Falcon 9: A Rocket for the 21st Century

 Spaceflight in 21st Century  

Much has changed in the spaceflight industry for the past few decades. During the space race, only governments (with the aid of contractors) were able to build and launch rockets. It took time and efforts just to get them working. It’s worth noting that more than the successes we saw, there were failures. But with the entry of private players, we have seen much changes in both the objectives and the demands of the industry, primarily being cost. The goals have changed from ‘Launching a rocket successfully’ to ‘Launching a rocket successfully at the minimum possible cost’. Moreover, the horizons have expanded. From just landing and returning from the moon to establishing permanent colonies. Access to space by private enterprises has opened new doors to commercialize activities, be it satellite launches, asteroid mining, transport of men and materials to other worlds. The customers to utilize these services have also changed. First it was mostly governments and huge telecommunication firms to build and launch satellites, but now, schools, colleges, small to medium sized businesses can also construct satellites. In order to meet these demands, the rockets of this generation must be reliable, technologically advanced and at the same time, be cost effective.   

What’s Falcon 9?  

   We have all heard about the Falcon 9 built by SpaceX. Why is it so popular compared to other rockets of the same generation? Well, this article attempts to explore several factors behind the success of Falcon 9 and how its advancing 21st century spaceflight. SpaceX Falcon 9 can deliver payloads with considerable mass into orbit and land the first stage back on Earth through propulsive landing. Usually rockets with multiple stages after separation leave their stages to burn up in the earth's atmosphere, here the engine which takes up to 70 % of the rocket cost, the fuel tanks and much of the rocket's framework is lost. That implies these rockets could just be used once, hence they're expensive. Elon Musk, the CEO and lead designer of SpaceX says that humans can't become a successful space faring civilization if rockets could be used just once. He believes more is the reuse technology, lesser is the cost for reaching space. Through reuse of the first stage of falcon 9, SpaceX could save up to 18 million USD[1] per launch. 

Coming to its capabilities, SpaceX Falcon 9 currently in its Block V iteration has a total height of 70 meters and a diameter of 3.66 meters. Its structure consists of mostly aluminum and lithium. Falcon 9 is a two staged launch vehicle with the first stage powered by 9 Merlin engines designed and developed by SpaceX, arranged in an ocataweb configuration and the second stage's powered by a single Merlin Vacuum engine. The sea level thrust of the nine Merlin engines in total is 7686 kN and of the Merlin Vacuum engine is 981 kN in vacuum. The rocket's propellants are Rocket Grade Kerosene (RP-1) as the fuel and Liquid Oxygen (LOX) as the oxidizer. The propellant from the tanks is fed to the engines through turbo pumps following a gas generator cycle. The fuel tanks are pressurized by helium whose tanks are within the propellant tanks. The Thrust Vector Component a.k.a. TVC consists of gimballed engines and Nitrogen Gas thrusters. The launch vehicle comes under two configurations, Falcon 9 and Falcon Heavy (One Falcon 9 core with two Falcon 9 first stages as strap-ons) including their reusable counterparts. Above the second stage it’s either the Human-Rated Dragon spacecraft or the usual payload fairing.  

Reliability at a lower cost?  

   Usually where other orbital class rockets have three to four stages, Falcon 9 adopts a much simpler two stage configuration that eliminates potential risks during stage separation (This is where much of the rockets fail). Also, instead of using explosive bolts during stage separation, SpaceX employs a simpler pneumatic release system to ensure reliability. Glancing through the fuel mixture (Rocket Grade Kerosene and Liquid Oxygen) one may realize it’s not the most efficient combination out there but then, it’s cheaper and much easier to handle. One of the major areas where SpaceX cuts down costs incredibly is by following a fault-tolerant system than a fault-redundant system. In other words, for the flight avionics, SpaceX doesn't use any exclusive space rated hardware, rather commercial off the shelf components [2]. They may fail easily due to the intense vibration of the launch vehicle during launch or by high radiation in space (solar radiation, cosmic radiation, ions trapped in the magnetosphere). But the key here is,  ha ving  backups.  In  place  of  having  a  single  subsystem  (a  processor  for  example),  there  are multiple subsystems that does the same calculation in the same instance of time. When an output is  produced  by  a  subsystem,  it  cross-checks  the  outputs  produced  by  the backup  subsystems and if  there's  any  discrepancy,  the  system  knows  an  error  has  occurred  and  it  can  correct  itself.  The biggest  means  through  which  SpaceX  cuts  down  launch  costs  is  reusability.  Yes,  SpaceX  in  fact is  known  for  reusability.  Also,  these  fir st  stages  require  minimal  refurbishment  before  they’re ready  for  their  next  flight. One  may  wonder  that  why  SpaceX  uses  nine  Merlin engines  on  Falcon  9’s  first  stage.  It’s  for  high redundancy.  At  most  with  the  failure  of  two  engines, Falcon  9  could  still  complete  its  mission  successfully. According  to  SpaceX,  this  high volume  engine production  has  enabled  SpaceX  to  manufacture  high quality  engines  and  with  so  much  engines  per  mission, they  could  substantially  build  flight  and  engineering data. Also, having  nine  Merlin  engines  enables  them  to land  the first stages much effectively. Each Mer lin engine produces  much  lower  thrust  compared  to  the  whole cluster ,  so killing all  engines  except  a  few  can give  Falcon 9  the  much needed  precise  control  and  maneuver-ability,  added  the thrust  level  ranges  from 100% to 20% ,  yet  even with  a  single  engine  at weight  ratio, meaning the  lowest  thrust  level,  it  still  has  a  higher  thrust  to the  vehicle  cannot  hover. During  the landing burn  when the  engine  cuts off,  the  vehicle  must  have  zero  velocity  ‘and’  it  must  be  on  the  landing  platform. burn  is  early,  the  vehicle  runs  out  fuel  before So, if  the  entry attaining  zero  velocity  and  crashes.  If  it’s  late,  the vehicle  won’t achieve zero velocity  at  ground  level  and  still it crashes.  The  Merlin  engines  are tested  rigorously,  several  times  their  working  capacity  because  these  engines  won’t  be  used  just once,  rather  many times which is very important  when  it  comes to  reusing  rockets.

Reusability, the new trend?

Coming  to  reusability,  it  makes  a  vital  part  for  SpaceX’s  goal  which  is  to  make  access  to  space cheaper.  To  understand  why  full-scale  reusability  can  reduce  costs  substantially,  Elon  Musk gives  the  example of a Boeing 737 which cos ts  90  million USD and can carry up to 180 passengers. If  the  737 were  to  be used  just once,  then  the  cost of  flight  per  person  would  be  500,000  USD  but if it were to be reused (which is actually the case) the cost of flight per person would be 43 USD[3]. Hence by reusing rockets spaceflight could become more affordable.  Now how does SpaceX reuse rockets? Earlier versions (Falcon 1 and some of the first Falcon 9s’) used parachutes for a water landing but proved to be unsuccessful. (The first stage couldn’t survive atmospheric reentry). Later SpaceX redesigned a powered descent system adopted in a version of Falcon 9 v1.0 called Grasshopper v1.0 (later v1.1) to perform takeoffs and landings at lower altitudes. However, for orbital recovery, a redesigned Falcon 9 v1.1 with four grid fins and landing legs for a complete recovery of the first stage was deployed.  This is how their propulsive landing works: After liftoff and MECO (Main Engine Cutoff), when stage separation occurs (usually at an altitude of 65 – 70km), the first stage flips by 180 degrees using nitrogen gas thrusters, then a boost back burn is performed where three of its nine engines ignite guiding the first stage back to earth. as it glides through the Earth’s atmosphere, an entry burn is performed slowing down the supersonic descent of the first stage. This is when the grid fins and with the help of the nitrogen gas thrusters, the first stage maintains its orientation. Based on its current velocity and altitude, the third and the final single engine burn is performed which slows down the first stage to land. As it nears the landing platform, the landing legs deploy which lands the stage upright. This completes the landing sequence. If the mission is to deploy a payload to LEO (Low Earth Orbit), the first stage lands usually lands at LZ-1 (Landing Zone 1) at Cape Canaveral, Florida or if the mission requires to deploy a payload at a slightly higher orbit or multiple payloads are to be deployed, the first stage gives a little more push to the second stage and the payload due to this the first stage cannot land directly at LZ-1, because of the low amount of fuel remaining onboard. Hence droneships (autonomous barges with huge landing platform) are deployed in Atlantic or Pacific Ocean upon which the first stage lands. Presently, SpaceX has got two operational droneships, ‘Of Course I Still Love You’ and ‘Just Read the Instructions’. If the payload is too heavy or is to be deployed at medium or high Earth orbits, the first stage wouldn't have any fuel remaining onboard, hence the stage is expended. Also, these non-reusable stages wouldn't have any landing gear in order to reduce the overall mass of the launch vehicle. Recently SpaceX is recovering the payload fairings too via parachutes. Yea, with Ms. Tree and Ms. Chief out in the ocean with huge nets catching the falling fairing. Now that the first stage and the payload fairing being recovered, but what about the second stage. Although SpaceX had plans to recover the second stage, it is uneconomical as adding extra fuel, landing gears to the second stage would reduce the payload mass considerably [4]. However, in their future launch vehicle (Starship), the second stage and the spacecraft would be integrated hence making a completely reusable launch system. In fact, Elon announced that the second stage spacecraft combo would be powerful enough that they could get to orbit without the first stage again with a relatively low payload mass. Yet that’s for the future.   But all these did not occur overnight. SpaceX indeed had to work very hard in order to land these first stages and they’ve encountered many failures until they could get this thing right which brings us to the next and the last segment: 

Rocket Failures, the Curse of Spaceflight. 

  SpaceX started landing rockets propulsively way back from 2012 through the Grasshopper. However, the first demonstration landing in an actual mission occurred 29th September, 2013 with the CASSIOPE mission which was also the first flight of the Falcon 9 v1.1. SpaceX continued to land these first stages but with near misses and explosions. Since landing the first stages were secondary objectives, these failures did not count as mission failures. However, SpaceX had learned a lot from each failure as a result of which on December 22, 2015 in the mission Orbcomm 2, SpaceX nailed it. Yes, for the first time in history SpaceX had landed the first stage of the Falcon 9 at LZ-1. And on April 8, 2016, during the launch of CRS 8, SpaceX had accomplished the first ever successful droneship landing on ‘Of Course I Still Love You’. Since then landing first stages has become a norm. Indeed, SpaceX had experienced two mission failures with Falcon 9 with the loss of payload. The first one was on June 28, 2015 during the CRS 7[5], where after 139 seconds after launch, the vehicle broke up leading to an explosion. However, the Dragon spacecraft atop the Falcon 9 not only survived the initial explosion but also communicated until it impacted the ocean. This event led SpaceX to ground all launch operations. Further investigations reveled that the explosion in the second stage was caused by a strut (structure holding the pressurized helium tanks within the liquid oxygen tank) failure leading the helium tank to slam the liquid oxygen tank and rupturing it. This over pressured the liquid oxygen tank of the second stage leading to mission failure. The next mission failure was on September 1, 2016 during the static fire test of the AMOS-6[6] mission when unexpectedly an explosion in the second stage caused the loss of payload and the vehicle. Since the rocket was stationed on the launchpad, Space Launch Complex 40 (SLC-40), it suffered some damage. According to SpaceX, due to the unwanted presence of solidified oxygen in the inner lining of the COPV (Composite Over-wrapped Pressure Vessel) structure, added to it the friction was responsible for the fire within the second stage leading to the explosion and the eventual loss of payload and vehicle. 

Falcon 9, A Standard for 21st Century Spaceflight. 

   Despite these failures SpaceX leads the launch market with the most advanced and reliable launch vehicle, the Falcon 9, so much so that it launched 6 astronauts to the International Space Station and bringing them back safely and with 103 launches at the time of writing, Falcon 9 has a success rate of 98.06%. Falcon 9, as I see, more than being an advanced launch vehicle, has single handedly changed the entire spaceflight industry, through bringing new private players, to making reusability, what people thought was impossible just decades back, a norm today. With Starship, the next chapter after Falcon, taking shape, the future of space exploration remains bright. I’d like to imagine of a time, in the future, when colonists of Mars and the Moons of gas giants, look back at Falcon 9, thinking, this is where it all began.   

- Aakash.P