Written by: Miguel Rico and Suzanne Monir, EIS Education Team Members, January 2016
Title of Lesson: Recent Space Development
Topic: SpaceX,
Grade (Age) Level: High School (Ages 14-18), University
Note: for greater detail on rocket reusability and spacecraft design, please go to the Rocketry course.
SpaceX’s long-term goal is to make life multi-planetary. To achieve this, a large transfer of people and material to Solar System objects such as planets, moons and asteroids, will be necessary. It is not enough, however, to have the rockets to put large payloads into orbit. Economic viability is also a very important consideration. Arguably, cost is the single greatest obstacle to widespread space exploration and use.
Rockets, and by extension spaceflight, are expensive. At best, the current price per kg for current orbital launch systems lies at around $4000 [1,4]. But why? Refuelling the rocket, even when accounting for expensive fuels, costs less than 0.5% of the total rocket cost [3,4]. Flight operations for relatively high flight rates also do not make a significant contribution. In fact, most of the cost of a rocket can be attributed to the cost of the rocket hardware (i.e. rocket structure, engines, avionics, etc.). The reason spaceflight is so expensive is that, traditionally, multi-million rockets have been discarded after flights lasting fewer than 10 minutes [5]. This means that a great percentage of the $60 million [1,3,4] - at the current best - needed to build and launch a multi-ton payload rocket ends up crashing into oceans or deserts or burning up in the atmosphere. This is the equivalent of disposing of an aircraft or car after a single journey and needing to build another vehicle from scratch for the next. The conclusion that we can draw from this analysis is that reusability has the potential to drastically lower the costs of using a given transportation system.
T
here have been several proposed concepts to reuse rockets, ranging from rocket booster flyback [6] to mid-air engine recovery [7] and propulsive vertical landing [8].
Figure 1: This table by Raymond James shows 3 of the many reusability concepts currently under consideration
or
development.
Some other noteworthy mentions include Skylon, an air-breathing single-stage-to-orbit (SSTO) spaceplane that makes use of a novel engine cycle called SABRE [9]; the Baikal booster, a Russian proposal that would make use of wings for a horizontal landing [6]; and Escape Dynamics' recently discontinued microwave-powered SSTO vehicle [10,11]. Also, please note that 80-90% of targeted rocket cost savings for the Falcon 9 rocket is likely to be an overestimate, as the first stage of the rocket (the only one under serious consideration for reusability) costs approximately 75% of the total rocket hardware cost. Moreover, SpaceX's COO Gwynne Shotwell has said that it could offer a Falcon 9 rocket with a reused Falcon 9 stage for around $40 million.The problem with reusability is that the addition of recovery systems subtracts from the useful payload you can carry into orbit. Already, it is difficult to design a rocket that is able to withstand strong vibrations, aerodynamic loads and hundreds of degrees of temperature difference and that can put approximately 2% to
4% of rocket mass into orbit [12,13]*. Adding recovery systems such as landing systems, aerodynamic flight control surfaces and extra fuel might result in zero or even negative payload to orbit. The challenge for rocket reusability has therefore been to engineer a rocket that, even when outfitted with recovery systems, is able to carry useful payloads to orbit*.
*To find out why rocket reusability is a somewhat easily achievable goal, check out the rocketry course!
SpaceX is one of the companies that has been pursuing rocket reusability. For now, the most easily recoverable part of the rocket, and also the most costly, is the first stage. It bears approximately 75% of the rocket's cost [14]. Through incremental steps, the propulsive landing recovery of SpaceX’s Falcon 9 first stage has become more and more likely. Firstly, there were the Falcon 9 Reusable Test and Grasshopper technology-demonstrator programs [15]. Modified Falcon 9 first stages were launched and subjected to different flight conditions, such as ascent to varying altitudes, hovering, diversion maneuvers, subsonic to supersonic speed transitions and landings.
Credit: SpaceX (click for greater resolution)
The insight gained during these tests was useful for the next stage of SpaceX’s reusability program: water landings (see above). After the first stage separated from the rocket, it performed a series of orientation maneuvers and burns needed to properly orient the rocket for atmospheric re-entry and to cancel out some of the velocity acquired during ascent [16]. Grid fins and one Merlin engine then guided the stage for its soft landing on the Atlantic Ocean [16].
These, partly successful experimental landings were then followed by landing attempts over stabilized, autonomous, floating landing platforms. During the first attempt, the first stage ran out of a hydraulic fluid needed to steer the grid fins and, as a result, the rocket impacted the barge in a forceful and uncontrolled manner [17,18,19].
The second attempt was marked by a thrust-controlling valve malfunction which resulted in the first stage tipping over after it descended with the incorrect attitude and excess lateral velocity [17,18].
The next rocket mission, which was intended to resupply the International Space Station, ended in an explosion (or a Rapid Unscheduled Disassembly (RUD) as SpaceX calls it) after a strut supporting a helium tank broke, leading to an overpressurization and subsequent disintegration of the second stage [19,20].
The next landing attempt occurred over land at SpaceX’s Landing Zone 1 complex. This mission marked the return to flight of SpaceX, the first flight of a significantly upgraded Falcon 9 rocket and the first successful landing and full recovery of an undamaged orbital-class first stage [19].
One of the more recent attempted landings on the autonomous barge platform resulted in failure when, due to ice buildup caused by fog condensation at the launch pad, a latch was unable to properly secure a landing leg [18,19]. This in turn caused the stage to tip over and explode.
As of late, however, two rocket stages were able to be recovered in the autonomous platform. The first of which occurred on April 8th, and was able to execute relatively comfortable recovery maneuvers as it came from Low Earth orbit (LEO), an orbit that does not require the rocket to reduce its available fuel margins to dangerously low levels. The second recent recovery, on the other hand, required a more demanding return flight profile. For example, to conserve fuel, the rocket used three rocket engines instead of the usual one to decelerate from supersonic speeds to touching down on the barge in just 15 seconds - 3 more seconds and it would have run out of propellant and crashed. The booster also dispensed with a boost-back burn, something that caused the booster to experience 8 times as much as heating as in previous recovery attempts.
April 8th mission's Falcon 9 first stage after landing on a ship in the Atlantic (high resolution) Credit: SpaceX
More such attempts are planned, and Elon Musk, co-founder and CEO of SpaceX, has said that a recovered stage may be reflow as soon as June of 2016. This would mark the first time any entity in the history of spaceflight has reflown an orbital-class rocket booster. Overall, Musk estimates that the success rate for rocket recovery will be about 70% for 2016 and rise to 90% in 2017 [21]. Second stage reusability should eventually follow suit (at least with future rocket designs), as will manned flights aboard the 7-crew Dragon V2, a spacecraft that will enable missions to the ISS in 2017, Mars in 2018 and possibly even the Jupiter moon Europa [22].
Hypothetical SpaceX rocket boosters landing on Earth, SHLV returning - Credit: Stanley Von Medvey
SpaceX Dragon V2 Mars Landing - Credit: ChrisMonson
SpaceX will continue to try to recover Falcon rocket first stages. While it is not the only company or government agency attempting to develop viable rocket reusability, currently, SpaceX is the company that can provide the highest potential launch cost savings in the shortest amount of time (see Rocket Reusability Initiatives above). Already, and even without reusability, SpaceX is at the forefront of lowering launch costs. With full, cost-effective and rapid reusability planned, launch costs should decline even further, low enough, in fact, to enable viable, long-term access to space, thus enabling greater space exploration and perhaps even a new, even more exciting, space age.
* SpaceX's upgraded reusable Falcon 9 v1.1 rocket has a payload fraction of 4.15% (payload 22800 kg and 549 054 kg total mass) while the Saturn V, the heaviest rocket to successfully reach orbit (flown in the 1960s and 70s), had a payload fraction of 4.71% (payload 140 000 kg and 2 970 000 kg total mass). How can two seemingly very different rockets with a more than 40 year age difference be so similar? Check out the rocketry course to find out!
