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Comment by HarryHirsch | Hacker News Reader

Where do you find a solid surface to land on except in Russia and China? You would like to fly across water or at least unpopulated areas in case a problem develops during flight and range safety needs to go for self-destruct. The USA, Europe, India and Japan all do that, because no one wants a re-run of the Long March desaster in 1996.

Flying back the opposite direction is no good because you are fighting the rotation of the Earth and are paying with extra fuel to carry the fuel needed to go back, besides coastlines are always the most densely settled areas of any country.

I can't imagine that the economics will work out, but perhaps someone else has genuine numbers to prove me wrong.

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I would suspect SpaceX has the numbers. I don't think they're just guessing that what they want to do will work. Rocket engineers are big on numbers.

They intend to do a return to launch site--at least eventually. There will be several intermediate steps to prove the system. If it proves unworkable, always landing on a barge is better than not, I guess.

RTLS will certainly require a good deal of margin (by rocketry standards), but first stages usually have some margin already, and SpaceX will have a nice range of options once the F9H comes on line so that they don't have to fly a rocket "fully loaded" except for certain extra-large payloads.

I imagine they can also optimize most flight profiles for RTLS, by sending the first stage more than usually straight up--which is close to the case anyway. Lateral velocity is mostly the job of the second stage.

While it's never been done before, it's not an unheard-of concept. One of the potential evolutions for the STS was "fly-back boosters".

Returning to launch site doesn't require that much fuel as the first stage is really light after separation.

Barnaby Wainfan patented an interesting alternative: after staging, re-enter, then do a burn and fly ballistically back to launch site. This kills the horizontal speed with air resistance instead of propulsion, potentially saving fuel. You have to regain the altitude though.

SpaceX owns some land in southern Texas. It seems that the initial idea is to have a spaceport in Brownsville, TX, and use Cape Canaveral, FL, as the landing location. In the mean time, they can practice by using floating platforms in the Atlantic.

http://www.faa.gov/about/office_org/headquarters_offices/ast...

oh I am sure there are a few islands out there both inhabited and not so much that would present some opportunities. Much easier to deal land on something that isn't moving up and down, let alone side to side.

If launched from Texas, would they have to move side to side to land the first stage in Florida?

my apologies for not be clearer, while the movement of the Earth is a concern I was replying to the idea of trying to land on a floating platform which I do not see as feasible. Both from the standpoint of trying to find such a small location to the fact it is not going to be a stable landing area because of wave action

Would an old oil rig work?

It would require significantly more fuel to get the stage to even the Gulf coast of Florida (much less the Atlantic coast) than it would take to return to Brownsville. Additionally, Brownsville launches will almost certainly be aimed between the southern tip of Florida and Cuba, in order to avoid traveling over any populated areas.

SpaceX has said that they want to fly the booster back to the launch pad from the first announcement of their reusability plans. Yes, the rocket launches with 30% extra fuel - Falcon 9 v1.1's first stage was made bigger to accommodate that need. Fuel is a small cost relative to everything else.

Fuel is a small cost relative to everything else.

But the extra weight is a concern, it limits payload to orbit. Try flying the first stage back to the launchpad in Kerbal Space Program while delivering a useful payload to orbit! It's instructive. KSP really develops your intuition for rocketry, everyone commenting in this thread ought to buy a copy.

You did note that the OG2 flight 1 payload was about 1/4 of the max? And the next 2 flights aren't going to try landing because they need all the fuel to get a big payload into GTO[1]? SpaceX seems to have all this under control, even if you can't imagine that it will work out.

[1] http://www.spacex.com/news/2014/07/22/spacex-soft-lands-falc...

You also always have the option of just building a heavier lift rocket - which is what SpaceX is starting to spin up for. I'd imagine the F9H's successor/bigger brother is going to be designed with provisions for the same type of return profile.

According to SpaceX the Falcon Heavy will only fly with reusable first stages. That will bring the payload down but the payload was very high before. The Falcon Heavy should still be able to lift more than 20 tonnes to LEO, which is equivalent to the Delta IV Heavy. And with reusability it should be able to do so at a cost comparable to or cheaper than today's expendable Falcon 9s.

Kerbal space program is a fun game, but you cannot use it for budgeting real launches, especially in this case. It only has minimalist physics, especially with fluid dynamics, which governs much of a first stage recovery.

It also "scales down" the universe and the time/thrust needed to leave orbit. still a fantastic educational resource!

Is definitely good educationally, but I wouldn't use it to argue against the plans of people who build rockets for a living any more than I would use experience of playing Risk to lecture an Admiral on international warfare.

I'd argue that KSP holds as an upper limit for what's possible - if it doesn't work in KSP, it won't work in the real world (while the opposite doesn't hold true necessarily).

That makes no sense. KSP hasn't got proper aeronautics and also hasn't got n-body physics. There are plenty of things that work in the real world that would fail in KSP.

edit - and for the same reason there are also things that would work in KSP that would fail in the real world.

> There are plenty of things that work in the real world that would fail in KSP.

Do you have an example for that?

> edit - and for the same reason there are also things that would work in KSP that would fail in the real world.

That's what I wanted to say with 'the opposite doesn't hold true'.

All of these take advantage of Lagrangian points, which aren't present in KSP: http://en.wikipedia.org/wiki/List_of_objects_at_Lagrangian_p...

Not really applicable to this discussion, but one instance which is highly applicable is that you can't control two ships in KSP at once. If your lower stage needed to do a powered land which an upper stage was still accelerating into orbit, you couldn't replicate that in ksp.

> If your lower stage needed to do a powered land which an upper stage was still accelerating into orbit, you couldn't replicate that in ksp.

.. Unless your name is Scott Manley ;-). But yes, good point about Lagrangian points [1]. I wasn't aware that KSP is only a two-body-simulation - interesting how the game can hide that with its sphere of influence implementation.

[1]http://forum.kerbalspaceprogram.com/threads/18623-Lagrange-p...

Quick save at stage separation, fly the upper stages to orbit. Go back in time (reload the quicksave) and fly the upper stage back to the landing site.

How about very tight hypersonic atmospheric slingshot maneuvers using aerodynamic lift from the shockwave generated by a flexible waverider lifting body? I think it would have significant problems modeling that without some serious modding. http://en.wikipedia.org/wiki/WaveRider#Hypersonic_Sail_Waver...

Also, my point with the logic is that when looking at the differences between physics in KSP and the real world, we are dealing with overlapping, rather than nesting sets.

> http://en.wikipedia.org/wiki/WaveRider#Hypersonic_Sail_Waver...

First time I've seen this, thanks. You certainly have a point that feasible designs in KSP vs. real world aren't completely nested. When I was arguing about KSP being an upper limit, I didn't really think about the SSTO case - at least the stock simulation certainly doesn't hold up to be able do any kind of feasibility check for horizontally launched vehicles.

I wouldn't say that. SSTO's are pretty trivial in KSP (even rocket SSTO's with only classical chemical engines, none of the RAPIER/SABRE stuff).

8 Rockomax 48-7S (little orange engine, stuck on with 8 small cube struts) + 1 Rockomax X200-32 Fuel Tank (the large gray tank, one size below the big orange tank) will put a capsule in orbit around Kerbin pretty easily. No such luck in real life.

Typically KSP engines have worse performance than real engines, but Kerbin is pretty small.

> 8 Rockomax 48-7S (little orange engine, stuck on with 8 small cube struts) + 1 Rockomax X200-32 Fuel Tank (the large gray tank, one size below the big orange tank) will put a capsule in orbit around Kerbin pretty easily. No such luck in real life.

That's what I meant by 'the opposite is not necessarily true'. My argument was that if something does not work in KSP, it probably won't work in real life (which was refuted by another commenter because of missing Lagrange points). IMO it still holds true for basic rocket designs and their capabilities for LEO / GEO. To my knowledge, all real world rocket designs have been replicated in-game (albeit with much lower complexity of course) and demonstrated to work.

With a truly cheap reusable system, it may finally make sense to send more smaller payloads and assemble in orbit.

On the topic of KSP: Some guy on /r/spacex modded KSP to hell for realism then recorded a video demonstration of how the boost-back will/could work: http://www.reddit.com/r/spacex/comments/1z6vyt/boostback_dem...

The first video explains the modifications to the game, mainly more realistic aerodynamics, realistic fuels and engine capabilities, a realistic planet and launch site. The second video is him actually performing the mission.

There are a few differences between what he did and what we now know is actually planned, but it's still pretty damn close and demonstrates the feasibility of the concept in a pretty intuitive fashion.

Sure, you limit the payload to orbit. But on the other hand you end up with an extra orbital rocket stage in your possession. Given that this is a roughly $45+ million commodity at present, that's a significant savings that justifies the payload reduction.

I would imagine if the rocket weren't traveling so fast, they could put wings on it and glide the thing back. That would save a lot of the 30% extra fuel if the weight penalty wasn't that significant. Adding more fuel definitely seems easier.

Long furled blades is another way to go, then you can have a heliroc. They work pretty nice as toys.

Heh, that would certainly put a positive twist on their spinning problem too. ;)

While it works nicely for recovering balsa and cardboard from a thousand feet or so, I have a sneaking suspicion that it might not scale all that well.

That sounds like the Roton by Rotary Rocket:

http://en.m.wikipedia.org/wiki/Rotary_Rocket

The trouble is that wings also cost a lot of drag on ascent...

If they launch from their Vandenberg launch complex they can "easily"[1] land in the high deserts of California or Nevada. Although I believe the plan is to return to the existing pad. And in answer to the expense question, 9 merlin engines are pretty pricey compared to just fuel costs.

[1] scare quotes because I know its never easy to land and California or Nevada east of Vandenberg for minimal delta-v requirements.

Vandenburg AFB is used for placing payloads in polar orbits, which means they launch north/south. However, because of the population centers north of the AFB, they generally launch all vehicles south. This mean launches will happen over the Pacific Ocean, and they would not recover the first stage in the high deserts of California or Nevada.

Unless they did something different from what others normally do.

As washedup accurately points out they only launch south because in the event of a scrub they are over the ocean. And while Spacex could request an eastward flight plan, its unlikely the FAA would grant such a request because it is unlikely that SpaceX could assure them adequately that Bakersfield would not be at risk. For grins and giggles I confirmed this with the Air Force (their response was "Safety concerns restrict the available launches from this facility."

Interestingly SpacePort America (http://spaceportamerica.com/) has a similar problem, although if you look (https://www.google.com/maps/place/Spaceport+America/@33.0125...) you will see there isn't much east of it.

I'm pretty sure they're planning to fly the first stage back to the launch site (Cape Canaveral in most cases for now). It wouldn't return to the same tower, but somewhere at the Cape.

Thing is, for orbital flight, you're adding a lot of sideways translation in order to achieve orbital speed.

In the case of the Saturn V, the first stage (S-IC) sequence completed after 2 minutes 41 seconds, at an altitude of 42 miles (68 km) a speed of 6,164 miles per hour (9920 kph). It continued ascending ballistically to an altitude of 68 miles (109 kilometers), ultimately landing 340 mi (550 km) downrange.

https://en.wikipedia.org/wiki/Saturn_V#S-IC_sequence

Reusability trumps everything.

EVERYTHING.

When you throw away the entire rocket every flight you throw away tens of millions of dollars in hardware. If you can recover and re-fly stages you can get away with a lot of compromises elsewhere. Especially since the first stage, for SpaceX anyway, represents 3/4 of the hardware cost of the launch vehicle.

Even reusing the first stage once means cutting the overall hardware cost of a flight by nearly 1/3. For SpaceX that translates to about $20 million per flight in savings. And that's from one and only one reuse flight of the first stage. In comparison the fuel is nothing. In comparison even a massive payload hit is acceptable. As long as the payload reduction is less than the cost reduction, everything is golden, and the rest is profit margin. For 2 reuses (3 total flights) the hardware cost per launch drops to 1/2 of current costs. For 5 or more uses per first stage the cost drops below 40%.

SpaceX's launches are already cheaper than the competition, dropping below 1/2 of their current cost floor makes it impossible for the competition to keep up and would enable them to own the launch market.

Which would then force anybody else who wants to be in the launch biz to develop their own version of the technology. Sounds like a win for humanity.

Well, cost trumps.

My comment above wasn't shooting down the concept of reuse, but showing that the lateral range of the stage 1 Saturn V booster was pretty significant. To re-land at Kennedy, you'd likely have to:

1. Change the launch trajectory to gain more initial vertical range.

2. (Possibly) reduce the first stage size to decrease its range. This means increasing comparatively the 2nd and 3rd stage sizes.

3. Use of strap-on boosters (themselves independently recoverable) which would reduce the mass of the remaining recoverable stage 1, and hence the momentum that would have to be re-vectored to KSC.

And while reusability is good, it's a bit like Amdhal's Law: your initial gains are the biggest, and likely you're going to see a cost function something like:

    (fixed booster cost + reusability engineering cost) / reuses +
    additional fuel cost + refurbishment costs + launch risk

The first two elements are going to decrease with reuse. The reusibility engineering costs will likely themselves be a function of reuses. And at some point you are below the increased incremental fuel, refurbishment, and launch risk (failure) costs.

Which doesn't say that the exercise is futile. Only that past 3-4 reuses you're gaining little for what's likely a large additional expense, or phrased differently, there's a minimum cost if you want to go to space today.

You could achieve very large savings in many things if you fly a lot and the oparations are routine. Also risk should drop a lot.

It's still not inevitable that such things will happen with reusability automatically. Most likely the first reusables have high maintenance. But they can be improved.

Launch risk does seem to be a function of experience -- the Russians have been flying pretty much the same rocket for nearly 50 years. That provides a lot of opportunity for incremental improvement. Combine this with early Russian practice (based largely on lack of sophisticated testing equipment) of testing rocket designs through live flight.

But risk in general is also a function of use and operational time. This has been borne out in many contexts, including both space flight and aviation. Systems degrade in nondeterministic ways over time, increasing failure risk. Even very minor variations in design -- a few mm of protrusion in a fuel-oil heat exchanger in the Boeing 777, implicated in the British Airways 38 Heathrow crash, given a specific set of circumstances in in-flight ambient temperatures and engine throttle settings resulting in ice-induced fuel starvation -- can have profound impacts.

https://en.wikipedia.org/wiki/British_Airways_Flight_38

It'll be interesting to see how re-use, risk, and cost play out with Space-X.

You just fly back to the launch area. It's immanently feasible to fly that route and always have a buffer of assurance that the rocket's trajectory will not head towards populated areas. If, for whatever reason, that changes then they press a button, the rocket zips apart and falls in pieces into the Atlantic ocean.

As for cost, fuel is cheap, it's one of the cheapest things in the whole operation. Think about the fact that every commercial airliner has the capability to make an unpowered landing and to save fuel doing so, why don't they? Because operational complexity adds cost much more than using up fuel does. By the same token, the greatest costs in orbital launch come from throwing away the hardware after every flight and operational complexity. If using extra fuel enables them to more easily reuse the stages and get more launches out of them as well as reduce overall operational complexity, then it makes economic sense.

If you can deliver a payload and recover and fully reuse the rocket then your costs drop by an order of magnitude. I can't imagine that the economics won't work out.

Depends on cost of refurb

They're shooting for no refurb on per mission basis and potentially to fly it the next day. Apparently the first stage is about 70% of the total $60M mission cost.

I will cry tears of joy when they send that first stage up in less than 24 hrs, return it and launch again.

I'm willing to bet that this will happen before the end of the decade (i'm guessing 2017)

I think they're talking about a barge or floating platform of some sort out in the Atlantic, at least for the initial tests.

Iceland, Bahamas or Canary Islands?

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