Aerospike engines. Why are not we using them Now?

 Aerospike engines. Why are not we using them Now?

Ever since the earliest rockets we've seen them working with a bell-shaped engine nozzle from the first German v2rockets right up until the space x falcon heavy but the ubiquity of the bell shaped rocket nozzle doesn't mean to say that it's the best way to do things infact they have a major drawback which is one of the reasons why we use multistage rockets, however since the early 1970s there has been an alternative rocket engine which has much greater efficiency and was to power the next generation of single-stage-to-orbit spacecraft to replace a space shuttle but if it's so good why hasn't even been flown yet. The engine in question is the aerospike, a design that goes back to the 1950s when it was first developed by Rocketdyne.

 

 Now whilst this might sound like an exotic new type of propulsion the aerospike is not actually a whole new engine it's actually just a different way to contain and control the thrust but any rocket be that liquid or solid fuel produces and it replaces the Bell type combustion chamber. Now if we've been using the Bell type rocket nozzle combustion chamber since the very first Rockets right up until now and what's wrong with them.

 

For a rocket to work correctly and produce enough thrust to lift it into orbit then you have to control the burning rocket fuel. If there was no rocket nozzle then the burning rocket fuel would just exp and uncontrollably in all directions and very little of it will be converted into useful thrust. What the rocket nozzle does is convert the high-pressure combustion of a rocket fuel into an ultra-high speed flow of gas exiting the nozzle in one direction and at atmospheric pressure. The ratio of the size of a narrow end to the wide end of the nozzle must also allow for the atmospheric pressure where the rocket will operate.

 

Now this sounds complicated but it's actually quite simple. Everything including you, me the ground and rocket engines has atmospheric pressure pressing on us in all directions due to the weight of the atmosphere as the air is pulled down by the Earth's gravity.

 

 If you imagine a column of air measuring one square inch or 2.54 square centi-meters from sea level to the top of the atmosphere, that air would have a mass of 6.67 kilograms or14.7 pounds. we don't feel it because we have evolved to live at that pressure but if you were to go up to 12 kilo meters that pressure would be just one-tenth of sea level and at 50 kilo meters it will be one hundredth of sea level simply because the column of air is now much shorter and thus weighs much less.

 

That air pressure also pushes on the gases exiting the rocket nozzle. For a rocket to work at its most efficient and produce the most amount of thrust the gases exiting the rocket nozzle must beat atmospheric pressure so where they exit out as straight as possible. This is fine if your rocket is traveling sideways through the atmosphere like a air to air missile then the air pressure doesn't change much. A rocket going into orbit however starts at sea level with a lot of air pressure and ends up in a vacuum with no air pressure. Because a bell nozzle has a fixed shape and size its maximum efficiency and thrust will only be achieved at one altitude.

 

Design it to have maximum thrust at sea level and take off and the exhaust gases will over expand and lose thrust at high altitudes because of a lack of air pressure, design it to have maximum thrust at high altitudes and at take off the air pressure is so much but it constricts the gas flow inwards which leads it to separating from the nozzle wall and becoming unstable usually resulting in the engine blowing up. In an ideal rocket nozzle it would change its shape as the altitude and air pressure changes this is what an aerospike does. It uses the pressure of the air surrounding it air surrounding it to have the same to have the same effect as the bell of a traditional rocket engine.

 

The two main types of aerospikes which have been developed are the annular or round ones and the linear or straight ones. The round ones are where we get the name from, if you imagine an inverted bell shape It becomes a round spike, this spike becomes one side of a virtual Bell whilst the air pressure surrounding it compresses the exhaust gas against the inner spike, although many now use a truncated spike which has a much shorter design than the equivalent engine bell. As the rocket goes from take off at high air pressure to the low air pressure at high altitudes the shape of the exhaust flow changes due to the changing air pressure to keep it at its optimum shape and optimum thrust. This is what's called an altitude compensating rocket nozzle.

 

Although it might not be as efficient as a bell any given altitude it out performs them at all others. The Space Shuttle main engines which were used from take off to space were much less efficient at sea level than they were in space. With a rocket engine like the aerospike you could use just one engine that will work efficiently from take off to space and avoid the need for multiple stages with different rocket engines optimized for different altitudes effectively becoming an SST o or single-stage-to-orbit.

 

 So if these are so good why aren't they used to date no major rocket launches have used an aerospike despite much research and development being done during the 1960s and 70s and then in the 1990s. As a follow-on to the successfulJ-2 engine which was used on the Saturn third stage Rocketdyne set about developing and building both toroidal and linear aerospikes using the turbo pumps and engine infrastructure of the J-2. one of the biggest problems with a aerospike engine is the cooling of the spike. In the toroidal or round design the spike is long and heavy which makes it difficult to cool the tip of a spike to stop it from melting. This was mostly overcome with the development of the new copper alloy called NARloy-Z in the1970s which allowed longer use of high temperatures.

 

The design were also changed with a truncated spike with some of the exhaust gases being passed through the center to achieve a similar result of a long spike but with much less mass. However the linear version was even more flexible. In this the round spike is straightened out into a v-shape with the combustion chambers on either side, the beauty of this design is that can be made modular so that it can have more combustion chambers added to make a longer a more powerful engine. Back in the 70s they use combustion nozzles made into small banks which were stacked side-by-side on both sides of the "V" center. Although aerospikes were proposed for the space shuttle, as the Apollo program was wound up in the early1970s development work on the aerospikes also stopped and the space shuttle went on to use conventional Bell engine nozzle designs. Things stayed pretty much like this for the next 20 years until NASA was looking to develop the next generation of space shuttle using an all-new single-stage-to-orbit design. The design brief was to be able to come up with a completely new launch vehicle that would be fully reusable and would greatly reduce the cost of getting in to space from $10,000 per pound to $1,000 per pound. Lockheed Martin won the contract to build the revolutionary design designated the X-33 and one of the key features was the use of the linear aerospike engines. Development work continued on the XRS-2200 linear aerospike and by now with the use of electric magnetic nozzles it meant that the fuel system could be controlled on a nozzle by nozzle basis a bit like the fuel injection on a car.

 

This allowed for much greater throttling control and allowed for the thrust vectoring by turning off different sections of the engine. This also removed the need to have heavy complex engine gimbals. Work progressed well up until 2001 when due to issues with the X-33's composite fuel tank and cost overruns the project was cancelled again taking the aerospike engines with it. Progress has been made with NASA testing small scale solid rockets with a toroidal full-length aerospike in 2004 but since then there have been no large aerospike engine developments. So why don't new companies like SpaceX and Blue Origin use the aerospike engines with all the efficiencies they bring wouldn't they be the perfect match for a low-cost route to space.

 

As far as we know SpaceX has looked at using aerospikes but given the fact that no large-scale aerospike has ever been flight tested it would be a very big risk when you're looking to set up a commercial orbital space company. One of the driving principles of the space race was "To do the job good enough and no more", basically mainly that once you have developed your spacecraft or your rocket engine to do what it was designed to do then that's it you stop there. The technology SpaceX and others are using is well known and tested. The way they use it might be different like relighting the main angels descend back to earth but the engines themselves area known quantity. Commercial companies have to make a profit in the end and taking on a major task like developing a new untested engine design is something but could quickly sap those profits away.

 

Although the X-33 project almost got them to flight testing, it's still new technology and needs more money and more development. It would also mean completely redesigning the Rockets away from the tried and trusted but yet limited traditional bell nozzle engines. All this takes time time which could be used launching with traditional engines and getting money into the companies. Although there would be a saving of up to 40%, the fuel is actually one of the cheapest parts of a rocket launch, Elon Musk said himself that the Falcon 9 costs $60 million to build but only $200,000 to fuel. The bringing back of the boosters and the central core with the engines brings far greater savings than could be achieved with a change to aerospike engines. It seems as though until the price of launches has been driven down to as low as conventional Bell nozzle engines will allow that arrow spikes will remain on the drawing board.

 

The single-stage-to-orbit vehicles which perfectly compliment the aerospikes capabilities seem a long way off since the cancellation of the X-33 and the renewed interest in returning to the moon using variations of conventional engines that date back to the early 1960s. There are smaller companies like Arco space which are developing small-scale aerospike engines for single-stage-to-orbit satellite launches but unless there is a radical change in the space market only time will tell if arrow spikes will ever get used for future space vehicles. So what does it take to design a rocket engine how much thrust does it take to lift a spacecraft and how fast must we launch an object from the surface of the earth to get it to leave. You can look up the answer if you want but if you like me then you want to create things by yourself our sponsor for this video, brilliant.org is dedicated to doing just that turning you into a living breathing and most importantly calculating scientist head on over there and prove for yourself just what it takes to get a rocket into orbit. Having a strong math and science skills set is crucial because it opens up so many ways to explore the universe.