[jessriedel]

For those of you who haven't seen it, here is SpaceX's fantastic animation of their ultimate goal for Falcon 9 (complete with soundtrack by Muse):

http://www.youtube.com/watch?v=sWFFiubtC3c

This will give you an idea of how grasshopper fits into the full flight plan.

[marcamillion]

I am not that much of a space buff....but for some reason...when I see this video and the one posted by the OP - I get SOOO excited.

Perhaps it is the innovation of never seeing a rocket do a full-blown take-off and then land vertically and then seeing what the long-term result will be - which is orders of magnitude better and more efficient than how it is today....that just gets me excited.

But I am truly glad to be alive today and glad Elon Musk is who he is....and doing what he is doing.

In 40 years, if he continues on his current trajectory, my kids will find it weird that space travel was "novel" in my generation.

That's VERY exciting.

[navneetpandey]

thats what got me excited, when I watched Elon Musk talking about this project on TEd, since then I am following this project of SpaceX.

[Florin_Andrei]

Absolutely. This is literally "rocket science" - the cutting edge of space transportation research.

[deepblueocean]

I find this confusing. It's awesome, that's for sure. But it seems like lifting all the fuel needed to do a soft landing is inherently a tradeoff for less payload capacity. What is the benefit, then? Is it just so much cheaper to be able to re-use the first two stages?

[InclinedPlane]

Almost the entirety of the cost of an orbital launch is in manufacturing and operational costs. Cost of fuel is basically just noise. If you can reduce operational turnaround time and operational complexity, even at a cost of payload, then you'll save so much money it'll be worth it in the long term. SpaceX is aiming for a re-assemble, gas up and go workflow. If they can pull it off it might reduce their per flight launch costs by a factor of 10 in the short term and perhaps as much as 100 if they get really good at it (although that's likely several generations of hardware down the line, at best). With that sort of thing on offer a reduction in payload is easily justifiable.

Imagine if somebody replaced your car with a version that had twice the carrying capacity, but would only run for one tank of gas. It wouldn't be a worthwhile trade would it?

[mikeash]

To put it in perspective, the fuel used to launch a Space Shuttle cost in the neighborhood of $1 million, while the cost of the entire launch was somewhere around $500 million to $1 billion.

[wiredfool]

Another perspective is that the Falcon9 carries about as much kerosene as a 747.

[747facts]

I decided to fact check you. 57,285 U.S. gallons in a fully loaded 747. Jet-A is 6.84 lb/US gal. That's about 196 tons. Looks like stage 1 of the Falcon 9 is estimated to use 239 tons of RP-1 and stage 2 49 tons.

http://www.boeing.com/boeing/commercial/747family/pf/pf_400_...

http://www.spacelaunchreport.com/falcon9.html

[simonh]

Cool, so they're well within an order of magnitude of each other.

[mikeash]

That's a good one, although a 747 gets its oxygen for free.

[schiffern]

LOX is literally cheaper than dirt (8¢/lb, or 67¢/gallon).

[DavidSJ]

Yes, but oxygen's pretty easy to come by it turns out.

[InclinedPlane]

For the Falcon 9, it costs about $200k for propellant per launch (Kerosene is cheaper than LH2 and the F9 puts about 1/10th as much mass in orbit) while the price of the launch is about $50 million.

[slacka]

Could and real or kerbal rocket scientists here explain why SpaceX isn't doing something like the curiosity rover landing? Sure there's 3 extra steps, deploy parachute, cut parachute, and evasive maneuver, but the fuel saving would massive.

[JshWright]

I'm not sure the savings would be as massive as you think (and would be offset by other costs).

Parachutes (and associated equipment) are heavy, so you'll burn fuel lifting that extra weight.

Parachutes are complicated, and would be an extra system to develop, test, and validate.

Parachutes are annoying to repack/replace (increasing turnaround time).

Parachutes put odd stresses on large objects when they deploy (increasing the amount of inspection you would have to do after each flight).

All that hassle to reduce the terminal velocity by a couple hundred miles an hour. That's not that big a win for a pretty high cost.

[Florin_Andrei]

> All that hassle to reduce the terminal velocity by a couple hundred miles an hour.

That's the TLDR. The delta-vee from the parachute is not worth the trouble.

[schiffern]

Not "massive". The empty stage is very light, so a little propellant goes a long way. It's less than 10% of the fuel for the entire descent phase (and the optimal upper stage length goes up to compensate somewhat).

[smackfu]

Well, one reason is that parachutes at this scale aren't that simple. For instance, check out the ones on the Shuttle SRBs: http://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Boos...

[samstave]

Also, the first stage put up the second and third stage units.

What if you have a payload container that was the size and weight limits of the second and third stage, which was put up into LEO by the first stage. The payload would then be picked up by vehicles already in orbit, and the first stage unit returns to earth.

Assuming that the payloads are simply building materials, to be assembled by bots in orbit....

[rbanffy]

The first sage alone won't take your payload to LEO - you'd still need a second stage. However, orbital tugs with electric engines could take your payload from LEO to other orbits with less reaction mass than a chemical rocket could.

You'd still have to refuel their reaction mass, unless we are talking solar or magnetic sails.

[6ren]

It's also extra weight to lift.

[EthanHeilman]

Fuel is very inexpensive (~2% the cost of a rocket launch), rockets are very expensive. If you can trade off fuel for not having to build a new rocket you drop the cost to orbit by orders of magnitude.

Additionally you can build more expensive rockets that are more efficient since rocket creation becomes a capital rather than a reoccurring cost.

[petenixey]

But why not just use a parachute and airbags? They're going to be a vast amount lighter than the fuel you'd need for a rocket landing.

[InclinedPlane]

Parachutes are a pain in the ass to replace. But more than that taking a dunk in the ocean isn't so good for rockets, and spending the better part of a day with a search and recovery team and all the relevant equipment necessary out looking for the rocket and hauling it back home isn't cheap, it also adds a lot of delay to the whole workflow. Compare that to a propulsive landing on the launch pad next to the assembly facilities. You don't have to have a whole special team of folks on hand. The first stage is back in your hands in a matter of hours, where it can be transported back to the assembly building using a simple crane or other specialized equipment. And it can be inserted into the processing workflow much faster than if it had been out in the ocean, and with less overhead of having to clean it, refurbish it, inspect it, etc.

[anonymfus]

But why everybody compares landing solely on parachutes in ocean with landing solely on engines on land? For example, Soyuz use multiple parachutes for slowing down and engines for guiding and final soft landing on land.

[InclinedPlane]

The Soyuz does not "soft land" anywhere. It basically crash lands at a speed which doesn't cause any injury. For a first stage rocket the advantages of coming down on land via just parachutes are pretty much non-existent to negative.

First off, if the goal is to save fuel from having to do a propulsive return to the launch site then that's not going to happen (except for the 2nd stage and capsule). The US has a lot of sparsely inhabited land but it doesn't have the same huge swathes of uncared for steppe that Russia/Kazakhstan have where they can just dump spent rocket stages everywhere with nary a care. There are range safety issues there that can't easily be avoided. Second, a giant rocket stage coming down on just parachutes is going to be damaged more on land than at sea. If you're trying to avoid the weight of landing gear you're just going to end up with the rocket engines crunching into the ground, which isn't going to be good at any speed. OK, so you can't save RTLS fuel, and you can't avoid having landing gear, at that point the only difference is a tiny little dribble of fuel to bring the stage in for a controlled powered landing. So you might as well just do that and be done with it.

[JshWright]

The Soyouz lands 'somewhere' in a fairly large target area. The recovery overhead is still non-trivial.

Something as big and 'light' as the F9 first stage will be slowed down tremendously by the atmosphere. No need to add all the extra complexity and weight of parachutes.

[EthanHeilman]

You can't direct where the rocket ends up, highly likely that the rocket will get damaged or much less likely that the rocket will damage something else (This is why NASA dumps it's rockets over the ocean). Then you have find a way to get the rocket home, which involves time and money. Why not just burn fuel (rocket fuel is only a few times more expensive that burning water) and bring the rocket home?

If you need more payload build a bigger rocket.

You could also add wings to the rocket and fly it home which has it's own set of trade-offs and benefits(see space shuttle).

[TeMPOraL]

> highly likely that the rocket will get damaged

And because you don't know whether it is damaged or not (sometimes the damage may not be obvious), it's likely that the rocket will have to get a long post-flight inspection to check if it is suitable for another flight. It's something you can almost completely avoid if you land the rocket gently.

I read somewhere, that the costs of recovering SRBs of Space Shuttle from the ocean and then inspecting and fixing them were many times greater than building another pair of boosters.

[InclinedPlane]

The SRB thing isn't true. The costs ended up being pretty much the same for new vs. refurbed. The thing is, with solid boosters the "rocket engine" tends to just be giant aluminum cylinders, almost all the complexity of the job is in casting the fuel and putting the segments together, which is completely orthogonal to the reusability aspects.

Also, part of the allure of SRBs is that they are cheap to manufacture, comparatively (this is a false savings, due to increased operational complexity, but it's still very tempting), so even if a significant amount of money could be saved per SRB through reuse it wouldn't have affected the cost a launch much.

[danielweber]

Maybe you are thinking of the costs of "re-using" the Space Shuttle. $250 Million to refurb it each time. But we do it for cost savings!

[rst]

Parachutes were the first thing they tried, on the first stages of the first Falcon 9 launches. It didn't work; the stages broke up on re-entry before they slowed down enough to deploy the parachutes. The response was to try active control to keep them intact --- and I guess they figure they might as well keep it all the way down.

[deletes]

Parachute and/or airbags are not necessarily lighter than fuel on a rocket as big as falcon. Parachutes don't scale linearly with the weight of the cargo. Plus they introduce an additional complexity and a point of failure.

[simonh]

On large heavy objects they also impose significant stress on it's structure.

[artursapek]

I don't think a parachute would give them the amount of control they want over where it lands. Maybe the idea is to eventually have many of these things docking in the same area, like an airport. If you have a fleet of multi-million dollar rockets, you want to be in as much control of them as possible.

[cryptoz]

You cannot land a large payload on Mars with a parachute due to the very thin atmosphere.

[EthanHeilman]

When I first read this I thought you were missing the point, then I remembered Musk's ambition to live on mars and the planned landing sequence for the Red Dragon. Good point!

[danielweber]

Parachutes are really hard. Ask Carmack at Armadillo, but make sure he's in a good mood first. It's not because he's an idiot or anything.

Plus, parachutes create a new failure mode: parachute deploying when it shouldn't.

[schiffern]

…or not deploying when it should.

[VLM]

Think like systems administrator. They've gotta have a rocket engine and a nav system or its not much of a 1st stage. However adding parachutes, airbags, etc, is new systems. Which means much lower reliability and higher expense.

[HodCarrier]

Elon wants rapid (think aircraft), turn around times. That's what's driving the decisions.

[jordan0day]

I imagine there's a lot less fuel required to land than is required to take off. That is, on launch, the rocket is probably (eventually) reaching speeds of several thousand miles per hour.

Upon landing, the atmosphere does most of the slowing-down for you, so you only need enough fuel to reduce the speed from terminal velocity (not sure what this would be for a rocket, probably several hundred miles an hour, at least) to 0. So, I'm sure it's not an insubstantial amount of fuel, but maybe less than you'd think?

[JshWright]

Not to mention that the giant flying fuel tank (er... 'rocket') is _substantially_ lighter now that you've burned most of the fuel off.

[InclinedPlane]

And separated from the 2nd stage and the payload, of course.

[Laremere]

Two other points to consider:

Atmosphere is slowing you down as you go up as well, so lift off requires fighting that as well. Therefore the energy you've expended is not entirely stored in potential energy. (Fun fact: If it weren't for the atmosphere, rockets would actually take off almost horizontally. They go up at first to get out of the thickest air before going sideways.)

The craft is much lighter, because it no longer has the payload, and also has used almost all of its fuel.

[Florin_Andrei]

> I imagine there's a lot less fuel required to land than is required to take off.

Right.

At take off, the delta vee is the escape velocity PLUS all the losses due to friction, which are tremendous.

At landing, the delta vee is only equal to terminal velocity. EDIT: Okay, plus some flying time on top of the landing point.

[Someone]

Another way to state this: at launch, air resistance is your enemy. When landing, it's your friend.

[cbhl]

"With an advanced rocket you can do maybe two to three percent of your lift-off mass to orbit, typically. And then reusability subtracts two to three percent. So then you've got nothing toward or negative and that's also not helpful. So, the trick is to try to shift that from two to three percent in the expendable configuration, to make the rocket mass efficiency, engines efficiency, and so forth so much better so that it moves to around three and half to four percent in the expendable configuration and then try to get clever about the reusablility elements and try to drop that to around the the one and a half to two percent level so that you have a net payload of about two percent."

http://youtu.be/vDwzmJpI4io?t=27m

(Watch from 27m for about 50 seconds.)

[stuff4ben]

I thought the same thing. One of the Youtube commentswas pretty insightful in that it mentioned that when you do a parachute landing into the ocean, it's landing in salt water. Therefore the craft has to be taken apart inspected, cleaned, and put back together. So I'm assuming they've done the cost benefits and figured it's cheaper this way.

I wonder if a combination parachute/thruster landing would be more feasible though? Sort of like the Mars Science Lab without the sky-crane.

[InclinedPlane]

Perhaps on Mars, unlikely on Earth. With our dense atmosphere a vehicle like a mostly empty Falcon 9 first stage is going to have a fairly low terminal velocity, in the low hundreds of km/s range. Slowing down from that speed to a controlled hover/landing is pretty easy. The cost/benefit on Mars might be different though, since the atmosphere is thinner.

[jlgreco]

> in the low hundreds of km/s range

I think you might mean m/s? low hundreds of km/s is very supersonic. ;)

[InclinedPlane]

What am I, a physicist? Jeez, boffing a few orders of magnitude like that is pretty bad, and it's too late to edit.

I meant kph, which was HOPEFULLY obvious from context.

I'll just leave this here: http://www.smbc-comics.com/?id=2679

[ryanbrush]

Not to mention the operational costs of a small navy to recover the rocket and transport it to a launch site.

[mikeash]

I believe the "small navy" was a political and technological requirement and not something you'd need today. A single ship ought to do now.

Political, because it was the Space Race, and a lot of it was about shows of force. And hey, why not? You have a bunch of carrier battle groups just waiting around for a hot war, and you need to have them out training anyway, so why not use them to pick up spacecraft from time to time?

Technological, because guidance wasn't necessarily very accurate. It's interesting to look at the miss distances here:

http://en.wikipedia.org/wiki/Splashdown_(spacecraft_landing)...

Some of these landed hundreds of miles from their target. However, by the time Apollo came around, they were all very close. You definitely want a big recovery fleet to cover a lot of area when you can't be sure it'll land on target, but that's not so much of an issue these days.

[emp_zealoth]

I remember reading about the issue in a book called "This New Ocean" apparently by the time Apollo was on the go NASA had to ask the military to position the ships off to the side of the splashdown zone - the guidance improved so dramatically they were afraid of hitting the carrier directly

[Lambdanaut]

Basically, yes. Fuel costs make up a marginal part of the rocket's cost. One reason the Space Shuttle was such a massive failure was because it was supposed to be "re-usable", yet it really wasn't. It threw away most of its volume every time it launched, yet it still had to carry more fuel into space for the soft landing. It had downsides from both sides of the equation.

[bdonlan]

According to http://www.popularmechanics.com/science/space/rockets/elon-m... :

> "The payload penalty for full and fast reusability versus an expendable version is roughly 40 percent," Musk says. "[But] propellant cost is less than 0.4 percent of the total flight cost. Even taking into account the payload reduction for reusability, the improvement is therefore theoretically over a hundred times."

[cdash]

There is an estimated 40% reduction in payload to become reusable but since the fuel is only less than 1% of the cost in launching the rocket it is pretty acceptable.

[Florin_Andrei]

As Elon Musk put it, it's the difference between filling up your car at the end of a trip, and then going on another trip immediately, versus having to build a whole new car almost from scratch at the end of each trip.

[mtgx]

Fuel cost is only 2-3% of the mission's cost, according to Elon Musk.

[JshWright]

Weird bit of trivia... The pad the the rockets are landing on is a helipad (green perimeter lights). I suppose that makes sense for the video, since there isn't, AFAIK, an FAA spec for rocketpad lighting...

[varjag]

Takes off and lands vertically, same thing really :)

[adlpz]

Well there is the whole fire thing going on too :P

[lutze]

I don't understand the need for the intermediary stages, couldn't the main rocket just deliver the payload and then land?

As it is in that video, it's having to carry engines and fuel for 3 separate stages, for no apparent reason.

[InclinedPlane]

It would be more convenient and cost effective to do so, if we could. But we lack the technology.

The rocket equation is a harsh mistress, it demands exponential amounts of fuel the faster you want to accelerate a given stage (specifically, as a ratio to the exhaust velocity of the rocket). Given that orbital velocity is quite high (about 8.5 km/s) relative to the exhaust velocity of the best chemical propellants (about 3 km/s for LOX/Kerosene) this results in impractical mass fractions to contend with (17:1 to get to orbit, and that's with no payload). But you can cheat. If you use staging and drop away the dead weight of empty fuel tanks and no longer needed engines from lower stages then you can make an end run around the rocket equation. You make a rocket that can accelerate a payload up to a certain velocity, then you make an even bigger rocket which can deliver the entire other rocket as a payload to a different velocity, and so on, until the sum total of all the velocities is the necessary total speed you require to get to orbit.

We're actually fairly close to being able to make single-stage-to-orbit (or SSTO) launchers workable, but it's a difficult problem. We can just about make a single stage with a high enough mass fraction to do it, but then there is almost no payload remaining. And the only way to make a vehicle with such a tiny payload cost effective would be to make it reusable, but enabling reusability would add additional weight which would destroy any payload whatsoever and probably prevent it from even reaching orbit, catch-22. Potentially we could use advanced engines, rocket fuels, and lightweight materials (like carbon fiber) to build a reusable SSTO which would have a reasonable payload, but such designs are hugely untested and very risky. So for now the best hope for reusability seems to be to incrementally advance the design of existing multi-stage rockets.

[elteto]

> The rocket equation is a harsh mistress

Love this, totally making it mine ;) Also, there is the problem of efficiency loss due to over/underexpansion as you switch from atmospheric to vacuum conditions, and I have never heard of a rocket engine with variable nozzle geometry to compensate for it.

[schiffern]

That was the promise of aerospike engines, but they have their own problems.

https://en.wikipedia.org/wiki/Aerospike_engine

[gus_massa]

I'd really love to see a spreadsheet with all this calculations. (Or better, an online html5 virtual rocket calculator tm.) With some default data (usual payload weight, fuel price, orbit speed, ...) and you can choose how many stages and haw much fuel each stage has and the spreadsheet calculate the total price of the mission.

You have a lot of information, but I understand that putting all of this in a xls would be a lot of work.

[numbsafari]

What you need is a transporter that can teleport you from a space-bound craft to a surface location and back without all of this rocket launching business to deal with.

[TheSOB88]

That would be quite convenient... A little too convenient?

[Florin_Andrei]

> Potentially we could use advanced engines

Nuclear rockets would be so awesome if they weren't so nasty.

[rst]

The video actually shows two stages and the Dragon capsule, which is payload. As to the use of staging, there are several reasons:

1) During the late stages of the boost, the one second stage engine is pushing only its single engine, fuel and tankage. You're no longer dragging around the nine engines of the first stage and their tanks. That weight saving means you get a lot more delta-V for each unit of expended fuel.

2) The engines themselves are also different. Rocket nozzles designed for optimal performance at sea level aren't optimal for high-altitude or vacuum conditions; those optimal for vacuum won't work at sea level (their exit pressure is so low that the exhaust has trouble pushing air out of the way). And compromise designs aren't optimal in either environment.

3) In the proposed SpaceX reuse architecture, they don't need to protect that long, thin first-stage tank from re-entry at orbital velocity. It's not clear how they could.

[lutze]

Good answer, thanks.

I've been playing too much Kerbal Space Program, I assumed the first stage had already reached orbital velocity.

[mikeash]

KSP is great fun and extremely educational, but it makes things way easier than they are in real life, presumably for the sake of fun.

One way this is done is by making Kerbin a lot smaller than Earth. Specifically, over ten times smaller. This reduces the speed needed for low Kerbin orbit to around 2000m/s, while low Earth orbit needs around 8000m/s.

A factor of four in speed probably already sounds bad, but it's much worse than one might naively expect. The amount of fuel needed to accelerate a given payload to a particular speed grows exponentially with the target speed. A typical rocket engine exhaust velocity, both in KSP and real life, might be 4000m/s. When the target velocity is half the exhaust velocity, as is the roughly the case with Kerbin, then you need about 40% of your rocket's mass to be fuel. In other words, you can orbit about 60% of your total mass. When the target velocity is double the exhaust velocity, as is roughly the case with Earth, you need 86% of the initial mass to be fuel, so you can only orbit about 14% of the rocket.

In other words, if you're putting 1000kg into orbit, then in KSP you need about 700kg of fuel, while on Earth you need over 6000kg of fuel. For a single stage rocket, that 1000kg that you're putting into orbit includes fuel tanks and engines. You need much bigger engines and fuel tanks to lift 7000kg than you do to lift 1700kg, so a lot of that 1000kg you get into orbit is going to be empty fuel tanks and spent engines, not actually useful stuff.

Staging lets you work around this problem by letting you reduce the amount of useless junk you put into orbit. By dropping large amounts of empty fuel tank and spent engine early on, you no longer have to lift it all the way up, and you have more orbited mass left over for actually useful widgets.

A single stage that goes from ground to orbit isn't too hard in KSP. On Earth, it's right at the limits of technology. Nobody has operated a rocket as a single stage to orbit, but a couple of pieces of larger rockets are theoretically capable of reaching orbit from the ground on their own if flown by themselves. It's possible, but the amount of useful payload that such a thing can put into orbit is so small that it's not cost effective.

[gamegoblin]

"A multistage (or multi-stage) rocket is a rocket that uses two or more stages, each of which contains its own engines and propellant. A tandem or serial stage is mounted on top of another stage; a parallel stage is attached alongside another stage. The result is effectively two or more rockets stacked on top of or attached next to each other. Taken together these are sometimes called a launch vehicle. Two stage rockets are quite common, but rockets with as many as five separate stages have been successfully launched. By jettisoning stages when they run out of propellant, the mass of the remaining rocket is decreased. This staging allows the thrust of the remaining stages to more easily accelerate the rocket to its final speed and height."

https://en.wikipedia.org/wiki/Multistage_rocket

[lutze]

Right, excellent.

Except the first stage doesn't run out of propellant does it, because it has to LAND again. Hence my puzzlement.

I know why multi-stage rockets are used... I'm asking why this rocket needs them, since they solve a problem it doesn't have by design.

[jessriedel]

Rockets don't have stages because they run out of propellant, they have stages so that you don't spend 99% percent of your fuel pushing engines and tanks into orbit which you don't need once you get there.

[jlgreco]

> "they have stages so that you don't spend 99% percent of your fuel pushing engines and tanks into orbit"

In fact you can kinda make a rocket that can reach orbit that doesn't have "stages" but rather jettisons engines themselves. Early Atlas rockets did this: they had one set of fuel tanks but two engines. About two minutes into the flight they would jettison one of the engines.

[nawitus]

Are you saying it should be single-stage-to-orbit? That's currently technologically nearly impossible, even if the rocket doesn't need to land back to Earth.

[nanofortnight]

Not as technologically impossible as you might think: http://www.reactionengines.co.uk/space_skylon.html

[tomelders]

I really don't understand why Skylon isn't getting more coverage.

[schiffern]

>they solve a problem it doesn't have by design.

This is where you're incorrect. Not having to accelerate the empty first stage to orbital speed (and back!) is a huge fuel savings.

[elteto]

A single stage rocket is desirable from a technological point of view since it sheds a lot of complexity: you don't have to deal with multiple engines, separation mechanisms, structural considerations, etc. However, multi-stage rockets can deliver more real payload to orbit, since they maximize the amount of fuel that is spent on taking the actual payload up there.

Single-stage rockets need to carry a lot of dead mass (empty tanks and engines) all the time, and that is a lot of wasted fuel. A multi-stage rocket can dispose the big first stage engines once it's cleared out most of the Earth's gravitational pull and use smaller engines to continue.

Another important consideration is that rocket engines don't run optimally during the whole burn. The first stages are optimized for atmospheric conditions, whereas later stages are optimized for vacuum conditions. Therefore having one big engine propel you up all the way incurs in an even grater loss of fuel due to the inefficiency at high altitudes. You could probably have a rocket engine capable of having a variable geometry to compensate for this but AFAIK is almost impossible to do it.

See the following for more details:

[1] http://en.wikipedia.org/wiki/Staging_(rocketry)#Advantages [2] http://en.wikipedia.org/wiki/Rocket_engine_nozzle

[schiffern]

>once it's cleared out most of the Earth's gravitational pull

There's about 90% as much gravity at the altitude of the ISS.

It's the atmosphere (and associated gravity drag), not the 1/r^2 scaling, that does it.

[elteto]

True! Really common misconception and yet I fell for it, thanks.

[dspeyer]

How does the first stage not get damaged by the backblast from the second stage seperating?

[JshWright]

SpaceX uses pneumatic 'pushers' to separate the stage. After a few seconds the two stages are far enough apart that igniting the second stage won't cause any real damage to the first stage.

Check out the video on this page for a good view of what that process looks like: http://www.extremetech.com/extreme/149741-spacex-falcon-9-la...