Author Topic: Starship!  (Read 5196 times)

Offline Zakalwe

  • Uranus
  • ****
  • Posts: 1526
Re: Starship!
« Reply #150 on: February 09, 2021, 01:04:11 PM »
I have no idea why they dont pull the flip manouver at a higher altitude and just run the landing burn longer

The header tanks are only so big - I don't know what kind of margin they have right now, but I don't think they can start appreciably higher without increasing their size.

Which may ultimately be the answer - these are prototypes, they're meant to test out ideas under real-world conditions, and it may be their modeling was just plain wrong and they need to rethink the tankage or the plumbing or the engines.  They may use larger headers on 16/17/18/etc.  They may find out the current Raptor design just isn't robust enough to be swung around that violently and has to be tweaked. 

I would not expect a successful landing before SN15, and I would not be surprised if they had at least one kaboom after demonstrating a successful landing.  They are going to burn a lot of hardware before they get this figured out.
i think that they have cut up a couple of prototypes that were in varying stages of being built. maybe that's the reason?
"The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge.' " - Isaac Asimov

Online smartcooky

  • Uranus
  • ****
  • Posts: 1855
Re: Starship!
« Reply #151 on: February 11, 2021, 05:11:28 AM »
I have no idea why they dont pull the flip manouver at a higher altitude and just run the landing burn longer

The header tanks are only so big - I don't know what kind of margin they have right now, but I don't think they can start appreciably higher without increasing their size.

Which may ultimately be the answer - these are prototypes, they're meant to test out ideas under real-world conditions, and it may be their modeling was just plain wrong and they need to rethink the tankage or the plumbing or the engines.  They may use larger headers on 16/17/18/etc.  They may find out the current Raptor design just isn't robust enough to be swung around that violently and has to be tweaked. 

I would not expect a successful landing before SN15, and I would not be surprised if they had at least one kaboom after demonstrating a successful landing.  They are going to burn a lot of hardware before they get this figured out. 

Let me start by saying I am not an aerospace engineer (although am a now retired aeronautical engineer - a different beast but with many points of convergence and commonality).

AIUI the whole (and sole) reason for the header tanks in the first place its to relight the Raptor engines due to the forces involved in the flip maneouvre to land creating a problm with fuel pressure from the main tanks. It seems to me that there is a more obvious solution; electric fuel booster pumps.

Peter Beck's "Electron" design at RocketLab uses electric fuel pumps powered by Li-Ion batteries. Why don't SpaceX take a leaf from RocketLab's book  - ditch the whole separate header tanks idea and use electric fuel booster pumps from the main fuel tanks to pressure-feed methalox to the Raptor engines? Is this idea perhaps so obvious that I am overlooking some even more obvious flaw in my thinking?
 
► What you can assert without evidence, I can dismiss without evidence
► When you argue with idiots you risk being dragged down to their level and beaten with experience.
► Conspiracism is a shortcut to the illusion of erudition

Offline molesworth

  • Mars
  • ***
  • Posts: 286
  • the curse of st custards
Re: Starship!
« Reply #152 on: February 11, 2021, 05:30:54 AM »
I have no idea why they dont pull the flip manouver at a higher altitude and just run the landing burn longer

The header tanks are only so big - I don't know what kind of margin they have right now, but I don't think they can start appreciably higher without increasing their size.

Which may ultimately be the answer - these are prototypes, they're meant to test out ideas under real-world conditions, and it may be their modeling was just plain wrong and they need to rethink the tankage or the plumbing or the engines.  They may use larger headers on 16/17/18/etc.  They may find out the current Raptor design just isn't robust enough to be swung around that violently and has to be tweaked. 

I would not expect a successful landing before SN15, and I would not be surprised if they had at least one kaboom after demonstrating a successful landing.  They are going to burn a lot of hardware before they get this figured out. 

Let me start by saying I am not an aerospace engineer (although am a now retired aeronautical engineer - a different beast but with many points of convergence and commonality).

AIUI the whole (and sole) reason for the header tanks in the first place its to relight the Raptor engines due to the forces involved in the flip maneouvre to land creating a problm with fuel pressure from the main tanks. It seems to me that there is a more obvious solution; electric fuel booster pumps.

Peter Beck's "Electron" design at RocketLab uses electric fuel pumps powered by Li-Ion batteries. Why don't SpaceX take a leaf from RocketLab's book  - ditch the whole separate header tanks idea and use electric fuel booster pumps from the main fuel tanks to pressure-feed methalox to the Raptor engines? Is this idea perhaps so obvious that I am overlooking some even more obvious flaw in my thinking?
With a similar disclaimer - I'm a semi-retired software engineer working on spacecraft data handling tech, but with a long-time interest in spaceflight...

As far as I understand it, the problem is the main tanks are large, and the small amount of fuel/lox left at landing would be moving about uncontrollably and unpredictably due to the flip.  Pumps may not be able to force through enough to relight the engines.

The header tanks are much smaller so should have less of an ullage problem, although pumps in addition to the nitrogen pressurisation may be worth looking into.  I haven't seen any report on why the engine on SN9 failed to relight, but it looked very different so may have been due to some other cause.  SpaceX have been pretty open about their development work so far, so I expect we'll hear more very soon.

I'd also add the comment that even 5 years ago I thought the chances of humans landing on Mars in my lifetime were vanishingly small.  Now I can see it happening within the next 10 - 15 years!   8)
Days spent at sea are not deducted from one's allotted span - Phoenician proverb

Offline JayUtah

  • Neptune
  • ****
  • Posts: 3709
    • Clavius
Re: Starship!
« Reply #153 on: February 11, 2021, 03:15:14 PM »
Yes, ullage is king here.  All the pumps in the world won't help you if the fluid you're trying to pump isn't anywhere near the pump inlet.  Not only do you have to consider pump head pressure, which you can solve with staged pumps, you have to consider that propellant slosh may deny you any use of pumps.  This is especially true if you have to plumb the tank for both horizontal and vertical operation.  The abstract principle of the header tank is very simple:  the fuller the tank, the less likely propellant slosh is a factor, and the less likely tank outlet location is a factor.  If the tank is sufficiently small, you can even introduce technologies like bladders, which effectively eliminate all the slosh, but I don't know if SpaceX are doing that.

So burning the Raptors longer on landing obviously means more propellant in header tanks that can't be touched until landing.  But since it all has to fit in the same fuselage, it means less fuel available for ascent -- the moneymaker.  Also, the flight dynamics would be altered by more mass in the header tanks.  And at a certain point, moving them around the fuselage for weight, balance, and moment-of-inertia purposes will typically either get you in structural trouble or plumbing trouble, or both.

And you aeronautical guys need to quit being so modest.  Fuel and oil flow paths and pumping strategies for high-performance aircraft are exactly the problems being solved here.  Rocketry used to be easy.  You were either ascending gracefully at 3+ g along the rocket axis, or floating in microgravity with no presumptions of favorable propellant placements.  Starship's propellant feed system reintroduces the problems of aircraft fuel-feed systems.  Hybrid thinking is needed.
"Facts are stubborn things." --John Adams