What’s difference between this ….. and this …. Well apart from the size and several million dollars, surprisingly there is not a lot.
When we think of iconic space programmes like Apollo and the latest generation of SpaceX reusable vehicles, we’re seeing liquid fueled rockets using cryogenic propellants and oxidizers, that is gases like hydrogen, oxygen and methane which are cooled until they become liquid’s and then stored in huge tanks inside the Rockets body.
From here they’re pumped in huge quantities into rocket engine’s and ignited producing the thrust which then propels the rocket.
But there is another method that doesn’t use liquid fuels and has been around since the 11th century, it’s both simple to use and manufacturer compared to liquid fuels and outside of the space industry propels nearly all surface to air and air to air weapons and missles, these are the solid fuel rocket motors.
So let’s see how these have been used space programmes to augment liquid fueled rocket engine’s of the shuttle and SLS and why we might not see these massive rockets in the future.
Solid rocket motors have also been around for a very long time far longer than liquid-fueled rockets which are only an invention of the early 20th century.
Solid rockets came about as a byproduct of the discovery of the explosive effects of early gunpowder, a mixture of saltpetre, sulphur and charcoal dust which was created in ancient China
To create explosions during religious festivals, bamboo tubes were filled with this gunpowder and then thrown onto fires, it’s thought that some of these didn’t explode but instead one end allowed the rapidly expanding gases to escape and they would fly across the ground.
This principle was the first military use of rockets which were the fire arrows. The rocket body will be strapped to an arrow and launched from hand held launcher.
It’s thought that the Mongols during their battles with the Chinese developed their own fire arrows and gunpowder bombs and these they took with them on their military campaigns as far as Eastern Europe.
Rockets were used in many places including the Kingdom of Mysore in southern India where they helped defeat the British East India Company in the 1780s. News of these triggered research in England, France, Ireland and elsewhere.
Later, during the War of Independence of 1812, the British Royal Navy used a modified version of Indian rockets to bombard Fort McHenry.
As for rockets to lift heavier items, nothing happened up until just before the beginning 20th century when in 1898 Russian schoolteacher, Konstantin Tsiolkovsky proposed the idea of space exploration using rockets.
In 1921 the Soviets started development work at the Gas Dynamics Laboratory into more powerful solid rockets which first flew in 1928 for about 1300 metres.
Over the next two decades the Soviets and the US created a whole range of solid rocket-based weaponry for use in ground to air, ground to ground, air to ground and air to air combat.
One of the most fearsome was the Katyusha multiple rocket launcher which could launch a salvo of upto 48 rockets upto 11 km and during World War Two 12 million rockets were produced in the Soviet Union, and this type of simple unguided missile system is still widely in service throughout the world today.
But it would be the development after the Second World War and into the 60s by both the Soviet Union and the United states into solid rocket motors for tactical, ballistic and intercontinental ballistic missiles that would lead to the largest boosters being made for orbital and deep space launches.
In the 1950s and early 60s, the US Airforce was doing research into increasingly larger rocket engines, although it didn’t really have any purpose for them at the time, it was more for research and to know that it might have something to fall back on in the future.
One possible use for these very large engines was for the proposed USAF Lunex project for a manned lunar landing, but after NASA was set up in 1958, Space Research would be moved from the Air Force to NASA.
A couple of engines which came out of this research were the Rocketdyne F-1 engine and the even bigger Aerojet M-1, although the M-1 only had parts of it tested, the whole engine was never actually built.
So, when NASA proposed the Apollo programme, the F-1 engine was an obvious choice to use for the Saturn rocket.
However, things were not plane sailing for its development, a liquid-fueled engine had never been built on this scale and there was no guarantee that it would work or be in time to meet JFK’s deadline to put men on the moon by the end of the decade.
So as a backup, work was done on creating a solid rocket motors or SRMs that would be a 1:1 replacement for the F-1.
A lot of work had already been done on SRMs with programmes like the Nike Zeus anti-ballistic missile system, the Pershing and eventually the Minuteman ICBM.
The advantage that solid rocket motors gave was that they were relatively simple to make and operate, they could sit in a silo for months or years with very little in the way of maintenance and be ready to go in a matter of moments.
However, early solid rockets had other problems, unlike a liquid-fueled rocket which could be throttled by controlling the amount of fuel delivered to the combustion chamber, once you start a solid rocket, it will run at full power until the fuel runs out. They also have a low specific impulse which means they use their fuel quickly, usually in about 2 minutes, so on the space shuttle they were ejected before they reached space.
In modern solid fuel rockets, there are ways using new types of propellants and techniques to control the thrust generated and even stop it but back in the 50s and 60s these were either full power or nothing.
Gimbling the nozzle, to control the angle of thrust was also something which hadn’t been done before with large solid rockets.
Also controlling thrust profile, in other words how thrust builds up, then sustains overtime and drops off was something that the space industry needed during the launch and control, for examples when the rocket goes through Max Q, or the maximum dynamic pressure during a launch.
However, the sheer size of these solid rocket motors meant that new techniques of manufacture and transport had to be developed
The development work was done by Thiokol and Aerojet, Although it be aerojet that built the SRMs starting with the Aerojet 100 inch diameter motor designs. This was gradually increased in size and significant advances were made culminating in a successful test of six 120-inch, nine 156-inch and three 260-inch diameter solid rocket motors.
The AJ-260 became the largest solid rocket motor ever to be built and tested. The 260 inch diameter, almost 6.63 metres was so large that it was too wide to fit onto a train for transport, so Aerojet set up a factory and test area in the Florida Everglades where they could be shipped by road and barge to the Cape Canaveral launch site 250 miles away.
That never happened because the Rocketdyne F-1 proved to be one of the most powerful and reliable liquid-fuel rocket engine’s ever made and there was no need for a solid rocket booster backup and there was no use for such a large rocket motor after the Apollo programme ended.
However, the AJ-260 was tested three times, the first and second test firing proved to be totally successful, with a maximum output of 3.6 million lb of thrust, over twice the 1,500,000lb Pounds of thrust generated by a single F-1 engine, more than enough to replace the eight H-1 engines of Saturn 1 rocket.
To test the rocket motors, they were lowered into a 150-foot silo with the nozzle pointing upwards, when the test was running the exhaust plume could be seen over 80 miles, 132 kilometres away.
The third test firing differed because it used a large ablative nozzle using the submerged nozzle configuration similar to those proposed with thrust vectoring control systems, the propellant formula was changed to that of one with an increased burn rate, and the chamber from the first test rocket was refurbished and refilled for this third test.
However, this was only partially successful as some of the propellant was ejected and caused the loss of the exit cone in the latter portion of the test, later tests showed that the flow characteristics of a new propellant were not compatible with the casting process for the propellant.
But it did generate a maximum thrust in excess of 5,000,000lb of thrust, over 3 1/2 times the thrust of a single F-1 engine and remains to this day the most powerful solid rocket motor ever tested.
Although the aerojet AJ 260 was never used, the test data and experience gained was put into use to create the solid rocket boosters for the space shuttle, though it would be Thiokol not Aerojet that would build these new boosters.
The shuttle SRB’s were slightly smaller at 146 inch, 3.71m in diameter, narrow enough to be transported by train for the 2000 mile journey from Utah to the Kennedy Space Centre in Florida.
The shuttle SRB’s were designed to be reusable as had been proved in the final AJ-260 test and all but four of the 270 SRB’s launched in the shuttle programme were recovered and over 5000 parts were reused after each flight.
They also used thrust vectoring which moved the nozzle up down and side to side to help control the vehicle in all three axes, something which was only in the very beginning of being developed when the AJ-260 was tested.
The shuttle SRBs were the most powerful solid rocket motors ever to launch humans into space developing 3.3 million lbs of thrust each and between the two of them generated 85% of the thrust at liftoff at take off for the space shuttle.
After the shuttle programme ended there were several proposals to use the SRB’s but none came to fruition until the Space Launch System of 2022 which used a pair of five segment SRB’s developed from the 4 segment versions used for the shuttle to give a longer burn time and 3.6 million lbs of thrust, although none of the SLS SRB’s were be recovered after use.
With SpaceX’s reusable rockets and more efficient less polluting liquid-fueled engine’s, solid rocket motors for the space industry will be limited to small launch vehicles and it seems unlikely that any completely new rocket design will use the large scale SRB’s like that of the shuttle or the SLS system let alone the AJ-260 for their first stages due to the inefficient use of their propellant for each kg lifted into orbit and cannot be controlled like liquid fuelled engines for complex manoeuvring.
However, solid rocket motors have a long history of being used for the final boost stage for satellites due to their simplicity, reliability and small size. Spin-stabilised rocket motors are used when extra velocity is required such as for missions to the outer solar system.
So solid rocket motors have a bright future here on earth in the military and for smaller space operations but it’s unlikely we’ll ever see anything approaching the AJ-260 in the future.
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