SpaceX since establishing in 2002 is one of leaders of space industry - not without a reason.
In 2002, after establishing SpaceX, Elon Musk had one goal - Mars. It was clear that it is ambitious target. For Elon Musk and SpaceX reaching Mars and further colonization was ultimate goal; first step was offering launch vehicle with low cost and potential and ease of use compared to ordinary airplane.
Since first commercial space rockets able to reach space and inject payload into orbit, launch vehicles were single use. Worth millions of dollars rockets were burning in atmosphere or falling and crushing on Earth. Costs of wasting expensive launch vehicles were charged space agencies (or to customers when commercial launches begun) - launching satellites were expensive and time-consuming venture. Manufacturing rocket and assembling on launch site took time. To make it more clear, it was just like after every flight, airlines would be forced to scrap plane and buy another when passengers were waiting until plane would be ready to go. Such idea of spaceflights was enough when there was no large demand. Launching communication, observation or military satellites was not as frequent as now. Even establishing space stations by USSR and USA, with demand for regular resupplying seemed not to exhaust possibilities and economical sense of utilizing single use rockets. Increasing number of commercial satellites related with intensive development of communication technologies like cell phones, Internet, satellite mobile communication and satellite navigation systems occurred in number of over 1100 operational satellites in 2014. Continuous development of space technologies resulting in more affordable solutions for creating spacecraft - Cubesat satellite bus is widely available; to create small satellites it is not necessary to possess huge facility. Most of elements are available on the commercial market. This intensive development stands in the opposition to launch vehicles. They were still huge, expensive, complicated and not widely available. Launch service providers are capable to ensure necessary number of rockets, but they are still very expensive. Good example is International Space Station which requires regular resupplying - costs of every cargo mission are very high - twelve cargo missions contracted to SpaceX in 2008 were worth $1.6 billion. Only twelve missions cost over $1.5 billion- if it would be asked, how much it will cost to build another, larger space station as an orbital base for Mars mission, costs will grow to astronomical values. Time is also important - twelve missions were contracted for eight years due the fact that every launch takes long time to be prepared. Such low frequency is a problem in Mars mission (still concerning version with Mars spacecraft build on orbit), even not acceptable for further exploration or colonization of Red Planet. Regular supply flights for Mars colonists will be necessary with higher frequency then now, not to mention when building Mars spacecraft on orbit will be performed. After reaching Mars and building Mars station with crew, it will be important to start continuous chain of supply for Mars base at the beginning. It is obvious that costs of Mars colonization (establishing base on Mars will be only beginning, resupplying and maintenance could outweigh the cost of establishing multiple times) will be ultimately high. SpaceX was always focused on reducing costs of launching payload to orbit to make future Mars mission possible SpaceX is successful in reducing costs - even before implementing reusable technology Falcon-9 offered much more better ratio kg/$ then ULA or ILS. An attempt to implement reusable technology into SpaceX rockets was concerned as significant step in further Mars missions.
Reusable technology by SpaceX is ambitious venture to improve profitability of rocket launches and help to make Mars mission possible as autonomously venture of SpaceX. History of developing reusable technology starts in 2011 when SpaceX announced about plans for recovering first stage of their Falcon rocket. Designing and manufacturing technology demonstrator was performed under reimbursable Space Act Agreement with NASA participation. In 2012 SpaceX performed first test flight of Grasshopper demonstrator rocket powered by one Merlin-1D engine with height of 32 m. Tests consisted of vertical starts, flight with low speed, reaching low altitude (744 m was record performed on 7 October 2013) and vertical landing on special landing pad. All eight test flights were performed in SpaceX test facility in McGregor, Texas. Next step of progress in reusable technology was creating new vehicle - this time it was larger rocket with height at 48 m based on first stage of Falcon-9 1.0v. Rocket called F9R Dev and performed five test flights between April and August 2014. Test flights took place in same facility as Grasshopper, but were much more complicated. They consist maneuvers such as hovering, moving sideways and testing new steering fins. Unfortunately, on 22 August 2014, rocket failed due the broken sensor and was destroyed. Just before introducing F9R Dev, SpaceX decided, that with starting utilizing Falcon-9 1.1v (enlarged version of Falcon-9 1.0v) every first stage of rocket will be equipped to perform descent tests. First attempt of controlled deaccelerating of 1.1v first stage, took place on 2013. On 18 April 2014 for the first time, first stage deaccelerated and it was possible to recover it from ocean. Still it was not landing, but controlled falling - due the stormy weather, SpaceX was not able to recover stage. Theoretically after recovery stage could to be utilized again. But additional time would be needed to recover it from Atlantic Ocean, bring back to factory and refurbish. It was solution closed to reusable solid fueled boosters of Space Shuttle lifting rocket. Of course, boosters were not using their engine for descent but falling on parachutes, but costs and time needed to make them operational again was comparable. It was not goal of Elon Musk and SpaceX. In 2015 next step was performed - this time first stage after separating and rotating for 180°, should try to land on special landing platform floating on the Atlantic ocean. Unfortunately first attempt on 10 January 2015 and second on 14 April 2015 were not successful. Rockets crushed during landing on sea platform because not sufficient precision of throttling the engine. In half of the year, on 28 June 2015, SpaceX planned to launch Dragon supply spacecraft to ISS and eventually perform another attempt. Falcon-9 failed and explodes after 139 seconds of flight. Next scheduled flight performed by SpaceX launch vehicle was planned for Falcon-9 1.2v, not for 1.1v. Mission was planned on 20 November 2015 and consisted lifting SES-9 communication satellite. Falcon-9 1.2v was equipped in enlarged tanks and nine Merlin-1D engines with modified thrust control. Mission was delayed and postponed to 2016, but in the beginning of December, according some sources, another attempt of landing was planned by SpaceX In fact works on rented by SpaceX launch pad were closing to finish and launch pad starting to look like landing zone. Scheduled for second half of December launching eleven Orbcomm OG2 satellites with Falcon-9 1.2v seemed to be most probably occasion for next landing attempt, this time not on floating platform but on landing zone. At least, on 21 December 2015, first stage of Falcon rocket landed on landing zone in Cape Canaveral after separating from second stage with Orbcomm satellites on atop. Just 25 days after first success of vertical landing performed by Blue Origin New Shepard, Falcon-9 1.2v
Achieving success on 21 December was not easy. SpaceX had to acquire different technologies which are necessary to create rocket with reusable first stage and possibility of soft landing. These technologies were possible to be adopted into different rockets so probably that reusability will become characteristic for SpaceX rockets in future. Bases of reusable technology are: restartable ignition system for the first-stage booster - it is necessary because, after stopping engines to start separation from second stage and performing rotation with utilization of cold gas thrusters, engines should start again. New attitude control - it is necessary to bring rocket through upper atmosphere and help rocket to stop rotating after 180 degrees, going from vacuum to Earth atmosphere and decelerating to supersonic velocities. Deaccelerating generates lot of heat so large surface heat protection system is required. Next innovation was adding hypersonic grid fins for better control during descending; engine had to be equipped with ability to be throttleable with high precision to provide capability of landing empty and light rocket body. Also affecting on new algorithm for landing where thrust-to-weight ratio of the vehicle is becoming greater than one. Accurate and very precision navigation was necessary for landing in correct place, and of course light and stable deployable landing gear resistant for heat and gases from rocket engine. First stage of rocket generally became spacecraft which able to return autonomously to Earth. It is impressive technical achievement without any doubts and puts SpaceX in leading position as far as technical possibilities are concerned.
Adoption of reusable technology into operating rocket and successfully testing it during commercial mission shows potential of SpaceX. But for Elon Musk and his team it is only the beginning. Next step is redesigning second stage of Falcon-9 to make it reusable and creating first in history operational VTVL launch system. Further plans of SpaceX consist, inter alia, creating more powerful engine for powering new heavy rocket. Engine called Raptor-1 will be up to six times stronger then Merlin-1D which is current propulsion of Falcon-9. Raptor will be fueled with non-toxic liquid oxygen and methane and will be fully reusable. It will be significant step in the way to Mars. Methane could be obtain by sintering it with Sabatier reaction (using hydrogen with carbon dioxide to possess water and methane in reaction in temperature 300–400 °C with nickel catalyst) on Mars, solving one of main problems considered during planning Mars mission - how to provide enough fuel for way back to Earth. At the moment SpaceX performs test of Dragonfly - prototype of suborbital reusable launch vehicle (RLV). It is prototype and demonstrator of propulsively-landed version of Dragon spacecraft. If it will be successful, SpaceX will be only company providing fully reusable rocket with reusable spacecraft with possibility of utilizing both next day after landing with possibility to design Mars lander with ability of returning back to Earth. Economic success is also impressive - SpaceX claims that price for launch after adoption of reusable technology will be at $5 million to $7 million - it means that cost of launch will be reduced ten times. Considering that instead one launch, there will be possibility to perform ten launches for same money and lifting ten times more payload, mission to Mars seems possible. But still there are many problems to solve; Mars spacecraft for manned mission should provide safe place to live for crew for at least one year. During this year, spacecraft will be exposed to cosmic rays, micrometeoroids, and contact with space debris, sun wind and many more. At the moment there is no clear conception how to build such resistant spacecraft (it is worth to mention how big issue were micrometeoroids even for ISS, which still is one of best protected against micrometeoroids object created by human). Also propulsion is a problem - electric propulsion is not strong enough to move heavy spacecraft, rocket engines need tons of fuel to reach Mars and return to Earth. Only these few aspects stand on the way to crew mission to Red Planet. Reusable technology solves only one aspect, which is of course very important - how to reach space from Earth in affordable way. But still many problems are not solved - SpaceX and Elon Musk are surely aware how huge job awaits them.