Safe Instantaneous Impact Point Tracks for Starship Launch and Recovery

[sdsds]

Let's just skip this, please.

The terminology quibble isn't really productive. On the other hand the discussion brought forth the (fascinating) idea that the re-entry trajectory SpaceX plans might include negative lift, so that at all times the uncontrolled ballistic impact point would be further down-range (and/or at higher velocity).

Is it possible to construct an example 'control law' that implements this, even for an overly simplistic atmospheric model? (The paper linked earlier seems to have been from a time when compute resources were comparatively limited.)

[InterestedEngineer]

Let's just skip this, please.

The terminology quibble isn't really productive. On the other hand the discussion brought forth the (fascinating) idea that the re-entry trajectory SpaceX plans might include negative lift, so that at all times the uncontrolled ballistic impact point would be further down-range (and/or at higher velocity).

Is it possible to construct an example 'control law' that implements this, even for an overly simplistic atmospheric model? (The paper linked earlier seems to have been from a time when compute resources were comparatively limited.)

What's the trajectory that accomplishes this?  Super shallow I guess, one that would nominally cause a skip unless negative lift was applied.  If anything is going wrong, cancel the negative lift, skip ahead to the Gulf.

i wonder how many Gs a Starship has to pull to stay in the curvature of the earth and not skip back out to space. 

Also you'd need to rotate somehow to get into belly flop mode.  At some altitude you've slowed down enough that you don't want negative lift you want positive lift.  I'm not sure how you safely do that kind of rotation at hypersonic speed.

[Jim]

i wonder how many Gs a Starship has to pull to stay in the curvature of the earth and not skip back out to space. 

none, non lifting entries and most lifting entries from LEO don't have the energy to increase altitude much less go into space.  A Starship can fold up its aerosurfaces and auger in.

[Jim]

What's the trajectory that accomplishes this?

Tail first, nose down

[LouScheffer]

The only way out of this is to make re-entry itself 1000x safer.

I don't think this is correct.   The exact same technologies that make re-entry safer *also* increase your odds of being able to abort to a safer crash site.

Using your example, suppose you add ablative shielding under the tiles.  This increases the odds of a safe re-entry, but also increases the information, and reaction time, you have to abort to a better (fewer fatalities) crash site.  Same with redundant actuators, increased margins, and other reliability improvements.

For a completely arbitrary numerical example, perhaps the same technologies could make re-entry 100x safer, but also make sure you could abort successfully 9 of 10 times.  Then you get a 1000x decrease in the odds of killing someone on the ground, with only a 100x increase in the odds of a safe re-entry.

[InterestedEngineer]

Kinda disappointing that the banking tests for Starship flight 11 are in the subsonic regime.

Some of us had been hoping for hypersonic and supersonic banking tests because that lends itself to landing trajectory tweaks - eg. avoid populated areas and then make a turn towards the landing tower after everything looks good (or after max-Q)

[wannamoonbase]

Kinda disappointing that the banking tests for Starship flight 11 are in the subsonic regime.

Some of us had been hoping for hypersonic and supersonic banking tests because that lends itself to landing trajectory tweaks - eg. avoid populated areas and then make a turn towards the landing tower after everything looks good (or after max-Q)

One step at a time.

[JaimeZX]

Indeed.  Hypersonic maneuvering could easily result in LOV, so you need to slot that into your prioritized mission objectives.

[Jim]

Kinda disappointing that the banking tests for Starship flight 11 are in the subsonic regime.

Some of us had been hoping for hypersonic and supersonic banking tests because that lends itself to landing trajectory tweaks - eg. avoid populated areas and then make a turn towards the landing tower after everything looks good (or after max-Q)

Why bother when it all be done in the lower less stressful velocity ranges like Falcon 9.

[InterestedEngineer]

Kinda disappointing that the banking tests for Starship flight 11 are in the subsonic regime.

Some of us had been hoping for hypersonic and supersonic banking tests because that lends itself to landing trajectory tweaks - eg. avoid populated areas and then make a turn towards the landing tower after everything looks good (or after max-Q)

Why bother when it all be done in the lower less stressful velocity ranges like Falcon 9.

Data from recent re-entry:   https://youtube.com/watch?v=bGFUowgKqmo


In a typical Starship re-entry, there are about 140 seconds past max-Q to supersonic, with about 2.2km/sec average speed.  That's downrange 300km.  That's quite a huge opportunity for maneuvering, on the order of 30km track change.

From supersonic to transonic, it's another 140 seconds with an average velocity of about 1km/sec.  That's another 140km downrange or another 14km of track change.

From transonic to firing up the engines there's about 150 seconds and it's moving at about 0.2km/sec.  That gives a possible track range (generously 0.2 ratio) of 6km.

So in total, the total track change capability is about 30km past max-Q to supersonic, 14km from supersonic to transonic, and 6km subsonic.

So most of the track change capability is in the hypersonic and supersonic regimes.

In answer to your comment Falcon-9 does maneuver in the supersonic regime.  Since it's barely hypersonic it's not really worth calculating how much maneuvering it does while hypersonic.

[Jim]

In a typical Starship re-entry, there are about 140 seconds past max-Q to supersonic, with about 2.2km/sec average speed.  That's downrange 300km.  That's quite a huge opportunity for maneuvering, on the order of 30km track change.

From supersonic to transonic, it's another 140 seconds with an average velocity of about 1km/sec.  That's another 140km downrange or another 14km of track change.

From transonic to firing up the engines there's about 150 seconds and it's moving at about 0.2km/sec.  That gives a possible track range (generously 0.2 ratio) of 6km.

So in total, the total track change capability is about 30km past max-Q to supersonic, 14km from supersonic to transonic, and 6km subsonic.

So most of the track change capability is in the hypersonic and supersonic regimes.

In answer to your comment Falcon-9 does maneuver in the supersonic regime.  Since it's barely hypersonic it's not really worth calculating how much maneuvering it does while hypersonic.

the 20 km from supersonic to subsonic is sufficient.  It just has to be off shore.

[meekGee]

In a typical Starship re-entry, there are about 140 seconds past max-Q to supersonic, with about 2.2km/sec average speed.  That's downrange 300km.  That's quite a huge opportunity for maneuvering, on the order of 30km track change.

From supersonic to transonic, it's another 140 seconds with an average velocity of about 1km/sec.  That's another 140km downrange or another 14km of track change.

From transonic to firing up the engines there's about 150 seconds and it's moving at about 0.2km/sec.  That gives a possible track range (generously 0.2 ratio) of 6km.

So in total, the total track change capability is about 30km past max-Q to supersonic, 14km from supersonic to transonic, and 6km subsonic.

So most of the track change capability is in the hypersonic and supersonic regimes.

In answer to your comment Falcon-9 does maneuver in the supersonic regime.  Since it's barely hypersonic it's not really worth calculating how much maneuvering it does while hypersonic.

the 20 km from supersonic to subsonic is sufficient.  It just has to be off shore.

It has other uses such as earlier return opportunities, to complement in-orbit maneuvering, and save propellant.

[DanClemmensen]

In a typical Starship re-entry, there are about 140 seconds past max-Q to supersonic, with about 2.2km/sec average speed.  That's downrange 300km.  That's quite a huge opportunity for maneuvering, on the order of 30km track change.

From supersonic to transonic, it's another 140 seconds with an average velocity of about 1km/sec.  That's another 140km downrange or another 14km of track change.

From transonic to firing up the engines there's about 150 seconds and it's moving at about 0.2km/sec.  That gives a possible track range (generously 0.2 ratio) of 6km.

So in total, the total track change capability is about 30km past max-Q to supersonic, 14km from supersonic to transonic, and 6km subsonic.

So most of the track change capability is in the hypersonic and supersonic regimes.

In answer to your comment Falcon-9 does maneuver in the supersonic regime.  Since it's barely hypersonic it's not really worth calculating how much maneuvering it does while hypersonic.

the 20 km from supersonic to subsonic is sufficient.  It just has to be off shore.

It has other uses such as earlier return opportunities, to complement in-orbit maneuvering, and save propellant.

How much cross range is needed to permit RTLS after a single orbit? Probably depends on the inclination.

[InterestedEngineer]

In a typical Starship re-entry, there are about 140 seconds past max-Q to supersonic, with about 2.2km/sec average speed.  That's downrange 300km.  That's quite a huge opportunity for maneuvering, on the order of 30km track change.

From supersonic to transonic, it's another 140 seconds with an average velocity of about 1km/sec.  That's another 140km downrange or another 14km of track change.

From transonic to firing up the engines there's about 150 seconds and it's moving at about 0.2km/sec.  That gives a possible track range (generously 0.2 ratio) of 6km.

So in total, the total track change capability is about 30km past max-Q to supersonic, 14km from supersonic to transonic, and 6km subsonic.

So most of the track change capability is in the hypersonic and supersonic regimes.

In answer to your comment Falcon-9 does maneuver in the supersonic regime.  Since it's barely hypersonic it's not really worth calculating how much maneuvering it does while hypersonic.

the 20 km from supersonic to subsonic is sufficient.  It just has to be off shore.

Looking at a map, there's a line that goes south of Monterrey, Mexico instead of north like the currently planned trajectory, and thereby avoids Matamoros and Reynosa, two population centers that could be affected by a breakup before max-Q (because that kind of breakup has an expanding cone of debris).

However, that leaves you 200km south of Boca Chica.

If one could steer 200km back to Boca Chica after max-Q (i.e. after the highest risk portion of the flight), one could have the debris trajectory over almost completely uninhabited Mexico.

There's about 300 seconds in which to make that change over 450km of traversal.  That angle is too much (200/450) or 24 degree trajectory change.

But in general the idea is to get as much cross range out of the post-high-risk phase so that the high risk phase can be pointed in a safer direction.  Probably won't work in this case.

At any rate, they aren't planning on doing supersonic banking (at least they aren't testing this flight), so they must be aiming at a spot less than 6km offshore from Boca Chica

[JaimeZX]

Can some of this discussion be illuminated by the TFR plots from the announcement page?

https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=63577.0;attach=2425026;image

[InterestedEngineer]

Can some of this discussion be illuminated by the TFR plots from the announcement page?

https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=63577.0;attach=2425026;image

The cone broadens and then narrows.  Maybe that narrowing is before Matamoros so perhaps I'm just chasing space dreams, err nightmares.

As long as the largest width of the cone doesn't touch Monterrey or its suburbs, it should be fine.

[Brigantine]

How much cross range is needed to permit RTLS after a single orbit? Probably depends on the inclination.

As stated, from the same trajectory as IFT-3 through 11, per my old scribbling-on-the-back-of-google-earth, 400±100 km.
(See attachment below. Thick line: great circle; thin line: ground track. Per mark 1 eyeball. Assumes instantaneous launch & descent, 90 minute orbit. Ignores precession)

But I'll broaden this question to the following case:

A launch from the cape on the descending node to the inclination previously discussed, does a single orbit, and then re-enters over Mexico.
How much cross range is then needed to reach Starbase? (I expect surprisingly little)

To what extent can a dog-leg on ascent mitigate the cross range requirement on descent? Or otherwise help route the hypersonic portion of re-entry track away from cities. (I expect almost 1 to 1)

Nice visualization of re-entry corridor: https://twitter.com/spacesudoer/status/1969128213625311532

[DanClemmensen]

How much cross range is needed to permit RTLS after a single orbit? Probably depends on the inclination.

But I'll broaden this question to the following case:

A launch from the cape on the descending node to the inclination previously discussed, does a single orbit, and then re-enters over Mexico.
How much cross range is then needed to reach Starbase? (I expect surprisingly little)

Thanks! That once-around may be useful for preliminary testing, but it's not sustainable, because it goes the wrong way for Ships built at Starbase. For (several of) those, you want to launch at Starbase and land at KSC. Better to just wait for 8 or 16 orbits, which is quicker and cheaper (when added to a Starlink mission) than using the barge.

[StraumliBlight]

The Starship flight path update has exactly 3 comments so far, with 2 weeks remaining.

Also the October 7th virtual meeting got cancelled due to the government shutdown.

EDIT: Closed on October 20th with 26 comments.

[Vultur]

The Starship flight path update has exactly 3 comments so far, with 2 weeks remaining.

And only one of the three appears particularly on point.