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#801 2020-08-25 12:18:35

Oldfart1939
Member
Registered: 2016-11-26
Posts: 1,912

Re: Starship is Go...

With the improvement in thrust from the Raptor engine, The Isp is finally approaching that theoretical figure of 330 sec.
I'm not sure about the weight of these vehicles, but restating the third law of thermodynamics in another perverted form: things will get heavier before they get lighter. I do look forward to the 20 km test hop by SN8. It shouldn't look like a flying silo by then, since it's been stated it will have a nose cone and flight control surfaces (fins).

Last edited by Oldfart1939 (2020-08-25 12:19:50)

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#802 2020-09-04 16:43:29

louis
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From: UK
Registered: 2008-03-24
Posts: 5,875

Re: Starship is Go...

So SN6 did its hop-thing! Great news!!  Looks like Starship is going from strength to strength.  Elon says they are starting on the build of the Superheavy in the next few days.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#803 2020-09-13 20:55:36

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 19,881

Re: Starship is Go...

Starship SN8 is to become the next prototype vehicle to undergo a test flight. CEO and Chief Designer Elon Musk confirmed a readiness schedule for the vehicle that will fly with three Raptors, a nose cone, and aero surfaces.

The update is an advance on SpaceX’s previous test schedule that was set to conduct multiple hops with SN5 and SN6. Following two successful 150 meter hops, SN8 will be launched to 60,000 feet (18.2 KM) as early as October.

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#804 2020-09-14 12:08:18

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 4,097
Website

Re: Starship is Go...

The Sea level Raptor is rated at max thrust as 2 MN,  and just about 330 s Isp. It's actually slightly less than 2 MN,  but let's ignore that.

3 Raptors totals 6 MN total thrust at max thrust setting.  Musk said at Boca Chica that for operational Starship/Superheavy launches,  they want a takeoff thrust/weight ratio of 1.5,  in order to get the trajectory kinematics they want.  But for a simple hop test to 60 kft,  let's assume thrust/weight only 1.2 at takeoff.  That's still a decent 0.2 gee vertical upward acceleration off the pad.

That means the takeoff weight is 6 MN / 1.2 = 5 MN = the Earth weight of some 509.9 metric tons.  Call it 510 tons at 2 significant figures. That's the max liftoff mass those three Raptors can lift at full thrust with a (presumed) adequate thrust/weight.

Musk also said at Boca Chica that the Starship prototype designs were running 120 metric tons inert dry mass.  He said the goal was 100 tons.  Not having flown yet,  they would still be a long way from that goal.  So assume the 120 metric ton inert dry mass figure.

Now,  also assume zero payload.  This is an early test flight.  Inert + payload + propellant = ignition mass.  We have a limit on ignition mass for this test,  so the propellant cannot be anywhere near a full load.  The max propellant they can carry for this test is 510 tons igition - 120 tons inert - 0 tons payload = 390 tons of propellant.  So the nominally-1200 ton tanks are nowhere near full.

Now,  I get a flowrate of 0.6 metric tons/sec propellant flow rate through a Raptor at 2 MN full thrust (at full thrust setting and 330 s Isp). For 3 Raptors at full thrust,  that is 1.8 metric tons of propellant burned per second.

If the entire test were flow at full thrust (and it will not be),  the 390 tons of propellant would last for only 216 seconds.  Bear in mind that both propellant flow and Isp reduce as you throttle back from full thrust.  At reduced thrust through much of the trajectory (up and down!),  this test might run somewhere around 300-400 seconds.  That's just a guess.

Toward landing, with the tanks nearing empty at, say,  130 tons (1.3 MN weight) of vehicle,  then at the same 1.2 thrust/weight margin,  they only need about 1.5 MN of thrust.  They could land on one engine at 75%,  or all three throttled to 25%.  20% would a thrust = weight touchdown with all 3 engines running.

What I have seen published about the Raptor is that the min-throttle condition is 20%.  These 20-100% limits are imposed by flame blowout inside the powerhead.  They actually have reached the 100% setting in tests now.

My guess is that they will shut down 1 engine and land on two for redundancy. Those two would be operating in the 31-38% range.  If one flames out,  the other could throttle up to 62-76% to handle the touchdown.

Beyond that,  what speeds they attempt and how they descend are unknown to me.  If it were up to me,  I'd go up slow (probably subsonic all the way),  and just do a descending hover from there to a touchdown.

120,000 feet of path length (60,000 up,  60,000 down) at an average velocity of 500 ft/s (just under half the speed of sound),  is a 240 sec flight.  In a gross sense,  that's just about right,  if most of the flight is done at reduced thrust.

GW

Last edited by GW Johnson (2020-09-14 12:14:25)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#805 2020-09-14 18:51:14

Oldfart1939
Member
Registered: 2016-11-26
Posts: 1,912

Re: Starship is Go...

Well--in any event, this first 60k ft should be nothing less than spectacular. I don't recall if Musk stated there would be a "skydiver descent" on this first flight or not.

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#806 2020-09-14 18:52:19

Oldfart1939
Member
Registered: 2016-11-26
Posts: 1,912

Re: Starship is Go...

Stay tuned tonight for the SN 7.1 pressure test; road closure is in place starting at 21:00 hrs CDT.

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#807 2020-09-15 20:01:57

Oldfart1939
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Registered: 2016-11-26
Posts: 1,912

Re: Starship is Go...

3 Tanker Truckloads of liq N2 arrived today in Boca Chica. Something is imminent!

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#808 2020-09-16 20:03:50

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 19,881

Re: Starship is Go...

No news yet

Lead_copy-1170x658.jpg

Sure looks ugly...

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#809 2020-09-18 11:15:51

Oldfart1939
Member
Registered: 2016-11-26
Posts: 1,912

Re: Starship is Go...

The pressure test to burst pressure of SN 7.1 was scrubbed last night (09/17/2020). Tonight is another possibility, since the road closures are still in effect.

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#810 2020-09-18 18:17:05

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,875

Re: Starship is Go...

Interesting video on Felix's "What About It?" You Tube channel - looks like Space X are now using their Florida site to test a 1 MW methane production facility (obviously to facilitate fuelling of a return to Earth flight for a Starship). This is marvellous affirmation of seriousness of intent. I don't think we can yet rule out a cargo flight to Mars in 2022 with human flight in 2024.

Musk is achieving Apollo style pace of development.  We could see orbital Starship flight jn early 2021.

So maybe they are planning to ship a fully operational methox plant to Mars in 2022!

One thing about space - as long as you can get into orbit and fire your engines,  the cosmos is pretty much your oyster. It's not like getting to Mars is really any more difficult than getting to the Moon as far as the transport system goes.  As long as you have comms in place they are much of a muchness.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#811 2020-09-18 19:48:35

Oldfart1939
Member
Registered: 2016-11-26
Posts: 1,912

Re: Starship is Go...

Louis-With all the activities at Boca Chica, many of the side projects aren't even noticed by most SpaceX watchers. The facilities in Florida haven't had much scrutiny recently. It won't be many more months and the Moxie unit on the latest NASA rover will begin operation. It's like a jigsaw puzzle with widely scattered pieces.

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#812 2020-09-19 18:47:47

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,875

Re: Starship is Go...

Yes the jigsaw puzzle is a good analogy. Space X's focus on developing a methox production facility suggests that plans for other essentials such as habs,  energy production and surface transport and mining capability may also be well advanced.

Oldfart1939 wrote:

Louis-With all the activities at Boca Chica, many of the side projects aren't even noticed by most SpaceX watchers. The facilities in Florida haven't had much scrutiny recently. It won't be many more months and the Moxie unit on the latest NASA rover will begin operation. It's like a jigsaw puzzle with widely scattered pieces.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#813 2020-09-26 07:19:27

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,875

Re: Starship is Go...

The "What About It?" You Tube channel (presented by Felix) has a great update video. Highlights for me were a glimpse of the first section of the SuperHeavy and an interesting illustrated discussion of possible landing leg designs for the Starship.

Last edited by louis (2020-09-26 08:10:11)


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#814 2020-09-26 08:23:34

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,875

Re: Starship is Go...

Here's the "What About It?" link...

https://www.youtube.com/watch?v=tG8KiXQahcg


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#815 2020-09-30 18:23:57

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,875

Re: Starship is Go...

Another good vid from WAI (Felix).

https://www.youtube.com/watch?v=tG8KiXQahcg


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#816 2020-10-11 17:22:33

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 19,881

Re: Starship is Go...

SN8 receiving Raptors as prelude to advanced Starship testing]NSF-2020-10-11-18-46-22-812.jpg and hoping for a test flight to 50,000 feet.

SN8 underwent a total of three cryo proofing tests during the week. The first was deemed acceptable per the test parameters, but a small leak “opened up near the engine mounts, possibly due to differential shrinking,” according to SpaceX Chief Designer and CEO Elon Musk.

The repair was completed in time for a second test the following night, which appeared to show SN8 fully loaded with LN2 (Liquid Nitrogen), but without any reference as to how it went from Elon. Roadblocks were then called for the third time, pointing to another test 24 hours later.

This time the test concluded with Elon noting cryo proofing was a success.

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