New Space-X rocket is the most Kerbal rocket I've ever seen in real life

[iueras]

All I could think about watching that video is "I wonder how much ISP does that engine have at sea level?"

275s, 420Kn at sea level. It uses 9 of these Merlin 1D engines. And it only weighs .63 !

EDIT: Oops. Those are the stats for the Merlin 1C. Ralathon's post has the correct stats for the 1D.

Edited August 15, 2013 by iueras

[Ralathon]

All I could think about watching that video is "I wonder how much ISP does that engine have at sea level?"

282s at sea level, 311s in vacuum.

Source

[Mr Shifty]

but why a rocket? seriously, what is this obsession with them?

In a word: staging.

The problem with spaceplanes is very similar to the rocket problem; where do you put the fuel? The fastest space-planes ever, the X-15s, only got to about 1/3 of orbital velocity. They carried about 20,000 pounds of fuel and oxidizer and started their flight 8.5 miles up at an initial speed of 500 mph, carried aloft by a B-52. (Virgin Galactic's SpaceshipOne and Two do the same thing.)

Note that to present, zero spaceplanes have ever used their wings for lift; all have used rocket engines for ascent and only used the wings to glide descend. And even if you made one that could benefit from aerodynamic lift, you still have the problem of getting to orbital speed.

Edited August 15, 2013 by Mr Shifty

[Geschosskopf]

SpaceX is trying to take a tried and true technology, tweak it, and make it better. A much more sensible approach business wise. Even if re-usability does not pan out they still have a damn good rocket that is cheaper than their competitors.

It'll certainly be interesting to watch. I wish them luck although I'm rather dubious about the whole reusability idea. I mean, the commercial rocket business is like the trucking business. You get paid to deliver a load, and the more load you can carry, the more you can get paid per trip. From this you subtract your costs of making the trip.

Reusable rockets carry less payload because they need a recovery system, so earn less money per launch than 1-shots. They require 2 infrastructures, 1 to build new rockets and 1 to refurbish old ones, so that's additional overhead. The refurbishment limits the production of new rockets, which decreases the efficiency of the manufacturing plant, so each rocket costs more to build, even before considering the added complexity of the recovery system. Then there's the sunk cost of the R&D for the fancy recovery system, which has to be recouped over the long term from the customers, meaning in the short term the price per launch has to be that much higher.

Meanwhile, the company next door is just making 1-shot rockets of the same general size. Because they lack a recovery system, they can carry more payload so can bring in more money per launch. They only have a manufacturing plant and it's mass-producing rockets at low unit costs. Because they didn't invest in a fancy recovery system, they don't have to pass that or the higher cost of lower-volume production on to the customer, so their price for lifting the same load is less.

That's the starting situation that reusability has to deal with. It therefore has to be SIGNIFICANTLY cheaper than building a new rocket to have much hope of competing with a non-reusable program. So, I'm skeptical. But like I said, I wish them luck with it.

[Mr Shifty]

That's the starting situation that reusability has to deal with. It therefore has to be SIGNIFICANTLY cheaper than building a new rocket to have much hope of competing with a non-reusable program. So, I'm skeptical. But like I said, I wish them luck with it.

Falcon 9's cost about $60 million, with about 75% of that for the first stage alone. Assume it takes $10 million to get a re-usable F9 first stage ready for launch and you've still got a $35 million per launch advantage over the guy building a new one every time. And economies of scale for rockets ain't gonna gain you that much, where total quantities per year are probably less than 10.

And that brings up another reason for re-usability. Musk boasts an eventual turnaround time from launch to launch of the re-usable first stages of 10 days. 30 launches per year would be incredible and if you're saving $30 million per launch by re-using them, the savings start to add up pretty quickly.

[koshelenkovv]

But even assuming SpaceX has this problem sorted (which remains to be seen), I still wonder why they don't use parachutes. Surely parachutes would be lighter than the fuel needed for a powered landing, so they could lift more payload.

Rocket stage is a very light, delicate and flimsy construction (and engines too).

They just can not survive impact with ground at 10-15 m/s, typical for parachuting.

So it will need airbags or even larger parachutes, which adds an additional weight.

Rocket stages have some extra fuel as security margin anyway. It can be used for landing.

Though, I have no idea how they going to return stage to launch site. Somewhere downrange when launching from VAFB is possible. But on the same launch site..

[koshelenkovv]

Some quick and dirty calculations:

Military D-6 parachute weights 12 kg and allows 120 kg paratrooper to land at 5 m/s.

It constitutes 10% of weight.

Lets' assume that empty F-9v 1.1 first stage weights about X tons. So we need 0.1*X tons of parachutes.

Merlin 1D engine have Isp about 282 at sea level. Lets's estimate how much dV F-9 can kill with 0.1*X tons of fuel:

9.8*282*ln(1.1) = 260 m/s

Much more than enough to kill terminal velocity and land softly. (if not do 'return to launchpad' trick)

[sojourner]

Reusable rockets carry less payload because they need a recovery system, so earn less money per launch than 1-shots. They require 2 infrastructures, 1 to build new rockets and 1 to refurbish old ones, so that's additional overhead. The refurbishment limits the production of new rockets, which decreases the efficiency of the manufacturing plant, so each rocket costs more to build, even before considering the added complexity of the recovery system. Then there's the sunk cost of the R&D for the fancy recovery system, which has to be recouped over the long term from the customers, meaning in the short term the price per launch has to be that much higher.

Elon is trying to eliminate most of this refurbishment cost. He want's it to be basically "gas and go". Something the shuttle never was. As for costs of "fancy R&D recovery system", what are they? Engines? already paid for. Tanks? paid. Legs? yeah, not an exotic new technology there. Software? while it can take a while, it's not a big cost monetarily. Grasshopper 1 and 2 are both built with spare parts that would basically be collecting dust right now. Essentially researching first stage recovery is costing Elon fuel and man hours from his employees. He doesn't have much to lose by trying.

Grasshopper 1 is mostly about testing software if that tells you anything. The hardware was never in question. Grasshopper 2 will actually include flights that will see if the hardware is up to questionable parts of the flight regime.

Landing the stage is actually the easy part. The hard part is getting it to survive return to subsonic speeds and then get it back to the landing/launch site. They may achieve both, but it could end up impacting payload capacity too much, or stress the vehicle to the point that re-use will not be practical. Regardless, it will be quite an accomplishment.

Now, the interesting work will be stage 2 reuse and recovery as it attains orbit. A much more difficult situation since every pound you add to the second stage takes a pound from the payload while on the first stage every 10 pounds you add removes a pound from the payload.

[Mr Shifty]

Though, I have no idea how they going to return stage to launch site. Somewhere downrange when launching from VAFB is possible. But on the same launch site..

Musk says it'll be back to the same launchpad. If you

, you can see that it gets to something like 100km downrange, 100km of altitude, and 2 km/s at MECO and 1st stage separation. If its velocity is 45 degrees from vertical at that point, it'll rise for another 2 minutes or so, with a max altitude of around 200km, about 300 km downrange from the launch site. Total distance downrange for a ballistic landing would be less than 600 km. Not sure how you get back to the launch pad while in flight.

[sojourner]

Falcon 9 1.0 did stage separation at mach 10. 1.1 is expected to do stage sep at mach 6, so the first stage will be much closer to the pad on the newer rocket.

[Mr Shifty]

Falcon 9 1.0 did stage separation at mach 10. 1.1 is expected to do stage sep at mach 6, so the first stage will be much closer to the pad on the newer rocket.

Thanks for that info -- I was having trouble finding information about launch trajectories and telemetry. Where'd you find it?

[HeadHunter67]

Some quick and dirty calculations:

Military D-6 parachute weights 12 kg and allows 120 kg paratrooper to land at 5 m/s. It constitutes 10% of weight.

Lets' assume that empty F-9v 1.1 first stage weights about X tons. So we need 0.1*X tons of parachutes.

I don't think that's quite how it works. If you double the weight, you should need four times the surface area for a chute to brake as effectively.

So yes, I think you're right that an equivalent amount of fuel would be better for braking.

[LameLefty]

If you guys want to get the best publicly-available SpaceX information and analysis (including some from some VERY well-qualified spaceflight industry people and scientists), there's a wealth of it here (most of which is a lot more well-sourced and analyzed than you'll find here, no slam intended).

http://forum.nasaspaceflight.com/index.php?board=45.0

[koshelenkovv]

I don't think that's quite how it works. If you double the weight, you should need four times the surface area for a chute to brake as effectively.

Imagine two paratroopers, descending near each other.

If they now hold hands, they weight doubles and total parachutes area doubles (not quadruples!).

Does it mean that they suddenly start do fall faster?

[HeadHunter67]

Probably, because their chutes aren't going to work properly that close together.

I see your point, but I think you have oversimplified it. I'd have to see some sources that support a linear parachute size like that.

[sojourner]

If you guys want to get the best publicly-available SpaceX information and analysis (including some from some VERY well-qualified spaceflight industry people and scientists), there's a wealth of it here (most of which is a lot more well-sourced and analyzed than you'll find here, no slam intended).

http://forum.nasaspaceflight.com/index.php?board=45.0

Yep, I have the same userID over there, though I mostly lurk. Those guys are the real deal rocket engineers.

It's also a very strict forum as far as staying on topic and not spamming up the threads. So tread lightly if you join.

[LameLefty]

Yep, I have the same userID over there, though I mostly lurk. Those guys are the real deal rocket engineers.

I post under a different name. And yes, some us are. (Or were, anyway, at one time).

It's also a very strict forum as far as staying on topic and not spamming up the threads. So tread lightly if you join.

Agreed! But for those here who are really serious about learning how stuff works in the real-world (where engines fail, computers fail, customers change requirements, weather doesn't cooperate and things like falling rocket stages are taken very seriously), it's THE best place on the 'net these days.

[MockKnizzle]

In regards to parachutes: We can agree that aerodynamic drag is proportional to surface area, right? Thus twice the area produces twice the drag force and can slow twice the weight. Koshelenkovv's analogy is accurate.

[HeadHunter67]

Yes, it's proportional, but is it on a linear or geometric scale? As I said, I'd like to see some actual data to support that, because it seems oversimplified.

[MockKnizzle]

Linear.

Technically A is the orthographic projection of the body in question and there are some simplifying assumptions about the shape of the body (rolling cylinders, odd shaped things, etc. can get a little weird)

[andrewas]

Its linear to the area of the 'chute. Double the radius and it takes four times the mass to drag it down at the same rate.

That said, mass of the 'chute is another problem, you need to use stronger fabric and lines on a larger 'chute, so the mass does grow faster than linear.

[HeadHunter67]

Fair enough. But skydivers don't tend to have triple-redundancy on their parachutes - maybe a reserve in case the first fails to deploy, but even the Orion uses a triple-chute system like they did in the Apollo days. It can make it down on two, or even one - but you're tripling that weight for safety.

And I think the heavier your descent payload, the greater your need for that redundancy. Not because it falls faster (it obviously doesn't), but strictly for structural reasons. That's why it's not simply a matter of scaling it relative to a skydiver. [EDIT: Ninja'd by andrewas...]

At any rate, I didn't want to digress, as I did agree with the underlying and more relevant point Koshelennkov was making - that being, such mass is better spent on fuel, which can make a descent as effective and likely more economical in terms of usage of that mass.

[Tygroux]

Yes, it's proportional, but is it on a linear or geometric scale? As I said, I'd like to see some actual data to support that, because it seems oversimplified.

I've thrown math, but i'm horrible at it and it's 2.30AM here.

What i've found, is that if the drag of a chute is constant, when you double mass and parachute, you double the force involved, but on both sides. So they compensate the same way, and you keep the same speed.

What might help: E=mc², or Ep = 1/2*m*v²

V is squared, and v depend of height in a lot of situations. But mass isn't squared.

[Kosmic Debris]

Just a correction - industry sources say cross-feed is not going to happen in the initial versions of F9H. They plan to add it (and they have to, to reach anywhere close to Elon's predicted performance numbers when the F9H was announced) but it won't be part of the first several launches at least.

Not really a correction - I simply noted that it was part of the design. Cross-feed is only required when doing a maxium payload lift...and there are no flights anywhere near that capacity on the launch manifest so far. The first launch will be a test only (although I suspect they'll carry a Bigelow Aerospace station), the second is a relatively light payload.

Should be a wild ride to watch

[zekes]

no Jebediah