How Do Rockets and Space Shuttles Work? Part 1
The news is full of stories about new rockets and missions. While most people get excited by space projects, they are a little sketchy on the details. This is intentional so people like me can sound smarter than we actually are, and keep all those lucrative rocket science jobs to ourselves😊.
There are some key principles of how rockets work that everyone should know. And as a former Space Shuttle engineer, I decided to compare and contrast how the shuttle works with what people think of as a “rockets”.
While there are MANY systems essential to making a rocket work, I am focusing on propulsion in this story.
So what is a rocket?
The word “rocket” can mean different things. We use really small rockets to launch explosive fireworks into the air or for signals. Many people fly small model rockets. Companies like United Launch Alliance (ULA) and SpaceX launch huge rockets to carry satellites, interplanetary spacecraft, and scientific instruments up into space. There are a range of rockets that come in many sizes.
What they all have in common is that they generally have a small payload on top, and have one or more sections, or stages. Stages have liquid or solid propellants which provide the fuel and oxygen that are combusted and propel the combusted materials out through a nozzle.
Thanks to Sir Isaac Newton and his third law — the force of the exhaust leaving the rocket causes it to move in the opposite direction. The type of fuel and oxidizer, they way they are ignited, and the complexity of the engines vary significantly between rockets.
We all know what fuels are. But what really is an oxidizer? Oxidizers are chemicals that allow a fuel to burn. On Earth we generally use oxygen for this purpose as it is all around us in the atmosphere. However in space there is no atmosphere to provide oxygen, so rockets need to carry their own oxidizers with them.
Most rockets use several types of engines. For example the Space Shuttle used four different types of engines and three different types of fuel/oxidizer combinations.
Space Shuttle Ascent — Eight and a half minutes of terror
While the majority of the fighter jocks that dominated the Shuttle astronaut corps would not readily admit in public to being frightened. I have heard some refer to launch and ascent into orbit as terrifying — whether it was on the Shuttle or the Russian Soyuz rocket. I don’t blame them.
Why is it so terrifying? Because passengers in a rocket are sitting on top of a controlled explosion.
From the time the main engines light till they shut down less than nine minutes later, the Shuttle lost about 94% of its mass — mostly by converting fuel and oxidizer into a controlled explosion that was directed through nozzles to get the Orbiter into the exact place in orbit it needed to be. That is ALOT of energy.
7..6..5..“We have main engine start”…
The final seconds of count down were very exciting in Mission control as so many systems had to come on line and show good status. Due to the inability of mission controllers to react to off nominal situations, automated sequences were in place to stop launch if necessary.
Things really kicked into high gear about 7 seconds prior to launch when the SSMEs ignited — one at a time. At this point, automated routines validated that everything was working before issuing the final SRB ignition. Several times in shuttle history, a problem was found and the SSMEs were shut down prior to SRB ignition.
If you look at you tube videos of shuttle launches you can see the igniters sparking as the fuel and oxidizers are released through the engine nozzle.
Once it was clear that the three SSMEs were running well, the two long solid rocket boosters (SRBs) on the sides of the Orbiter were lit. Once these two behemoths of the deep fires of hell were lit, the shuttle was off an running. There was no way to shut these down. Also, there was no way for the crew to safely escape in an emergency for the two minutes that the SRBs ran.
Once the Solid rocket motors lit — the shuttle was GOING.
Since the SRBs could not be shut down, the automated launch control system had many checks and balances to be sure that all was ready before the ignition signal was sent.
The hydrogen / oxygen combustion of the SSMEs resulted in a clean, almost transparent exhaust. The less efficient but more powerful solid rockets generated most of the dirty exhaust plume during ascent.
Since rockets have to carry all their propellants with them, they are really heavy at launch. The Shuttle, including one Orbiter, two SRBs, and a full fuel and oxidizer tank weighed about 4.5 million lb at liftoff.
A mission ready Orbiter with payloads, supplies, propellants, crew etc weighed less than 300,000 lb. Typically rockets only deliver on the order of 5% of their liftoff mass into Orbit. (This, and the economics of re-use, is why SpaceX’s capability to retrieve and re-use boosters is such an important breakthrough.)
Each of the Shuttle SRBs had 2.8 million lbs of thrust(!), while all three SSMEs together had about 1.2 million lbs of thrust. All told, the Shuttle had 6.8 million pounds of thrust pushing it off the pad. Since it only weighed 4.5 million lb at liftoff, it jumped off the launch pad once the SRBs were lit and accelerated rapidly.
As scientists and engineers like to say — When you have more thrust than weight — you go fast.
The SRBs burned out after two minutes, and separated from the Orbiter and External Tank. Once the SRBs were away, the passengers felt a little better! However, they were not home free yet.
The three SSMEs were still pumping out 1.2 million lb of thrust and the Orbiter and tank were a lot lighter. As more and more propellants were burned, and the vehicle go lighter, the shuttle accelerated faster — eventually reaching 3 g’s.
At this point the guidance computers throttled the engines back to keep from accelerating more rapidly. Exceeding three g’s was deemed bad for astronaut photo shoots (no one likes that drooping plastic face look) and keeping scientific instruments and payloads from getting damaged. The Three SSMEs finally shut down about eight and a half minutes after ignition. Mission Control loved catchy acronyms. When the SSMEs shut down, we called it “Main Engine Cutoff”, or MECO.
Not time for a break yet
While the passengers breathed a major sigh of relief at MECO — they were not done yet. At MECO the Orbter was in an orbit that would bring the Orbiter back into the atmosphere as it rounded the planet. To gain the final orbit needed for the mission, the shuttle had to separate (but not recontact) the External Tank, and then use yet ANOTHER propulsion system.
I noted that the three SSMEs were located on the tail of the Orbiter. The Orbital Maneuvering System (OMS) had two smaller rocket engines located in pods on either side of the vertical tail, and above the SSMEs called Orbital Maneuvering System, or OMS engines. Only a few minutes of run time was required to place the shuttle in a stable orbit. The acceleration levels were much lower than during ascent. making OMS maneuvers a bit more relaxed than powered ascent.
While the SSMEs used cryogenic (super chilled) liquid oxygen and hydrogen, and the SBRs used a solid mixture ammonium perchlorate (oxidizer) and atomized aluminum powder (fuel), the OMS engines used monomethylhydrazine (MMH) as fuel and dinitrogen tetroxide (N2O4) as oxidizer. This last combination was chosen because these propellants ignite on contact very reliably. While these propellants are very hazardous on the ground, they were extremely reliable in space. In fact, most spacecraft and satellites today use these propellants for that reason.
Tiny little thrusters
Yet a fourth propulsion system was used to keep the shuttle pointed in the right direction at all times. The Shuttle Orbiter had 44 small liquid-fueled reaction control system (RCS) thrusters.
The “small thrusters” used the same propellants as the OMS system. In addition to pointing the Orbiter in the right direction, the RCS thrusters allowed the Orbiter to station keep in orbit, maneuver slowly while docking with another spacecraft or space stations, and provided a backup for the de-orbit burn if the OMS failed.
“Regular” rockets Vs the Shuttle
The space shuttle was a one of a kind vehicle — sort of. The Soviets built a reusable space shuttle called Buran (“blizzard” or “snowstorm” in Russian). While the Orbiter, tank and strap on boosters looked suspiciously like the shuttle. However, the resemblance was superficial. The system was actually different in very significant ways.
Buran only flew once, but flew unmanned — and landed successfully on a runway. There are many stories about why the Russians abandoned Buran. It may have been as simple as funding. Buran not withstanding — Shuttle was pretty unique and flew 135 times. Conventional rockets look different, but essentially have the same kind of systems and do the same kind of things to get payloads into space.
A key distinction is that the Shuttle Orbiter was really equivalent to the payload on a rocket. The downside of a reusable Orbiter was that the lions share of the 300,000 pounds or so that the Shuttle put into orbit came back down. So the delivered payload was much smaller. Because of this the actual payload delivered for use in orbit was a fraction of the 5% we noted above.
The shuttle did a lot of incredible things, but as a delivery truck it was much less efficient than a conventional rocket. That is why it took over 40 shuttle missions to deliver the US portions of the Space Station into orbit.
Bigger rockets have multiple stages. By accelerating to high speed, and the dropping part of the structure, machinery, engines and tankage used so far, the upper stage of the rocket can more easily lift the payload into orbit.
There have been many attempts to develop a single state to orbit spacecraft (including my senior design project in aeronautical engineering many decades ago!), but with conventional chemical propulsion systems (which is all we have) this is how the math and physics work out.
A potential game changer
I am watching the development of the Reaction Engines Sabre engine with interest as it could potentially make single stage to orbit feasible. Sabre is a hybrid air-breathing rocket engine. Sounds contradictory? Sabre changes the way it operates in flight. Initially it is an air breathing engine that uses atmospheric oxygen for combustion. After accelerating the spacecraft to hypersonic speeds in atmosphere, it acts as a rocket engine using internally stored oxidizer.
Differences between the shuttle and conventional rockets
Conventional rockets sometimes use strap on solid or liquid rocket boosters for additional performance when carrying heavier payloads. Like the shuttle, these separate when they burn out or run dry.
Then, as the first stage of the rocket runs dry minutes later, it too separates — and the upper stage motors ignite.
Shuttle was unusual in that the Orbiter (payload) hung on the side of a large fuel and oxidizer tank that fed the engines on the end of the Orbiter (Rockets carry payloads on top). After the SRBs burned out and separated, the Orbiter and tank continued on until orbit together.
Some payloads carry a third propulsion stage stage if they need to reach a high orbit, or are destined for an interplanetary mission. Similarly, Shuttle carried payloads that had their own propulsion system on many occasions.
Similarities between rockets and the Space Shuttle
Conventional rockets have reaction control thrusters to keep pointing the rocket in the right direction as well. However, unlike the shuttle — once the payload is delivered into orbit — the job of the rocket is done.
Rockets use pyrotechnic (explosive) separation systems during flight to rapidly and safely separate strop on boosters and rocket stages — as did the shuttle.
All rockets rely on computer avionics systems with sophisticated guidance, navigation, and flight control systems — as did the shuttle. In fact, ascent phase through MECO was hands off for the astronauts unless emergency procedures were required.
All rockets rely on communication systems to provide updated and real time commands and information.
All rockets carry range safety systems. In the event that the rocket leaves the pre-programmed and approved flight plan — ground operators have the ability to blow the rocket up before it can approach a populated area. The shuttle had a system like this as well.
All big rockets have some sort of thrust vector control to use the engine thrust to steer the rocket in flight. The shuttle main engines and SRBs did as well.
All big rockets have electrical power systems and utilize batteries.
The Rocket Revolution
There is a revolution going on in the rocket world. There are many more rockets available today, and the prices have come down considerably.
SpaceX is largely responsible for setting a new commercial tone and stealing business from the traditional rocket companies.
Blue Origin has added more competition and there are many smaller rocket companies. Even the Japanese and European space agencies have initiated new big rocket developments aimed at cheaper pricing. The venerable United Launch Alliance has been re-tooling as well to meet this threat. While the Atlas and Delta rockets have a fantastic track record — they are far too expensive in this new market reality. ULA hopes to re establish their market dominance with the new Vulcan rocket that will replace Atlas.
This is an exciting time to work in the space industry.
NASA is building a new “big rocket” as well. However, NASA seems to be following the old school approach. The Space Launch System (SLS) will have tremendous lift capability, but it will be very expensive to operate, requiring a large logistical support base for a limited number of missions. With the exciting expansion of launch vehicle providers, and multiplicity of options for launch into space, it is hard to understand this approach.