine operated only in a vacuum, and the nozzle is not small relative to the size of the craft it propelled.

https://upload.wikimedia.org/wikipedia/commons/thumb/c/c0/Apollo_CSM_lunar_orbit.jpg/1920px-Apollo_CSM_lunar_orbit.jpg

Maybe we really should think rocket scientists are smart!

Meteor Showers 2024

Posted on by nealsumerlin Posted in Uncategorized

For most people, if they are aware of a meteor shower, it is the Perseid shower in August. It’s pretty reliable, and it takes place at a time of year when being outside in the wee hours of the morning is actually pleasant.

But meteor showers occur at predictable times throughout the year, and there are some good ones coming this month and next. They are due to a comet (or rarely, an asteroid) that warms up when approaching the sun and kicks off dust particles. Those particles spread out all along the comet’s orbit. When the Earth intercepts that stream, a meteor shower takes place.

https://griffithobservatory.org/wp-content/uploads/2021/03/444.2_LM-PS-MC-12.003newwork_v2-1200×1185.jpg

The interception is at a specific place in Earth’s orbit, and therefore at a specific time of year. The remaining showers in 2024 and their peak times of activity are:

  • Leonids: November 16-17.
  • Geminids: December 12-13.
  • Ursids: December 21-22.

And the objects responsible for each are:

  • Leonids: Comet Temple-Tuttle
  • Geminids: 3200 Phaeton (one of the few asteroids responsible for a shower)
  • Ursids: Comet Tuttle

Meteor showers are named for the constellation from which they appear to radiate. For example, the Leonids appear to track back to the constellation of Leo.

https://dq0hsqwjhea1.cloudfront.net/Leonids_radiant_2020_900px.jpg

But this is a trick of perspective. The particles entering the atmosphere are actually on parallel tracks. They appear to diverge for the same reason that parallel railroad tracks appear to do so.

http://www.starlight-nights.co.uk/wp-content/uploads/2015/11/Meteor-Radiant-Diagram.jpg

The best time to see meteors is between midnight and dawn. That’s when the Earth encounters the particle stream “head on” without their having to catch up to the Earth in its orbit.

https://www.starryhill.org/graphics/meteorshowerbesttime_700.jpg

This is not a great year for any of these showers, unfortunately. The moon will be just past full for the Leonids, making it difficult to see any but the brightest meteors. For the Geminids, if you are willing to be outside around 4:30 am, the 88% full moon will have set, and you will have another 80 minutes or so before the sky begins to lighten up before dawn. The Ursids have the moon in the sky from a little before midnight and well past dawn.

Oh, well! Just as with your favorite baseball team that isn’t the Dodgers, wait ‘til next year!

 

 

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