Using Lasers to Get Moving

In the chain of crazy ideas of how to get to space and how to get from one planet to another, there is an idea to use lasers. Actually, there are a couple of ideas on how to do this.  This is the first of a two-parter where I talk about this idea.  The first part will cover one project that has actually gotten off the ground (literally) and an idea on getting to Mars, while the second post will look at interstellar travel with lasers.

The first idea on using lasers makes a tiny bit of sense.  It is called Lightcraft (get it – light and craft?).  The general idea with this is that you have an object that has a very specific shape on the bottom side. Then you shoot a laser at it and the shaped bottom focuses the laser so that it superheats the air that is touching the object.  The air then is propelled away from the object, resulting in a net thrust that is towards the top of the object.

Interesting, eh?  They have actually tested this with some very shiny objects that are about the size of a fist and are pretty light. Here is a picture:


That is actually almost real size, too! These little things have flown about 75 meters into the air.  That is not, um, unimpressive, I guess. There are several problems with this technology, since it is hard to keep the Lightcraft pointed in the right direction and keep the laser pointed directly at it, and all sorts of other things. My guess is that they have not had the right public relations people and the large amount of funding that is needed to take a project like this from the tiny prototype stage to anything of real size.

Recently, another team has also been working on using lasers to move things about in the solar system.  This idea with this team is to use very high powered lasers in a similar way as we would use the sun and a solar sail.

A quick aside on solar sails (boy, I really need to write a post about solar sails):  When light hits an object, it actually imparts a super, super small amount of momentum.  When you feel the sun beating down on you on a very bright days, it is totally because it is actually beating down on you. Well, technically it is, but in reality, the amount of force on you by sunlight is less than a paperclip put on you.  Like, way less.  But, if you were out in space, and you had a huge reflective “sail”, the light would shine on it and impart a very small force – something like a pound for a sail that is about 1 km². But, imagine if you could turn the brightness of the sun up by a factor of 100. Or 1,000. Or 1,000,000! Then you could get some real force to act on your spaceship!

So, the general idea with using lasers is that you could have a reflective surface on your ship that you would point a really really really intense laser at.  This would impart a large force on the ship and accelerate it. The beauty of this plan is that the lasers all would need to be powered here on Earth, so we could generate it using a nuclear power plant or hydroelectric or even good old-fashioned coal.  The ship could be very small, since it wouldn’t need a lot of fuel to accelerate it, since that power is coming from Earth.

In the article that I linked to above, the researchers say that they could envision getting to Mars in 3 days using this type of technology. Please excuse me if you heard a cough that subtly masked my slight doubt of this claim.

The first (and most obvious) issue with this is that you would have to have some sort of a laser system that would be on Mars to slow down the ship.  So, you would have to build something like a nuclear power plant on Mars. I am sure that this is not really likely to happen soon, since we are so successful at building them here in the United States (sarcasm). But, there are probably less regulations on Mars, so it should be easier. But then there is the whole getting all of the (highly radioactive) materials to Mars to actually build the plant.  Well, any ways, we will get there eventually!

Ok, so now that we have a laser system on Mars and a laster system on Earth, how much acceleration would we need to get to Mars in 3 days?  Well, we would accelerate for half the distance and then decelerate for the other half of the distance.  If we make a very simple approximation that the acceleration is constant, the problem is easy to solve.  Let’s assume that Mars and Earth are the closest they can be together, which is 0.3 AU, or about 45 million kilometers. We need to accelerate through about half that distance in about 1.5 days. Do a little math and we get that the acceleration needs to be a constant 5.3 m/s², which is about half of the acceleration of Earth on the surface.  This is extremely reasonable!

The problem with this is that the power from the laser falls off as the distance squared. This means that the acceleration that the laser system could supply would have to start off extremely large, then would fall to almost nothing, or that the power that is consumed by the laser would have to start off relatively small, and would have to increase dramatically.

Let’s think about how high-powered of a laser you would have to have in either case. I am going to simplify the problem significantly, since I am a relatively simple person. The sun, for reference, exerts about 4.5667e-6 Newtons of force per meter squared of area. This is an incredibly small force! Like, really, really, really small.  In order to exert that much force, the energy in that light is about 1350 Watts, which is a LOT of energy.  So, this idea is not very efficient at all!

Let’s say that we want to send something to Mars that is a 100 kg, or about 220 lbs. This is an extremely small satellite. If we want to accelerate it at a rate of 5.3 m/s², like the example above, we would have to use 530 Newtons. If we had a sail hooked up to this object that was, say, 100m by 100m (about the size of a football field), how much force would the sun exert on it?  About 0.0457 Newtons.  That is not much! And that is taking about 1350 W, as described above.  So, we would need a laser that is about 11,600 times more powerful than the sun to give us our 530N of force.  That would require a 15.7 Mega-Watt laser.  And this would only accelerate it at the 5.3 m/s² for a little while, since the distance between the laser and the satellite would increase and the received power from the laser would decrease.

Let’s say that the laser delivered the 15.7MW (or 530N of force) at a distance of about 10 Earth radii away from the surface of the Earth (I had to choose a distance, and this was quite arbitrary, but whatever). If you wanted to continue to accelerate the satellite at 5.3 m/s² all the way to the halfway point between Earth and Mars, the power of the laser would have to increase by a factor of about 500,000 times while it was shining on the craft. This means that in order to accelerate it all the way to the halfway point, the laser would have to be a 7,800,000 MW (7.8 Tera-Watt) laser, and would have to fire (ramping up in intensity) for about 36 hours.

Practical?  I don’t know.  This website talks about a 2,000 TW laser that was fired for 1 pico-second (not very close to 36 hours). Another website talks about getting a 10 TW laser that fires for about a femtosecond (that is also pretty short), but fits on a desk.

Where could we get the power? Well, if the sun delivers 1,350W of power per 1m x 1m area, then we would need about 5,800 km² of solar panel area to get that much energy.  Oops, solar cells are not perfectly efficient (more like 25% efficient), so we would need about 23,000 km² of area, which is about 150 km by 150 km of solar cells. This is about the size of New Jersey.

Anyways, the idea is that power on Earth is very cheap, while getting that power into space is really painful.  So, it is ok to take a HUGE hit on efficiency to accelerate something up to enormous speeds in space using Earth-based systems, instead of trying to haul some sort of chemical rocket engine up to space. In fact, chemical rockets will never get us to another star, so it is a non-starter. But, that is a conversation for next week (I promise!)



Reinventing Innovation With Small Satellites

When NASA first formed, getting to space was extremely dangerous and no one really knew how to do it. Therefore, there was an acceptance of a large amount of risk. Part of the reason for this is that the amount of money invested in NASA was enormous – at one point in the late 1960s, NASA’s budget was 6% of the total federal budget. This allowed NASA to rapidly solve problems by trying new and innovative things and iterate on the design over and over until it worked. For example, the Atmospheric Explorer satellites, built in the 1970s, had four versions that each lasted only a few months until AE-E lasted for many years. The IMP series of satellites measured characteristics of space between the Earth and the Sun. There were seven satellites that lasted only a few months until IMP-8 lasted 30+ years. The reason that NASA could do this is because the ratio between the cost of a satellite to the total budget of the agency was quite small. Therefore, failing on a few satellites didn’t matter very much.

NASA today is quite different. The budget is 0.5% of the total federal budget and dropping every year. The total number of satellites launched is significantly smaller, and each one needs to succeed, since they cost so much compared to NASA’s total budget. While failure still happens at NASA, it is not something that is taken as a lesson; it is taken as a failure. Therefore, there are a plethora of lessons learned, sets of rules that need to be followed, and significant numbers of reviews that must be held to move the project to the next phase. While this helps to ensure success, and it is a natural outcome of a system with limited budgets, it stifles innovation and greatly increases the cost of each satellite.

When a satellite is launched, the rocket (or launch vehicle) has a certain capability. For example, a given rocket may be able to launch a satellite with a mass of 1000 kg to a 700 km altitude. If the satellite that is being designed is 950 kg, the rocket then has excess capacity and, in order to get the satellite to the exact desired orbit, 50 kg of ballast needs to be added to the rocket. Sometimes, this ballast can be another satellite.

The idea of a CubeSat was created when Bob Twiggs suggested that there could be a standardized deployer for very small satellites that could be used as ballast on almost any launch vehicle. The deployer could be very strong and could ensure that if anything went wrong with the little satellite inside it, the primary payload (i.e., large satellite) would not be affected. The P-POD deployer was invented to allow cheap access to space for extremely small satellites as secondary payloads. It created a standard for a tiny satellite, dubbed a “CubeSat”, that could fit into the P-POD deployer and be launched into space wherever a P-POD could be used. A P-POD accepts a satellite that is roughly 4 inches by 4 inches by 12 inches, or 3 satellites that are roughly 4 inches on a side (hence the name “CubeSat”).

The invention of the P-POD deployer has revolutionized space, since is allows cheap access to space for anyone who can build a satellite that can fit within its extremely limited confines. At first, it was considered too small to be of any use for any real science to be conducted. This has since changed, though. The National Science Foundation has embraced CubeSats as both an educational tool and a science tool. They have funded over 10 CubeSats that have been used to conduct science ranging from lighting detection to radiation belt monitoring.

CubeSats have been successful because that have placed the community into a place where the satellites are once again quite cheap compared to the total budget of the institution. This has allowed innovation and rapid technology development to reenter the satellite industry. Because of this, a large number of companies have been created to support such an industry, creating smaller and smaller supporting hardware for tiny satellites. For example, small, high-speed radios have recently been introduced that can be used to downlink massive amount of data. Without such radios, it would be extremely expensive to get all of the data from the satellite to the ground.

With the emergence of small-satellite hardware, agencies such as NASA and the Department of Defense could start to look at creating missions that are designed in a radically different way – instead of launching a single satellite that was quite expensive, they could explore how to get the same science return with two or more smaller, cheaper, satellites.

Another technology that has led to the creation of missions such as CYGNSS, a satellite constellation mission that I am involved with, is the Global Navigation Satellite System (GNSS), which is the general name for constellations such as GPS. These satellites were designed to provide precise position and time information for military systems, but their use has become significantly more general. GPS is ubiquitous, with everyone having a receiver in their phone, which has done two things – pushed GPS receivers to smaller and smaller sizes and lowered the price for those receivers.

In space, a GPS receiver can be used to do many different things. One of the most common is to use the delay between when the GPS satellite sent the signal and when the monitoring satellite received the signal to tell something about the atmosphere. This can be done because electromagnetic waves travel at different speeds in different medium. So, for example, if the waves travel through the ionosphere, they slow down a bit and take longer to reach the satellite. This difference can be measured and the amount of ionosphere between the two satellites can be determined. Just like the ionosphere, the waves slow down when they travel through water vapor, so, when the waves have to travel through the atmosphere, the amount of water between the satellites can be determined. This is the primary use of GPS on satellites today – radio occultation to determine atmospheric characteristics. Because of the inexpensive nature of GPS receivers, constellations can be launched that take advantage of this. The COSMIC satellites are an example.

The CYGNSS satellite mission uses GPS signals, but in a very different way than other satellites – it measures how much of the signal is reflected off of the ocean’s surface, which says something about the roughness of the ocean. The amount of scattered signal is dependent upon the surface roughness, which itself is dependent upon the winds over the water. Therefore, the scattered GPS signal strength that is measured by a satellite such as CYGNSS depends on the wind speed over the ocean. CYGNSS takes advantage of there being the GPS satellites in orbit around the Earth continuously sending signals towards the Earth.

Past satellites that have measured the wind speed over the ocean have had to both transmit a radio wave and receive the wave back. In order to measure the signal, the transmitter had to be quite large and powerful. Since CYGNSS does not contain the transmitter, it can be significantly smaller.

In addition, CYGNSS takes advantage of a number of components that were designed for very small satellites (like CubeSats), such as the radio for communication with the ground station, the star trackers that provide information on the orientation of the satellite, the momentum wheels, which keep the CYGNSS satellites oriented in the proper direction, and the computer systems that run everything on the satellites. This has allowed NASA to launch eight very small satellites for cheaper than a normally priced large satellite, and it is all due to the shrinking of space technologies and the global availability of the GPS network.

By pushing the boundaries of what can be done with extremely small satellites, NASA has started to shift to considering constellation missions that can give us an idea of what is happening over a large portion of the globe instead of in a single place.  It has allowed innovation and rapid development of cheap technologies to flourish. This will result in some amazing new science to take place in the next decade!


Space Shuttle Atlantis

Here are some pictures from Kennedy Space Center of Space Shuttle Atlantis.

A view from the front with a wide angle lens.
A view from the side with a horribly distorting 8mm fisheye lens.  Interesting effect, but it makes you think that the shuttle is small, while it is gigantic.
The three main engines. Feel the burn.



Project SpaDE – Deorbiting Satellites with Bombs and Balloons

In 2011, a person from Raytheon called me up out of the blue and asked if I wanted to be involved in a project to help eliminate space debris.  I was interested.

A little background (the first): The majority of space is not really a vacuum.  There are particles pretty much everywhere in space.  The question really is what is the density of those particles – or how many particles are there in a given volume. The atmosphere of Earth is a relatively narrow shell around the Earth. We know that if we climb Mt. Everest there is not much air up there.  But, to us, “not much air” is still a whole lot of air.

We think of space as having no air at all. Astronauts have to wear space suits to protect them from the “vacuum of space” after all. There are a number of very interesting things about this, but it should really be a totally separate post.  Let’s simply leave it that there is, indeed, atmosphere up in space where the International Space Station (ISS) orbits.

Since there is air up there, objects such as satellites and the  ISS feel a drag force, as we have discussed in a few other posts. The density in the atmosphere up there is very very small, but the objects in orbit are moving extremely fast.  This means that the drag force that the objects feel is not really large, but it can strongly affect their orbital motion.

As a rule of thumb, objects that are orbiting below about 200 km (120 miles) will deorbit extremely quickly, in a matter of days to weeks, because of the atmospheric drag. The atmosphere is so thick there that the objects just can’t stay aloft and they ultimately decrease their altitude rapidly and (hopefully) burn up in the atmosphere around 70 km altitude.

Satellites above about 700 km (420 miles) or so will basically never deorbit because of atmospheric drag.  The atmospheric density there is so low that drag is not an important force on objects in orbit. Don’t get me wrong – there is still an atmosphere, but it is so tenuous that orbiting objects don’t care about it.

Between these two altitudes, drag is important. In fact, the ISS, which orbits at around 400 km (240 miles) altitude has to be moved up every couple of weeks, because atmospheric drag pulls it down.  The ISS is constantly being pulled back towards the Earth due to drag, and we lift it back up to 400+ km using propulsion to keep it in orbit.  If we didn’t do this, the ISS would fall back to Earth and burn up in the atmosphere in 6-12 months.

A little background (the second): Space is filled with a lot of crap. There are about 20,000 objects in orbit around the Earth that are about the size of a softball or larger. This stuff is pretty dangerous, since it is moving at speeds of 7,600 m/s (17,000 MPH). It can cause a lot of damage (Gravity, anyone?).  Here is a picture of the space shuttle window after being hit with a piece of debris that was significantly less than a millimeter in size:


There is a realization that we need to do something about all of this space debris.  One of the worst things that has happened because of the amount of debris is that an Iridium satellite was destroyed due to a collision with a retired Russian spy satellite. This caused the creation of thousands more pieces of debris, which could then impact other objects. There is real concern.

I was called by this guy from Raytheon to see if I wanted to help solve this problem.  I said, sure, that would be great.  We wrote a proposal to NASA to see if we could get money for it.  They gave us some seed money to see if the idea could pan out.  The project was called “Space Debris Elimination (SpaDE)”.

The general idea of the project was to see if we could increase the atmospheric drag in front of an object in orbit around the Earth enough to cause it to deorbit. Here is a graphic to illustrate the idea:


At first, this idea seems very interesting and plausible. My main profession is a modeler of the upper atmosphere.  A long time ago, I wrote a large-scale model of the atmosphere that looks at what happens when large amounts of energy is added to the atmosphere in the form of the aurora (i.e., the northern/southern lights). I continue to work on this model today, and have a bunch of graduate students who use the model to do research. For project SpaDE, I took the model and made it so we could run it over a very small area (like a couple of hundred kilometers by a couple of hundred kilometers) and inject a huge amount of energy quite low in the atmosphere. The model then simulated what would occur.

You might ask, what is “a huge amount of energy”?  Well, that is a good question.  The model can handle almost any amount of energy, but for this project we were looking at roughly nuclear bomb types of energies. The general idea would be to take a large explosive device to the stratosphere, or about 30 km, and explode it there.  We would use some sort of device to direct the majority of the energy upwards, creating a very large density perturbation that would propagate upwards to the upper atmosphere where a piece of debris would travel through the density enhancement and deorbit.

When people find out about this project, they always ask: why did you stop working on it? Well, this is really the point of this post – to describe why this type of debris mitigation strategy is unlikely to work.

Problem, the first: You have to take an extremely large explosive device up on a balloon and explode it in the atmosphere. That is unlikely to be ok with pretty much anyone.  Also, directing the blast so that the majority of energy would go upwards could be rather heavy, making the balloon quite big. Basically, there would be a lot of logistical problems.  You could envision that each balloon launch could run upwards of several hundreds of thousands of dollars or more. For comparison, if you wanted to do something like capture debris with a satellite and deorbit it, it might cost 10s of millions of dollars.  So, a balloon with a large bomb is much cheaper, but a logistical nightmare.

Problem, the second: This is somewhat more technical. The atmosphere breathes. When it warms up, it expands, and when it gets cooler, it contracts. The upper atmosphere absorbs a bunch of energy from the aurora. The aurora deposits about 40 Giga-Watts of energy into the  upper atmosphere continuously. During extremely disturbed times, it can deposit over 500 GW of energy for several hours. Those are big numbers, so what does it mean? Let’s say we have an extremely disturbed aurora (500 GW) for 6 hours. That is about 1e16 Joules of energy, which is 2.6 megatons of TNT, which is roughly 10 times LESS energy than a very large nuclear bomb. With the aurora, it is distributed over a very large region, and will cause an increase in density at around 400 km altitude of about a factor of 10. That is a pretty big increase in density. A coworker noted to me that a hurricane is caused by a density change of about 10%. This is a 1000% change in the thermosphere due to the aurora. The thermosphere is a pretty interesting place!

A nuclear bomb will cause a much larger change in the density, clearly.  But, the problem is that it will change is only in a very small volume. You get a mushroom cloud that goes up into the atmosphere and causes a (lets say) 100 times increase. By the time the mushroom cloud gets up to orbital altitudes, it will be about 200-ish km across. A satellite will pass through this mushroom cloud in about 25 seconds. Then the density is back to normal.  For a large auroral event, the entire atmosphere is increased by a factor of 10 for 6 hours, meaning that the satellite goes through 6 hours of 10 times larger drag.  This is enough to change the satellite’s orbit.  Definitely.  But, it will really only change the altitude of the satellite by a meter or two. Not much in the grand scheme of things.  So, a 100 times increase in the density for 26 seconds doesn’t change the orbit very much at all. Simplistically, you would need about a 1 million times increase in the density at satellite altitudes over the 26s in order to get the same effect as a 10 times increase for the storm over six hours.  That would be a very large bomb indeed. And it would only cause something like a meter or two altitude change.  Not deorbiting.  For that you would need on the order of a hundred or so of these events.  That is a lot of bombs.

Hopefully that was not too technical. Sorry if it was.

Problem, the third: In order to actually effect the single piece of debris, you would have to launch the balloon several hours before hand to allow it to get to a high enough altitude, and be over the pretty much the exact spot in which an object will be orbiting.  Then you have to time the explosion perfectly, so that the orbiting object passes directly through the shock front of the blast.  This is probably the easiest of the three problems, since with a model, you can pretty easily determine how quickly the blast wave will move through the atmosphere in all directions. Controlling the path of the balloon is more of a challenge, but I have actually written software that will allow you to figure out the trajectory of high altitude balloons, since this is a sort of a hobby or mine. (I will say that I have never exploded a nuclear device on one of our balloons!) Check out a video of one of our balloon launches where we launched two balloons at the same time.

What basically happened with Project SpaDE is that we did not get selected for funding for round two.  This was not too surprising, since the concept is just not very scalable.  You could actually see this working if you wanted to slightly nudge a small object that was going to hit a major satellite or something.  If you perturbed the orbit a couple of days before the collision was supposed to occur, it could actually change things.  Possibly. But, it would not be able to deorbit anything.

Sorry that this post was so long.  Hopefully it was still entertaining.

The Craziest Way to Get to Space

Hopefully, over the past few posts, I have convinced you that using chemical rockets to get to space is a pretty horrible way of doing it. And, just as I am certain that I will continue to post about this technology, I am certain that we will continue to use them, since it is really the only way to actually climb out of this gravity well that we call home (at this time!). But, my friends, trust me when I tell you that there is something better.  Actually, there are a number of technologies that are being worked on that may be better.  All of them have some really major issues, but it is good that we are trying.

This is the first post in a series that is going to explore some alternate ways of getting around the solar system and off of this rock. And, because I like you guys, I am not going to start off with #10 and work up to #1.  I am going to start off with the craziest possible way of getting us into space. (There are crazier ways of getting around the solar system, though!) Let’s get started.

Once upon a time, there lived a guy named Gerald Bull. Yes, he was sort of short and stocky.  And Canadian.  Here is a picture of him:

Gerald Bull

Gerald Bull came up with a fantastic idea. In 1961 he bought a 16-inch battleship gun from the US Navy for about $2000.  That isn’t 16 inches long, that is 16 inches in diameter. He moved it to Barbados and started running tests with it. He put atmospheric sensing instruments into the noses of the shells, and then fired them into the atmosphere.  These shells were about 150 kg and could go to altitudes of about 100,000 ft, or about 20 miles. In the air. By the way, these “rockets” were called Martlets. His program was called the High Altitude Research Program, or HARP.

Ok, let’s step back for a minute and think about this.  Bull was firing a gigantic cannon straight up in the air with things that weighted about 300+ pounds. I launch weather balloons.  These go up to the same height with packages that weigh 12 lbs.  Bull was doing some crazy stuff! Interestingly, there is really not a great way of sampling this part of the atmosphere, since it is really hard for airplanes to fly this high. Satellites can’t orbit this low because the atmosphere is incredibly “thick” there. So, rockets are about the only good way to take in situ measurements in this area (well, above about 100,000 ft, or 30 km to about 200+ km) of the atmosphere. Bull fired about 1,000 of these Martlets into the atmosphere in just a year or so.

But it really doesn’t stop there.

In 1963, Bull created the Martlet-3, which reached over 100 km altitude. He could launch a “rocket” that could go up to space for about $5000. Considering that rockets can cost over a million dollars that do the same thing, this is super freaking cheap.

He then extended the length of the cannon to about 110 feet with the ultimate goal of launching things into orbit. (The reason that you extend the length of the cannon is because you can get the force of the expanding gas for longer, allowing the projectile to accelerate for longer.) His idea was to build a rocket that would be shot up to about 100 km, and then the rocket would fire and take the payload into orbit.  This would be extremely cheap, since the majority of the mass to get something into orbit is used up just to get up to the right altitude.  If you can get the “third-stage” of a rocket up to 100 km altitude with a big gun, then it is super cheap to get to orbit! His Martlets got up to 180 km altitude for a world record that is still in existence.

The HARP 16″ gun firing a Martlet-3

Unfortunately, Bull never reached this goal. There was a ton of red tape, with the US and Canadian government involved.  Bull did not really believe in red tape and so he left the program. Bull continued to love big guns and started working for some shady people, developing highly accurate guns that could be used by one country against another. Ultimately, he worked for Iraq in helping them develop the Scud missiles that were supposed to be used against Israel. It turns out that Israel doesn’t really like this type of behavior, and Bull ended up with a few bullets in him in March of 1990.

The moral of the story (besides “don’t screw with Israel”) is that we could actually use a big gun to get us to outer space and ultimately into orbit. We don’t even need gunpowder to do this anymore – we can use the same technology that drives super-fast roller coasters and trains: linear induction motors. This technology has led to the development of rail guns by the Navy.  You seriously have to watch this video. This is a massive increase in our technological capabilities.  Basically, you accelerate the “bullet” up to speeds of about 4,000 miles per hour in the barrel of a gun. Serious horsepower.

If we can use this technology to knock things out of the sky, why aren’t we using it to put things into orbit?  That is a fantastically good question!

There are two problems with this idea (beyond Israel killing you for trying):

  1. If you got something up to orbital speeds as it left the gun, it would slow down extremely quickly because of atmospheric drag.  Really, you want to have it launched upwards, and when it gets well above the atmosphere, have it accelerate up to orbital speeds using fuel. This is somewhat complicated.
  2. The accelerations that take place with this are just unbelievably horrendous. A human would be a pancake if they were launched like this.  So, humans will NEVER be launched into space using a bug gun.  Maybe a very very very long runway, but never something super efficient like a gun.  But, supplies and fuel and other things like instruments could be launched into orbit using this technique.

There are researchers who are working on techniques that could be used independently or in conjunction with a big gun, so that you wouldn’t have to actually take fuel to get to orbit either.  While that is interesting, it is no where as cool as the Martlet. Seriously.  Gerald Bull. What a guy.


Why Are Rockets So Heavy?

One of the big problems with rockets is their size.  They need to be truly humungous to get anything into orbit.  Interestingly, the reason for this was explained back before modern day rockets were even invented. A Russian scientist named Konstantin Tsiolkovsky described why rockets need to be really big way back around the turn of the last century (like 1900).

The graphic below helps to understand what is going on.  Let’s say you want to lift a blue cube into space. The blue cube has some mass to it. In order to accelerate the blue cube up to some speed it takes a total of two bricks of red fuel. Let’s put some pretend numbers to this to make it a bit easier to understand.  Let’s say that you want to reach a speed of 4, and using two bricks of red fuel will give you a speed of 1.  That is too slow.  So, we need more fuel.


Now the problem is that we have the blue cube plus two red bricks of fuel, which is more massive than just a blue cube.  So, we will need even more fuel to accelerate this.  In order to accelerate the blue cube plus two red bricks by 1, we will need four red fuel bricks:


We can keep going on this. Now we have a blue cube and 6 red bricks. In order to accelerate all of them by a speed of 1, you need 8 red bricks of fuel.


After that, we will be going at a speed of 3.  We are very close to 4! To accelerate all of those red bricks (we have 14 now!) plus the blue cube by another 1, it takes 16 red bricks of fuel! Yikes!  This is growing out of control!


For a rocket, what would happen is that the 16 red bricks would burn to allow the rest of the fuel plus the blue cube to be accelerated by 1.  Then the 8 red bricks would fire, accelerating the 6 red bricks plus the blue cube by 1, giving a total speed of 2.  Then the four red bricks would burn and accelerate the two red bricks and blue cube by 1, giving a total speed of 3.  Finally, the last two red bricks would burn, accelerating the blue cube by 1 and giving a total speed of 4.

The point of all of the above is that the amount of fuel that you need grows very quickly, since you have to have more fuel to lift the other fuel that lifts the other fuel which lifts the other fuel, etc.  Tsiolkovsky realized this more than 110 years ago and came up with a formula that describes this phenomena (of course, he knew calculus, which helps to explain things a bit).  There are two forms of his equation:



They are exactly the same equation (but probably don’t look like it because of the “e” and the “ln”), but just re-arranged to allow two different questions to be answered:

  1. If we need the rocket to change speeds by a certain amount (V), and the empty rocket has a given mass (Mempty), how much mass does the rocket have to have at the start (Mfull)?
  2. If we have a given amount of fuel and a rocket that has a given mass (Mempty and Mfull), how much change in velocity (V) can we get out of the rocket?

One detail that I left out, which was talked about in the last post about chemistry, is that there is a term in the equation that represents the exhaust velocity of the rocket (Ve). If we take the top equation above, there are simplistically two terms of the right hand side: the exhaust velocity and the ratio of the full mass of the rocket to the empty rocket.  What this multiplication means is that if you want to reach a given speed (V), you can use less fuel (smaller ratio of masses) if you have a larger exhaust velocity (Ve). The amount of fuel still exponentially increases (this is sort of what the “ln” means), but if you use a fuel with a higher exhaust velocity, you can use significantly less of that fuel. So, you want to get a fuel that will really leave the rocket with as much speed as possible. Then you can use less of it!

You can also use these equations to prove that a rocket with stages is much more efficient that a single-staged rocket.  I won’t do this here, but you can think of it conceptually given the diagrams above. Let’s pretend that the black boxes around the fuel and blue cube are different stages of the rocket and that they have mass, which is pretty much exactly how it works. For the biggest rocket (with the blue cube and the 30 red fuel bricks), the rocket will be quite heavy and it will really be hard to get the fuel and everything up into the air. When we burn the 16 red bricks, we then get to drop the gigantic storage tank and some motors and plumbing and all sorts of stuff.  The rocket then has significantly less mass. The next 8 red cubes have a MUCH easier job to do in this case, and they can accelerate the rocket much faster.  The same is true when the 8 red cubes are done burning and the rocket drops the second stage with the motors and plumbing and stuff for that.

Rockets typically have three or four stages, each with smaller motors (or less motors) and smaller fuel tanks, just as illustrated above. The most efficient rocket in the world would destroy itself as it burned, having an infinite number of stages. That is quite difficult to engineer, though.

Chemical rockets that use fuel like this are about the only thing that we have ever used to get something off the ground.  But, there are other methods. Some of them are just scary, and could get you killed by the CIA. Let’s talk about that next time.


The Limits of Chemistry

In the last post, I talked about how it was basically impossible for humanity to get to another star using modern technology. For this post, I would like to talk about why that is, and why we don’t have space hotels or moon bases yet.

The whole reason comes down to chemistry. The vast majority of rockets that exist and all rockets that take anything into outer space use chemistry to make the rockets go.  A few posts ago, I talked about thrust. Thrust is a pretty simple concept – basically, a rocket moves forward by expelling things quite quickly out the back.  There are two terms in the thrust equation, the mass flow rate (how much stuff the rocket spits out), and the exhaust velocity (how fast it spits it out).  Simple.

The mass flow rate is pretty easy to understand also.  It basically is just how much fuel the rocket uses per second.  In some ways, it is like hitting the gas pedal on your car: the harder you push on the gas pedal, the more gas flows into the engine and the faster you go.  That is a pretty simplified version, but it is about right.  A larger rocket really just has a larger mass flow rate.  The space shuttle had pipes that fed into the main engines that were about a foot in diameter.  That is a LOT of fuel!  The Saturn V used roughly 1000 gallons of fuel per second.  They actually had a very hard time mixing the fuel with oxidizer on the Saturn V, since the flow rate was so high (they didn’t have great fuel injectors in the 60s!), and they would get explosions in the engines.  Instead of giving up, they simply made the combustion chambers more sturdy to handle the explosions.

Anyways, the mass flow rate is how much fuel the rocket uses per second.  This is set by how big the engine is, and there is no real limit, except how big you can build the engines (or how many engines you can stick on a rocket – yes, I am talking to you Space-X with your 27-engine Falcon Heavy rocket).

The other term in the equation is the really tricky one – this is the exhaust velocity, which is how fast you can expel the mass out the back. Simplistically, you would think that this would be easy to turn up, but it is not. There has not really been any big revolutions in the exhaust velocity in a long time (like the 60s). The most common way to make a large exhaust velocity is to make an extremely hot gas, and direct it into a nozzle.  You mix fuel with an oxidizer, and you get an explosion. Then you turn the explosion into directed energy using a nozzle.

We can design pretty good nozzles.  They can be something like 90%+ effective at turning the thermal energy into kinetic energy.  That is great.  There is no factor of 10 improvement or anything that can be gained from nozzles.

The big problem behind this is chemistry. Let’s take the space shuttle’s main engine. This engine used two of the most abundant elements we have on Earth: Hydrogen and Oxygen.  You cool them both down until they are liquids, store them until the rocket is ready to fly, then combine them in the engine.  What is the result?  Water!  The space shuttle’s main engine exhaust is water!  Crazy, eh?  The amount of energy that is released when 2 molecules of Hydrogen are introduced to one molecule of Oxygen is exactly the same every time – about 6 eV, which is a tiny bit of energy.  The fundamental issue here is that we get only a very specific amount of energy out of the reaction.  If we take the 6 eV of energy and we turn that into an exhaust velocity, it ends up being about 3,000 m/s.  This is very fast at first glance, really it is not.

Space Shuttle Columbia taking off for the first time.  There are really 5 engines that you can see if you look really closely.  The big white stick things are solid rocket boosters – they don’t burn hydrogen and oxygen). On the back of the shuttle proper (orbital vehicle, to be more precise), you can see three engines.  The huge white thing that the shuttle is attached to os a gigantic fuel tank.  That is where the hydrogen and oxygen are located.

This small amount of energy totally limits us so that rockets have to be huge.  If the chemistry were such that these elements released 10 times more energy, then we could (in theory) make rockets that were much smaller (more than 10 time – by a lot). In fact, we play around with different chemicals to try to make a larger exhaust velocity, but the problem is that the chemicals that produce the most wickedly large exhaust velocities are horrific to work with – like super caustic and really, really bad for humans. So, there has to be a balance between safety (which costs a LOT of money or lives) and exhaust velocity too. This huge Russian rocket explosion that killed over 100 people, was while they were trying out new fuels that would have larger exhaust velocities.

We have not invented a better way to get off the ground than using a chemical rocket engine.  There are a TON of other ideas out there, but it is this fundamental limitation of the exhaust velocity that limits our ability to actually go very many places far away from Earth.

Next time, I will go through a simple formula that was invented in the early 1900s that predicted this whole problem. It was a good 40 years before modern rockets were even invented! And then, I will start posting about all of the absolutely crazy ideas that could possibly get us to the stars. Well, ok, maybe not.  But, they are awesome anyways!