# What is the intent of attaching legs to the rocket?

SpaceX Adds Landing Legs to Falcon 9 Rocket for Next Launch

Still thinking why they added legs to the rocket's first stage?

What are the advantages of attaching the legs to the rocket?

Does that increase the structural weight thereby decreasing the payload mass?

They're part of the SpaceX reusable launch system development program. The intent is to make both first and second stages of the Falcon 9 and the Falcon Heavy reusable by returning them both to the launch pad, where they are to make vertical landings. Their technology demonstrator, Grasshopper, made eight successful test flights.

Yes, it reduces the payload fraction, but if they can make the technology work, the cost per pound to orbit can be substantially reduced.

If Elon Musk can pull this off, then maybe the State of California should take another look at his Hyperloop.

• Whether or not they can reduce launch cost through a returning first stage still needs to be found out. A returning stage provides less acceleration than an expendable stage of the same size, because it needs to keep some fuel for returning. Also, liquid fuel rockets are currently designed to be ignited and used exactly once. When you want to design a rocket which is durable enough to be used multiple times, cost will increase and efficiency will decrease. – Philipp Feb 28 '14 at 10:11
• The engines of the Falcon have been designed from day 1 to be reusable for at least 10 missions. Likewise, the NASA SSME engines were designed for reuse. – aramis Feb 28 '14 at 11:13
• @aramis The SSME's were designed for reuse with no consideration of cost. Merlin is designed for cheap/fast reuse. But to be fair, anything is better than the SSME's record. In defense of the SSME, it was one of the highest performing engines ever (by Isp, not by sheer thrust). And that is hard to make quickly reusable. Or cheap. – geoffc Feb 28 '14 at 11:16
• Yes, the intent is to make it reusable and come back to the pad, but they have to first prove it can fly with the legs on before they can land with the legs on. This one will go into the water. This answer should clarify that – Brian Feb 28 '14 at 14:05
• +1 for the mention of hyperloop! Also for great answer! – John Odom Feb 28 '14 at 14:53

As noted, the SpaceX plan is to build a rapidly reusable rocket. That is the only real way to get costs down.

At the moment, no one has ever done what they are trying in several ways. What I find most interesting is the regular testing.

Grasshopper and Grasshopper 2 (officially named F9R-Dev1 and it was tested into a lovely explosion as they hit an edge of the test envelope, the purpose of testing) are testing the control software at the low end of the landing event. (GH completed testing up to 1/2 mile, F9R-Dev1 was lost during testing, and F9R-Dev2 will go much higher in New Mexico).

Every flight they can, SpaceX will be testing the landing system, as a 'bonus' use of the first stage until they succeed. Once they do, they will have the evidence in hand to demonstrate to the FAA that they can safely land a stage on land.

Thus first they will test it over water to demonstrate control and the entire system end to end. Since their original testing, they added a landing barge, the ASDS (Autonomous Spaceport Drone Ship), which provides a 170X300 foot target to try and land on. The first attempt on the CRS-5 mission hit the corner of the barge, but ran out of hydraulic fluid for the grid fins. But boy they got close! Following missions aim to land on the drone ship.

Going forward they are building a West Coast drone ship for Vandenberg launch recoveries.

This gives them additional flexibility. If the ASDS is far downrange and they launch a heavy payload, the fuel required to return to launch site (RTLS) is less, if they actually land on the barge down range.

Testing on the barge makes the FAA certification for landing at KSC much easier, if they have proven repeatadly that they can land on target. (They are building a landing site at KSC at LC-13).

The Falcon Heavy center core will be flying much higher and faster than a normal F9 core, or the two side cores, so using the ASDS for recovery will help payload as well.

Their plans for reuse do involve a 15-30% payload hit but as part of the move from F9 to F9 v1.1 was a big boost in available payload. Thus they will have some flexibility in the future. If your mission requires 100% of the payload, they could launch expendable and charge you more. If it fits in the reuable mode payload range, charge much less.

If they can get Falcon Heavy reusable, then instead of expending a F9 it might be more effective to launch bigger things on the F-Heavy.

The legs on a water landing mission seem sort of silly, but are really needed for several reasons.

1) Test out the leg mechanism on a full up flight, end to end. 2) They need the angular momentum control to handle stage spin that they encountered on flight 6, with the Cassioppe mission.
3) Land on the ASDS and prove their ability to hit a small target to the FAA.

In Sept 2013, they tested the recovery model by firing three engines after MECO to slow down and aim vaguely back to base. Then once it reentered the atmosphere, restarted the center engine (third firing of the mission for that engine!) above the water for the final deceleration, but the vehicle had been spinning and the reaction control system was overwhelmed and could not control it so the fuel centrifuged away from the intakes and the engine flamed out. By extending the legs, like a figure skater, they expect to better be able to control the spin with the existing RCS system and land it on the water.

And if it fails on the Mar 2014 CRS-3 mission, they will try again on the next mission until they succeed. (See above, what no one else seems to be doing, serious experimentation!)

As Jerard mentioned, the goal is to land the first stage, and eventually the second stage, to improve re-useability. The legs provide an important piece to the puzzle to making this happen. And yes, it does reduce the lifting capacity, but there are many cases where the rocket is not completely used. It seems evident that SpaceX was avoiding doing re-usability testing for satellites which used a significant portion of the carrying capacity (Geo birds), while they seem to do more testing with smaller satellites, which are often LEO (CRS, Cassiope). I would expect this trend will continue, seeing more re-usability test on missions with surplus, which I would expect to be the next Orbcomm and CRS missions.

It should be noted that they will not attempt to land on land, but rather have a controlled landing on water. They would like to land on land, but they haven't been given the appropriate clearance to attempt the landing, and won't be until the system can be proven reliable in a low risk situation. Still, if successful, landing on water will provide valuable information on the post thrust lifetime of the rockets, as well as performance in air. Maybe SpaceX will even just set up some sort of a barge to land on to keep the rockets from landing in populated areas.

• Yes, but first they have to prove it can fly safely with the legs, which they haven't done yet. So while this one will end up in the water, its just incremental steps until they can prove it is stable. – Brian Feb 28 '14 at 14:07
• Sure. I expect it will take several test flights with legs landing on water before trying on land. The legs now are just a test – PearsonArtPhoto Feb 28 '14 at 14:14
• Yep, I was commenting because it wasn't clear to me first either when I read the article about them adding legs. I thought they were going to finally land it until I determined that it is just a sort of stability test. – Brian Feb 28 '14 at 14:18

SpaceX is intending to make the Falcon series a reusable rocket system.

The landing legs are to allow the 1st stage to perform a vertical landing at or near the launch site, for refurbishment and reuse.

The financials are based upon the theory that the fuel is not the most expensive component (Elon Musk, in a television interview).

According to SpaceX’s Elon Musk, \$200,000 in fuel and oxygen make up just 0.35% of his firm’s launch costs. The rest is in their \$56.5 million Falcon 9 rocket. Musk likens this to throwing away a \$300 million 747 after a one way flight. If you give this a minute of thought, you’ll realize that Musk’s completely right and more effort into reusability should have been given a long time ago. ZMEScience.com Article dated 20 Aug 2013. M\$0.2 of M\\$56 is just a hair under 0.36% If operation and refurbishment costs are in the 5% range, that means launching 10 missions on one rocket body is 15.3% of the cost of 10 rockets... 1/6th the cost. At 20 launches, it would be 1/10 the cost.

Since the lost payload capacity isn't needed this flight, it's a suitable flight for the experiment. And if it can soft land, it will benefit SpaceX immediately.

• Just a quick note to reaffirm that 20 flights per unit seems to be the target, here's a video from a PBSO NewsHour report on SpaceX where Gwynne Shotwell, president and COO of SpaceX, mentions they've tested engines to 20 flights. – TildalWave Feb 28 '14 at 18:23