SpaceX to begin testing on Reusable Falcon 9 technology this year

[jabe]

Can someone explain why merely nulling the horizontal speed, falling mostly straight down, and decelerating to a gentle splashdown in the vicinity of a prepositioned recovery barge seems to be considered not the best idea?

It requires much less fuel than boost-back.
No range safety concerns with stage returning to populated areas.
Stage definitely can survive it wrt thermal loads.
No pinpoint landing needed: +/- mile is ok.
Heck, even terminal speed requirements are much relaxed compared to land landing.

Is short duration salt water bath THAT bad?

i have thought that also..
my 2 cents..
my WAG is they will do something like that first to test system..but in end refurbishment is easier if kept salt free
they want 1st stage vertical so 2nd stage can land on it to make relaunch easier and quicker
jb

[meekGee]

they want 1st stage vertical so 2nd stage can land on it to make relaunch easier and quicker

I thought of that too, but was afraid of Jim

[modemeagle]

Going back to drag again.

Are your models taking into account the drag from the landing legs?

It strikes me that they could be designed to provide significant drag, but that would be at the front for base first re-entry which I think makes it aerodynamically unstable.

I don't account for the surface area needed for the legs as that should be minimal and won't have any real effect on entry drag.

[cordor]

Running the center engine on gases (once tank pressure reaches a certain pressure threshold) could reduce the fuel usage rate to far below 70%, while providing a plume for reduced g-loads on re-entry as well as some thrust for deceleration.
 

I'm sure they will test the technology. But  I think it will be quite some time before they will have something to bring to market.

After all spacex is not a research lab getting grant from gov, and they have many more things needed to be done with limited resources atm. It makes no sense to try if they don't think there is a huge reward in front of them. We just don't know the detail of their approach.   And don't you ever forget for one second, Elon musk is a business man, not a scientist.

[peter-b]

After all spacex is not a research lab getting grant from gov, and they have many more things needed to be done with limited resources atm. It makes no sense to try if they don't think there is a huge reward in front of them. We just don't know the detail of their approach.   And don't you ever forget for one second, Elon musk is a business man, not a scientist.

Actually, he's both; he majored in Physics, IIRC.

[docmordrid]

After all spacex is not a research lab getting grant from gov, and they have many more things needed to be done with limited resources atm. It makes no sense to try if they don't think there is a huge reward in front of them. We just don't know the detail of their approach.   And don't you ever forget for one second, Elon musk is a business man, not a scientist.

Actually, he's both; he majored in Physics, IIRC.

Degrees in Business and Physics from the U. of Pennsylvania..

[RDoc]

Transferred from the 1D thread.

Actually, if the reentry engine is the center one, it's probably going to be right at the stagnation point, so the pressure will be even higher than the average Q over the entire base.

During a nominal entry for the suborbital trajectory you're going to want the stage at least angle toward the side to increase frontal area.  They can always start the engine while angled and then pitch to the desired angle for slowing down.

I'm not sure what your point is. To slow the stage, the thrust has to be opposite the velocity so the engine would be facing directly into the air stream.

If you can supply altitude, velocity, pitch, and dynamic pressure at ignition then we can go over the numbers.  At no time in my model was thrust directly into the flight path angle since my original trajectory was designed to be short of the pad to account for a failure to restart.

Sure, I'm using this calculator: https://engineering.purdue.edu/AAE450s/trajectories/Point%20Mass%20Lifting%20Earth%20Entry%20Trajectory%20-%20Time%20Integration.xls from Purdue, and starting at 70km, 70 degree angle, 1500 m/s velocity and an end on entry with Cd .8 and Cl 0.

The stagnation pressure exceeds 1Atm at about 26km, 1800m/s, 73 degrees, Q is about half, 58kN/m^2, deceleration .7G. It never gets below this for the rest of the return without braking.

The max stagnation pressure without braking is 680kN/m^2 at about 8.8km, 1250m/s, 74 degrees, Q 360kN/m^2, deceleration 9.3G.

If you think changing the pitch will significantly change these numbers, please provide the pitch angle, altitude, velocity, Q, Cd and Cl you're using.

I'm still wondering if anyone knows what the implications are for starting and operating an engine in a high dynamic pressure environment would be. Are they going to do something like the SSME narrowing the end of the nozzle to avoid problems with over expansion?

http://forum.nasaspaceflight.com/index.php?topic=27748.msg914446#msg914446

583 seconds

pitch angle - 25 DEG
flight path angle - -89.7 deg
altitude - 3.4KM
velocity - 109 M/S
Q = 5,207 kpa
Cd = .75 per your request
Cl = zero, your not going to get lift from a cylinder

As mentioned before, that's a bit cryptic. Are you suggesting that the reentry would be pitched up? That is, assuming engines forward, engines high, stage top low by 25 degrees? If so, the CG would be forward of the CL, generating a torque tending to drop the engines, decreasing the pitch. Countering that torque would require a control force tending to keep the engines raised, either aerodynamic or thrust. What are you assuming would create that force? It wouldn't be small.

[RDoc]

http://forum.nasaspaceflight.com/index.php?topic=27748.msg914446#msg914446[/url]

583 seconds

pitch angle - 25 DEG
flight path angle - -89.7 deg
altitude - 3.4KM
velocity - 109 M/S
Q = 5,207 kpa
Cd = .75 per your request
Cl = zero, your not going to get lift from a cylinder

Zero angle of attack along the long axis.  No more questions, you can extract your answers from the graphs.

I'm not sure how zero angle of attack is consistent with 28 degrees of pitch. Nor how it's consistent with your idea of pitching the stage up during reentry to increase drag.

Keeping Cd approximately constant isn't just because I requested it, it's how aerodynamics works. Previously your model was quite incorrect.

The numbers in the charts are only consistent with an engine first reentry. Using the Purdue speadsheet shows the stage hitting the ground at about 570 m/s with Q at 186kPa and stagnation pressure at 392kPa, so there's still a question of how the engine would run at those pressures.

If you've got your spreadsheet working more correctly with Cd, try a sideways entry and notice that the stage is subjected to over 5kN/m2 at 40km. A thin cylinder would almost certainly crush at that point if not sooner, so the breakup would be at high altitude if the stage came in sideways out of control.

[douglas100]

Can someone explain why merely nulling the horizontal speed, falling mostly straight down, and decelerating to a gentle splashdown in the vicinity of a prepositioned recovery barge seems to be considered not the best idea?

It requires much less fuel than boost-back.
No range safety concerns with stage returning to populated areas.
Stage definitely can survive it wrt thermal loads.
No pinpoint landing needed: +/- mile is ok.
Heck, even terminal speed requirements are much relaxed compared to land landing.

Is short duration salt water bath THAT bad?

Well, maybe. You certainly take a much smaller payload hit by landing downrange, but you've got extra costs for recovery and transport back to the launch site. And I think that even a short dip in the sea will increase refurbishment costs as opposed to fly back.

It's a problem with multiple variables, some engineering, some financial. It's difficult at this stage to get a feel for what is the best way to go.

But they've stated they're going for boosted fly back followed by a powered landing. It will be interesting to see if they can make this approach work.

[cordor]

Can someone explain why merely nulling the horizontal speed, falling mostly straight down, and decelerating to a gentle splashdown in the vicinity of a prepositioned recovery barge seems to be considered not the best idea?

It requires much less fuel than boost-back.
No range safety concerns with stage returning to populated areas.
Stage definitely can survive it wrt thermal loads.
No pinpoint landing needed: +/- mile is ok.
Heck, even terminal speed requirements are much relaxed compared to land landing.

Is short duration salt water bath THAT bad?

Well, maybe. You certainly take a much smaller payload hit by landing downrange, but you've got extra costs for recovery and transport back to the launch site. And I think that even a short dip in the sea will increase refurbishment costs as opposed to fly back.

It's a problem with multiple variables, some engineering, some financial. It's difficult at this stage to get a feel for what is the best way to go.

But they've stated they're going for boosted fly back followed by a powered landing. It will be interesting to see if they can make this approach work.

They never show the recovered stage 1(at least i never saw it).  I guess they lost them.

[douglas100]

That's right. They've yet to recover a stage. Recovery turned out to be quite a bit harder than they originally thought.

[dcporter]

They never show the recovered stage 1(at least i never saw it).  I guess they lost them.

Elon has spoken to this several times; the attempts to recover anything at all from the first stages failed miserably, and he has expressed a great deal of embarrassment about it.

[cordor]

They never show the recovered stage 1(at least i never saw it).  I guess they lost them.

Elon has spoken to this several times; the attempts to recover anything at all from the first stages failed miserably, and he has expressed a great deal of embarrassment about it.

I heard that too, but what does "failed miserably" mean? The only report i read was f9s1's parachute fail to open, and that was only one instance. I doubt that alone is good reason to build a fly back rocket.

[clarkeo]

By "failed miserably" he means the stages broke up on re-entry despite their cork TPS. ie. The parachutes failed to open because the stage was literally broken into pieces by the forces of re-entry not because the parachutes themselves failed to open. I think Elon's words were that the stages basically "belly flopped on the atmosphere" and they came to the realisation that the stage just wasn't strong enough to do this sort of re-entry without serious modifications. Therefore better to uprate the merlins so they can use that extra performance to loft extra propellant to MECO that can be used to boost back/shed velocity.
To be honest Elon looks to have had a re-awakening to his goals and his recent interviews he keeps mentioning that the stage has to be rapidly re-usable(multiple uses per day). Fishing it out of the sea just doesn't mesh with this so there is an additional reason beyond the problems they were having with stage re-entry.
Performance is not everything, eeking a little bit better performance to orbit out of rockets has been the modus operandi of the space industry for decades and it doesn't bring down prices (significantly).Better to sacrifice even 20% of your payload to get re usability which will still bring down cost per kg down by at least a huge factor. I mean it just makes sense cost is the ultimate limiter and will be for ever (not just in the current economical climate) the cold war is a historical blip and you cant ever expect to get funding anywhere near that again.

[Idiomatic]

I never understood the point of quickly reeusable. CHEAPLY reuseable seems more important.

If I had to spend 3 months refurbishing but only spent 5% of the cost of a stage that is a sweet deal!

Need more flights? Then build more rockets! Have 10 rockets in the refurbishing pipe and you can launch every 10 days. It isn't like they'll have only the one rocket. And their factories are scaled to produce plenty anyways.

The time element once you have a fleet of rockets costs you almost NOTHING.