Shotwell supposedly said first BFR flight will be an "earth hop"

[RDoc]

"The altitude at which the Concorde can go supersonic was limited by the regulations. For example, the aircraft was only allowed to go at 400 knots during the climb phase between 6,000 and 32,000 ft; also, the aircraft was allowed to go supersonic only after it has crossed the coastline.
<snip>
The problem with the Concorde was it was noisy as all get out at ground level going no faster than a Boeing or Airbus would at takeoff. It wasn't the sonic boom people complained about it was the god awfully loud engines. at barely missing the cars and buildings heights.
<snip>

There wasn't a problem with the Concord sonic boom for the simple reason that there are explicit regulations prohibiting supersonic flight over populated areas pretty much everywhere. That will apply to SpaceX as well (when in the atmosphere), and I cannot see that being changed.

If you think that anything close to 5 miles would be far enough to avoid issues with sonic boom, take a look at this video with the camera jumping 6 miles away from a SpaceX landing, and then think about the relative masses of the BFS and Falcon 9 first stage.

 

[RDoc]

<snip>
Incidentally the one on the ground could have been escaped successfully by the Dragon module save for the fact that the software wasn't set to enable that mode on the fateful day. All launches since have that feature enabled in software and the hardware supported it before that happened.

Actually, it was the other way around. The ground explosion was a satellite, the one that exploded in the air was an ISS resupply with a Dragon.

In either case though, if it were a BFS stage that blew up, either on the ground or in the air, there would have been no escape options.

 

[dhanson865]

Actually, it was the other way around. The ground explosion was a satellite, the one that exploded in the air was an ISS resupply with a Dragon.

In either case though, if it were a BFS stage that blew up, either on the ground or in the air, there would have been no escape options.

You are right that it was an in flight not launch issue.

SpaceX CRS-7 Launch performance was nominal until an overpressure incident in the second-stage LOX tank, leading to vehicle breakup at T+150 seconds. The Dragon capsule survived the explosion but was lost upon splashdown because its software did not contain provisions for parachute deployment on launch vehicle failure.

As to BFS sure if the ship itself blows up that is it. But if the first stage aka BFR blows up there is still the option of BFS being the escape system.

As to the concorde you deflect about sonic booms not acknowledging the flight path issues it had.

I don't trust that shaky cam video, I don't think the camera was firmly attached to anything. As to miles, it's only about 7 miles to the nearest public structure (around Titusville) from SLC39A and about 6 miles from SpaceX landing zone 1 to public structures at Port Canaveral / Cape Canaveral. So whatever the Falcon 9 is doing and Falcon Heavy will do will be at those distances if done in Florida.

BFR will launch from 39A first? (not counting test flights from McGregor)

Now think about Boca Chica! Something from the choices of Falcon 9, Falcon 9 heavy, or BFR will launch from Boca Chica in Texas and there are some houses 1 mile or so from the launch pad (if they lauch from land not a drone ship). I can't imagine how much the house at 17 Esperson St, Brownsville, TX will shake if that camera shook at 6 miles with just a falcon 9 single stage.

I guess we'll know how close is too close once the Boca Chica site launches something.

here is a random redditors quote about putting it at Kwajalein Atoll for some thoughts on current restrictions at Boca Chica.

Vandenberg and Boca Chica have noise and environmental restrictions. It has been suggested that the best answer is to go back to Kwajalein Atoll for testing and initial flights. This actually works pretty well. You have to ship the spaceship and booster by sea anyway. You have to build new facilities anyway. You want to do the tests far from population centers. Only Boca Chica is cheaper, and that is only true if they can do their tests within the 12 flights a year restriction at Boca Chica.

 

[jkn]

I'm certain BFS will have an escape system. BFS cannot be used as an escape system in case of 1:st stage fails at low altitude. Vacuum engines cannot be started at sea level. For vacuum engines sea level ISP is not given. They don't know it, because they cannot even test it. Test burn at sea level must be done with different nozzle or by adding something inside vacuum nozzle to prevent air mixing with exhaust inside of the vacuum nozzle. That would cause turbulence and vibrations which would tear engine and rocket to pieces.

Main engines of the Space Shuttle had special nozzle design to prevent air entering in to it. Because of this it had not best possible vacuum ISP.

BFS accelerates too slowly to be a good escape capsule in case of explosion. It would not remove need for an escape system in case of BFS explodes.

Boca Chica seems to have many disadvantages. Perhaps Kwajalein Atoll would be better.

 

[dhanson865]

I'm certain BFS will have an escape system. BFS cannot be used as an escape system in case of 1:st stage fails at low altitude. Vacuum engines cannot be started at sea level. For vacuum engines sea level ISP is not given. They don't know it, because they cannot even test it. Test burn at sea level must be done with different nozzle or by adding something inside vacuum nozzle to prevent air mixing with exhaust inside of the vacuum nozzle. That would cause turbulence and vibrations which would tear engine and rocket to pieces.

Main engines of the Space Shuttle had special nozzle design to prevent air entering in to it. Because of this it had not best possible vacuum ISP.

BFS accelerates too slowly to be a good escape capsule in case of explosion. It would not remove need for an escape system in case of BFS explodes.

Boca Chica seems to have many disadvantages. Perhaps Kwajalein Atoll would be better.

Why do people keep saying this drivel about vaccum engines not starting at sea level. It was never true and never will be.

QUESTION 21

Can BFS vacuum-Raptors be fired at sea level pressure?

The BFS will have four Vacuum-Raptors and two sea-level Raptor engines, embedded in a protective skirt.

Will it be possible to start the vacuum Raptors at s/l pressure as well (with reduced efficiency due to over-expansion), for example in case of an emergency launch escape and landing event, or to allow a higher return payload mass than ~50 tons?

Or can they only ever be fired in low air pressure?

A: The "vacuum" or high area ratio Raptors can operate at full thrust at sea level.

summarized elsewhere as

Q: Can BFS vacuum-Raptors be fired at sea level pressure?
A: The "vacuum" or high area ratio Raptors can operate at full thrust at sea level. Not recommended.

Emergency escapes from an exploding stage 1 are not recommended but that doesn't mean they are impossible.

"BFS accelerates too slowly to be a good escape capsule", that may be true. May be very pertinent. Just don't say that the engines won't light at all.

 

[jkn]

Why do people keep saying this drivel about vaccum engines not starting at sea level. It was never true and never will be.



summarized elsewhere as

Q: Can BFS vacuum-Raptors be fired at sea level pressure?
A: The "vacuum" or high area ratio Raptors can operate at full thrust at sea level. Not recommended.

Emergency escapes from an exploding stage 1 are not recommended but that doesn't mean they are impossible.

"BFS accelerates too slowly to be a good escape capsule", that may be true. May be very pertinent. Just don't say that the engines won't light at all.

That FAQ has an error. Nobody would use SL-engines if they could use vacuum optimized instead.
Space Shuttle main engine - Wikipedia
"At sea level, a nozzle of this ratio would normally undergo flow separation of the jet from the nozzle, which would cause control difficulties and could even mechanically damage the vehicle."

 

[dhanson865]

That FAQ has an error. Nobody would use SL-engines if they could use vacuum optimized instead.
Space Shuttle main engine - Wikipedia
"At sea level, a nozzle of this ratio would normally undergo flow separation of the jet from the nozzle, which would cause control difficulties and could even mechanically damage the vehicle."

That isn't a FAQ, it's from Elon's AMA. It isn't an error. It's a fact.

What you are quoting says it's a bad idea to do it, not that it can't be done.

What you said before was "Vacuum engines cannot be started at sea level"

Change that to "Vacuum engines should not be started at sea level" and we have no argument.

 

[jkn]

That isn't a FAQ, it's from Elon's AMA. It isn't an error. It's a fact.

What you are quoting says it's a bad idea to do it, not that it can't be done.

What you said before was "Vacuum engines cannot be started at sea level"

Change that to "Vacuum engines should not be started at sea level" and we have no argument.

I wrote "Vacuum engines cannot be started at sea level." in post 64. There is no need to change that.

 

[RDoc]

Whether a vacuum engine can be started at sea level or not is irrelevant. The question is if NASA, and/or the FAA, will approve crewed rockets without an escape mechanism. AFAIK, the BFS when fully loaded doesn't have enough thrust to escape a first stage explosion. After first stage separation, there is no escape mechanism as currently designed that I'm aware of.

 

[Doug_G]

747 does not have an escape mechanism.

The solution is reliability.

 

[Nikxice]

747 does not have an escape mechanism.

The solution is reliability.

+1 for reliability. Floating seat cushions and O2 masks aren't likely to help much either. Automation, thereby eliminating human pilots, will also help to make BFS safe. There will likely still be a 'Captain' onboard, but hopefully he'll have nothing to do but look out the window. Space shuttles could never be successfully flown back into the atmosphere by a human, only during the last few minutes of the approach. Even that proved too interesting once for STS-37 commander Steve Nagel. Not much noticed at the time, he landed 600 feet short of the threshold at Edwards.
Astronaut Steve Nagel's Unforgettable Landing

 

[RDoc]

747 does not have an escape mechanism.

The solution is reliability.

Very true, but what the 747 has that the BFR almost certainly won't have for a very long time, if ever, is an FAA airworthiness certificate. It takes many years, lots of design reviews, and many test flights in many conditions to get one.

 

[jkn]

BFS cannot escape from exploding first stage. It does not have enough trust. I'm certain BFS will have an escape system.

It might be possible to make BFR as reliable as airplane. Proving it is will take very long time.

 

[mongo]

BFS cannot escape from exploding first stage. It does not have enough trust.

Grain of salt time, but according to Wiki BFR (rocket) - Wikipedia, the max mass of BFS is just above the max thrust (non-derated for vacuum engines in atmosphere). Elon said you could run the vacuum engines at full thrust at sea level, so if they under load the BFS when carrying people, it could potentially escape (albeit slowly).

 

[jkn]

Grain of salt time, but according to Wiki BFR (rocket) - Wikipedia, the max mass of BFS is just above the max thrust (non-derated for vacuum engines in atmosphere). Elon said you could run the vacuum engines at full thrust at sea level, so if they under load the BFS when carrying people, it could potentially escape (albeit slowly).

Slowly is not enough, in case of exploding rocket. See video of Dragon test. They gave it that trust, because it is necessary.

You mean this:"
Q: Can BFS vacuum-Raptors be fired at sea level pressure?
A: The "vacuum" or high area ratio Raptors can operate at full thrust at sea level. Not recommended.
"

Elon was tired of giving obvious answers and made a joke! "Not recommended" is a very important part of that answer. Those engines would start then there would be a RUD.

 

[dhanson865]

Slowly is not enough, in case of exploding rocket. See video of Dragon test. They gave it that trust, because it is necessary.

You mean this:"
Q: Can BFS vacuum-Raptors be fired at sea level pressure?
A: The "vacuum" or high area ratio Raptors can operate at full thrust at sea level. Not recommended.
"

Elon was tired of giving obvious answers and made a joke! "Not recommended" is a very important part of that answer. Those engines would start then there would be a RUD.

It literally will fire at sea level. If it's for escape use only it doesn't matter if it will stress the engine unduly and need to be replaced. I don't think it would RUD.

Here is a video of an engine that wants vaccum but starts at sea level where you can see the negative side effect in action

See What makes the Merlin vacuum engine so deeply throttleable? and the answer is that if you don't run close to full throttle the oscillations are worse.

I think the key phrase was "full thrust" not "not recommended". If they fired the vacuum engines at half thrust at sea level that would be bad. It's all or nothing.

 

[Electroman]

The only way to know for sure is to make it happen atleast once with dummy payloads. Like the crash test for automobiles

 

[mongo]

Slowly is not enough, in case of exploding rocket. See video of Dragon test. They gave it that trust, because it is necessary.

You mean this:"
Q: Can BFS vacuum-Raptors be fired at sea level pressure?
A: The "vacuum" or high area ratio Raptors can operate at full thrust at sea level. Not recommended.
"

Elon was tired of giving obvious answers and made a joke! "Not recommended" is a very important part of that answer. Those engines would start then there would be a RUD.

Oh, I'm in no way saying it's ideal, only that is could potentially get away. In a conflagration event, the speed of exit is less critical (v.s. explosion).

I took his statement to mean that at 100% thrust, the exhaust pressure is above the 40% of ambient stability limit, but start up/ shut down would be not so great. In an escape situation, damage to the engines is better than loss of 2nd stage. Is there a reason for RUD other than output instability?

And regarding the SSME, I read that the output end of the nozzle was returned to make the outer ring pressure a better match with atmospheric.