A specific type of rocket booster is so widely used during orbital rocket launches that it almost seems to be a regular occurrence. But this raises the question of what strap-on boosters are and why they are used.
Strap-On boosters are supplementary rocket thrusters typically attached in a parallel configuration to a launch vehicle’s central core. They provide additional thrust, allowing the rocket to lift heavier payloads or reach orbits that fall outside the capability of a rocket’s main or core thrusters.
From the Titan IIIC, first flown in 1965, to more recent examples like the Space Shuttle’s iconic solid rocket boosters and also the Ariane V and Long March 7 launch vehicles – strap-on boosters have been playing a crucial role throughout the history of orbital rockets.
The added fire and smoke, especially from solid-fueled rocket boosters, not only contribute to the drama and spectacle of an already visually dramatic event, but they play a crucial role in making many otherwise impossible launches possible, as the following section illustrates.
The Purpose Of Strap-On Boosters
The majority of modern orbital rockets are medium-lift launch vehicles, meaning they can carry a payload of 2 000 to 20 000kg (or 4 400 to 44 100 pounds) into Low Earth Orbit (LEO), according to NASA.
(Russia’s definition varies slightly, classifying medium-lift capacity as between 5 000 and 20 000kg or 11 000 to 44 100 pounds.)
This capacity is sufficient for most satellite launches and even crewed missions to the ISS. Occasionally, though, heavier spacecraft need to be placed in orbit, or a satellite at the upper limit of a vehicle’s carrying capacity needs to be put into a higher orbit.
Instead of trying to solve this potential problem by designing a larger capacity rocket or compromising on payload, adding smaller strap-on boosters can provide a vehicle with the required thrust or even turn it into a heavy-lift launch vehicle.
Examples of the former include launch vehicles like the retired Titan IV and recently retired Delta IV+, as well as the Alas V rocket family. SpaceX’s Falcon Heavy and United Launch Alliance’s Delta IV Heavy are examples of the latter.
Using strap-on boosters is a much more practical and cost-effective way of increasing the launch capacity of an existing medium-lift launch vehicle.
As the following section will illustrate, there are primarily two types of strap-on boosters typically used to increase a rocket’s thrust and carrying capacity.
Types Of Strap-On Boosters
Strap-on boosters can primarily be divided into two categories:
- Solid Rocket Boosters
- Liquid-Fueled Rocket Boosters
1) Solid Rocket Boosters
Most strap-on boosters are off the solid-fueled variety. Not only are they already filled with propellant during the manufacturing process, but the fuel is stable at room temperature, and the boosters can be stored for prolonged periods until needed.
As a result, solid rocket boosters (SRBs) are the ideal “off the shelf” option for existing launch vehicles needing varying degrees of additional thrust, depending on the spacecraft’s payload or intended destination.
(For example, the United Launch Alliance’s workhorse, the Atlas V rocket, can have any combination of up to five solid rocket boosters to accommodate most launch requirements.)
Examples of launch vehicles using solid rocket boosters include the now-retired Titan IV and Space Shuttle, as well as rockets presently in use like the European Space Agency’s Arianne 5, China’s Long March 5, and the Atlas V rocket family.
2) Liquid-Fueled Rocket Boosters
An alternative to solid rocket boosters some launch providers are opting for is the use of the first-stage thrusters of the primary launch vehicle as strap-on boosters. Three individual first stages of a rocket are essentially connected in parallel and ignited simultaneously at launch.
For example, SpaceX uses three Falcon 9 first-stage boosters attached in parallel to form the Falcon Heavy launch vehicle. All three first stages ignite simultaneously to provide the rocket with enough thrust to lift a payload of 63.8 metric tons (141 000 pounds) to Low Earth Orbit.
In a similar fashion, United Launch Alliance uses three Delta IV Medium first-stage boosters in parallel to form the Delta IV Heavy launch vehicle. It also ignites all three thrusters together at launch and can lift a payload of 28.8 metric tons (63 470 pounds) to Low Earth Orbit.
The boosters of both vehicles are essentially identical to their central core’s first stage. The Falcon Heavy uses a combination of refined kerosene (RP-1) and liquid oxygen (LOX), while the Delta IV Heavy uses liquid hydrogen (LH2) and liquid oxygen (LOX) as fuel.
(Orbital launch vehicles use several different fuels as propellants for their engines, from liquid and solid to hyperbolic fuels. To learn more about the fuels orbital rockets use, as well as the advantages and drawbacks of each, read the full article here.)
Apart from being liquid-fueled, the strap-on boosters of these launch vehicles serve the same purpose as their solid-fueled counterparts, and the procedure followed is the same for both types of strap-ons during launch, as the following section will illustrate.
What Happens To Strap-On Boosters Once Expended
During launch, the strap-on boosters ignite simultaneously with the central core’s first-stage thrusters to provide the maximum amount of thrust to gain altitude and speed as quickly as possible to allow the spacecraft to reach space in the shortest amount of time.
(In the case of liquid-fueled strap-on boosters like the Falcon Heavy and Delta IV Heavy, where all three first stages are essentially identical, the central core’s main engines do not run at full power during the initial phase of the launch.
This is necessary since they have to continue propelling the spacecraft to space after the strap-on boosters are expended and jettisoned away, and need to save the necessary fuel for this final burn before separating from the second stage later in the flight.)
Approximately 2-3 minutes after launch, shortly after the rocket goes through Max Q (the period where the vehicle experiences the maximum amount of dynamic pressure), the strap-on boosters are expended.
At this point, both boosters separate in parallel fashion from the central core by means of small explosive charges or a pneumatic system, allowing the empty thruster to fall away safely to the side of the launch vehicle’s main body.
(This separation is a normal part of rocket staging, where a launch vehicle’s first stage typically separates from its upper stage. Learn more about rocket staging and why it is so crucial for spaceflight in this article.)
Traditionally, the empty boosters would have partially burned up in the atmosphere and disintegrated before falling into the ocean or, when launched over land, in mostly uninhabited areas like Siberia or the steppes of Kazakstan (depending on the launch location.)
In more recent times, though, launch providers started making a more concerted effort to retrieve and reuse spend strap-on boosters.
For example, during the Space Shuttle Program, NASA attached parachutes to the 45.46 meters (149.16 feet) long solid rocket boosters, which deployed shortly after separation from the main vehicle, allowing them to plunge gently into the ocean.
If possible, many of the recovered boosters were disassembled, refurbished, and refilled with solid propellant before they were prepared for reuse in upcoming launches.
SpaceX took the concept of reusability one step further by using a combination of the boosters’ main thrusters, grid fins, and reaction control system (RCS) to allow them to return to the planet’s surface and make a soft landing before being refurbished for future launches.
Conclusion
As mentioned in the introduction, the added fire and smoke created by strap-on boosters not only contribute to the drama and spectacle of an already visually dramatic launch event, but they play a crucial role in enabling launch vehicles to perform beyond their normal capacity.
By adding strap-on boosters to an existing orbital rocket, they greatly increase its capacity to lift heavier payloads or allow it to be placed into a higher orbit that would otherwise not be possible with the standard specifications of a specific medium-lift launch vehicle.
As a result, strap-on boosters are and will remain an essential part of any launch provider’s arsenal to make a large number of current and future rocket launches a reality.
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