Starship's Third Flight Test [video] • Hacker News Board

Re: Starship's Third Flight Test [video]

#241

Post by dotancohen » Thu, Mar 14, 2024, 4:47 PM UTC

wolf550e wrote: post #192

DarmokJalad1701 wrote: post #167

Earlier quoted context omitted.

> They can't leave the Starship up in orbit, and where it reenters needs to be controlled. Why? Plenty of boosters re-enter uncontrolled and burn up all the time.

Starship is huge and heavy and made of steel, it will not burn up on reentry, if it falls on a populated area it will kill people. They will not be allowed orbital trajectories until they demonstrate they can control the deorbit burn.

An empty Starship has a very low ballistic coefficient, it will be torn apart by the atmosphere if not carefully controlled. Add to that the FTS and there is no real danger to population on the ground.

Re: Starship's Third Flight Test [video]

#242

Post by exDM69 » Thu, Mar 14, 2024, 4:50 PM UTC

dotancohen wrote: post #235

exDM69 wrote: post #218

Earlier quoted context omitted.

No, that's inefficient and real spacecraft don't do that for typical low earth orbits. Works in Kerbal Space Program, though. Normal orbital insertion is a single burn to orbit (with staging). With the correct initial roll and pitch, the spacecraft follows a perfect gravity turn and ends up in a near circular orbit at main engine cut off.

Do they typically burn softly until near apogee, then put on the power until raising the orbit on the other side to their current altitude? In KSP, I can easily get an orbit on a single burn (with staging) but getting it circular obviously requires adding energy at apogee.

No, most rocket engines have quite limited amount of throttle capability and run near maximum thrust until cutoff.

The rocket yaws and pitches in the first seconds of flight while the vehicle is still subsonic, then flies a gravity turn trajectory at zero angle of attack (facing the direction of travel) at near maximum thrust. Any errors accumulated during early part of the flight will be corrected by adjusting the timing of the second stage cutoff based on radar tracking.

The initial pitch over is just a few degrees off vertical, but must be precise to a fraction of a degree (KSP tolerances are higher due to small planet).

You can get a pretty good circular orbit in Kerbal Space Program with one burn if you do a few attempts and trial and error binary search for the optimal initial pitchover angle, but it's very difficult to do without throttling the 2nd stage burn. If I recall correctly, the MechJeb mod can do a precise single burn to orbit.

Re: Starship's Third Flight Test [video]

#243

Post by DarmokJalad1701 » Thu, Mar 14, 2024, 4:53 PM UTC

neffo wrote: post #196

golol wrote: post #184

Earlier quoted context omitted.

One technical modification: They need engine relight in orbit to work to deliver payloads, otherwise Starship will stay in orbit and they can not test reentry.

You need a second burn just to enter orbit. You burn at the top of the sub-orbital arc (opposite side of the earth) to enter orbit.

> You need a second burn just to enter orbit

Not necessarily - it is completely dependent on the ascent profile. For example, during the Apollo program, the Saturn V would fly directly into a parking orbit and only relight the S-IVB for the Trans-Lunar Injection burn.

Re: Starship's Third Flight Test [video]

#244

Post by numpad0 » Thu, Mar 14, 2024, 4:54 PM UTC

ta1243 wrote: post #8

BryanLegend wrote: post #6

Earlier quoted context omitted.

Thanks, I like the Spaceflight Now commentary the most. Best analysis.

I find those youtube channels far too grifty for my tastes. Sadly as twitter doesn't seem to work ("Something went wrong. Try reloading.") I have lost quite a lot of the excitement I used to have for spacex.

It's SpaceX's problem that they're not where the public expect them to be.

Re: Starship's Third Flight Test [video]

#245

Post by DarmokJalad1701 » Thu, Mar 14, 2024, 4:55 PM UTC

johnyzee wrote: post #138

travisgriggs wrote: post #126

Earlier quoted context omitted.

I wouldn’t classify the re-entry survival problem as icing. But otherwise, I agree with you.

Yeah, breaking up in-air during re-entry (at 65 km going by last telemetry) seems like a potential big issue to fix.

> Yeah, breaking up in-air during re-entry (at 65 km going by last telemetry) seems like a potential big issue to fix.

That's what pretty much every non-SpaceX rocket does today with very few exceptions.

Re: Starship's Third Flight Test [video]

#246

Post by dotnet00 » Thu, Mar 14, 2024, 4:57 PM UTC

travisgriggs wrote: post #142

I was surprised that during the initial ascent footage from the fin-cam, you can see that the grid fins are out. Do they not fold back on ascent? Or possibly drag on one side to arc over? Just seems weird to have your “drag device” deployed during ascent.

I think that between the high TWR allowing the vehicle to spend less time in the thicker parts of the atmosphere and the lower pressure 'wake' the ship on on top would produce, there isn't as much drag over the fins as one might think. IIRC it was one of their various "delightfully counter-intuitive" discoveries during earlier testing.

Re: Starship's Third Flight Test [video]

#247

Post by DarmokJalad1701 » Thu, Mar 14, 2024, 4:57 PM UTC

ajross wrote: post #174

DarmokJalad1701 wrote: post #167

Earlier quoted context omitted.

> They can't leave the Starship up in orbit, and where it reenters needs to be controlled. Why? Plenty of boosters re-enter uncontrolled and burn up all the time.

Stated simply: zero-thrust "orbits" repeat the same trajectory again and again. So if you end your burn in the outer atmosphere near your launch pad, the next time around you will be back in the outer atmosphere (near where the the launch pad "was", ignoring the rotation of the planet). And since there's air there providing resistance, you'll re-enter and crash. Getting to orbit requires at least one more burn near t…

> Getting to orbit requires at least one more burn near the apogee of the original orbit to circularize it and ensure the spacecraft doesn't approach the atmosphere again

The Saturn V went direct to Earth orbit without requiring relighting the third stage engine.

Re: Starship's Third Flight Test [video]

#248

Post by numpad0 » Thu, Mar 14, 2024, 4:58 PM UTC

exDM69 wrote: post #218

neffo wrote: post #196

Earlier quoted context omitted.

You need a second burn just to enter orbit. You burn at the top of the sub-orbital arc (opposite side of the earth) to enter orbit.

No, that's inefficient and real spacecraft don't do that for typical low earth orbits. Works in Kerbal Space Program, though. Normal orbital insertion is a single burn to orbit (with staging). With the correct initial roll and pitch, the spacecraft follows a perfect gravity turn and ends up in a near circular orbit at main engine cut off.

IIUC real launchers do a single burn to orbit because S2 TWR is not very high and relight is finicky. Launchers that has relightable S2 and/or hypergolic S3 routinely do circularization burns.

There are reasons "apogee" is more recognized word than "apoapsis".

Re: Starship's Third Flight Test [video]

#249

Post by jumpman_miya » Thu, Mar 14, 2024, 5:01 PM UTC

Man this has me crying like a baby. I do not know why.

Re: Starship's Third Flight Test [video]

#250

Post by dotancohen » Thu, Mar 14, 2024, 5:04 PM UTC

exDM69 wrote: post #242

dotancohen wrote: post #235

Earlier quoted context omitted.

Do they typically burn softly until near apogee, then put on the power until raising the orbit on the other side to their current altitude? In KSP, I can easily get an orbit on a single burn (with staging) but getting it circular obviously requires adding energy at apogee.

No, most rocket engines have quite limited amount of throttle capability and run near maximum thrust until cutoff. The rocket yaws and pitches in the first seconds of flight while the vehicle is still subsonic, then flies a gravity turn trajectory at zero angle of attack (facing the direction of travel) at near maximum thrust. Any errors accumulated during early part of the flight will be corrected by adjusting the t…

Thank you. Yes, I figured that real life engines could not throttle enough for the maneuver as stated. I am familiar with the gravity turn, but I just don't see how energy can be added continuously, uniformly right up to a circular orbit. But I've not really put much effort into trying to understand that, I'll start looking more at real life pitch angles at various altitudes. Maybe I just need to start that gravity turn sooner - you mention that it already starts in the first few seconds. Thank you.