Communications and PNT Support for Starship Mars EDL Testing

[crandles57]

Virgin Galactic found feathered format at high speed didn't work well? Well I think that was deployed when outside design limits, so you 'just' have to design it to stay within its limits, but that might be difficult at orbital entry speeds?

If the atmosphere varies in thickness, the time you need it, is if the atmosphere is thinner than you expect causing the speeds to be higher, giving you less time, less effective deceleration and higher speeds so more energy to dissipate. Doesn't sound easy to me, not that that means much.

[TheRadicalModerate]

Virgin Galactic found feathered format at high speed didn't work well? Well I think that was deployed when outside design limits, so you 'just' have to design it to stay within its limits, but that might be difficult at orbital entry speeds?

If the atmosphere varies in thickness, the time you need it, is if the atmosphere is thinner than you expect causing the speeds to be higher, giving you less time, less effective deceleration and higher speeds so more energy to dissipate. Doesn't sound easy to me, not that that means much.

If you're ejecting a black box from a disintegrating Starship, the design limits are kinda hard to specify.

There's a pretty good chance it's being ejected at a low speed, closer to max-q than to peak heating.  That probably helps with maintaining an acceptable thermal profile.  But then you have two options:

1) Send all the telemetry before the box hits the ground, which obviously limits the volume of data.

2) Find a way to make it OK to hit the ground, either by:
a) A soft landing, or
b) A crash landing, with a structure that preserves the data and the transmitter to send the telemetry at its leisure.

As with all things Starship, size is not necessarily a problem, so you could have something with a fairly low ballistic coefficient and a lot of crush core.  But I'm not sure how you get viable RF through a bunch of crushed crush core.

Remember, this is an alternative to something like the trailer, which is fairly close to a Starlink with an interplanetary antenna grafted on.  That still seems like it's easier than doing crasher engineering.

PS:  For a 50m/s terminal velocity on Mars, you need a ballistic coefficient of 6.7.  For 10m/s, you need BC=0.27.

[Twark_Main]

Virgin Galactic found feathered format at high speed didn't work well? Well I think that was deployed when outside design limits, so you 'just' have to design it to stay within its limits, but that might be difficult at orbital entry speeds?

If the atmosphere varies in thickness, the time you need it, is if the atmosphere is thinner than you expect causing the speeds to be higher, giving you less time, less effective deceleration and higher speeds so more energy to dissipate. Doesn't sound easy to me, not that that means much.

If you're ejecting a black box from a disintegrating Starship, the design limits are kinda hard to specify.

There's a pretty good chance it's being ejected at a low speed, closer to max-q than to peak heating.  That probably helps with maintaining an acceptable thermal profile.  But then you have two options:

1) Send all the telemetry before the box hits the ground, which obviously limits the volume of data.

...

Oh no, whatever will we do without unlimited data!      Space engineers these days are so spoiled.....


All these complex solutions have to be weighed against

3) just increase the transmit power of the transmitter to offload more data before impact m

4) it fails. That's fine. Remember, this was a Plan B on top of a Plan B anyway! Repeat after me: failure is an option here. 

[TheRadicalModerate]

4) it fails. That's fine. Remember, this was a Plan B on top of a Plan B anyway! Repeat after me: failure is an option here. 

Failure is an option, but it also means an extra synod before you can do anything interesting.

I'd put the the chances of a successful Mars EDL the first time out at under 20%.  So the data is the payload--and that payload has to be clear of the Ship before it burns up / tumbles / fails structurally / crashes / hard-lands / just plain loses its mind.  Spending some time to both optimize that payload and ensure its delivery seems well worth it.

[Twark_Main]

4) it fails. That's fine. Remember, this was a Plan B on top of a Plan B anyway! Repeat after me: failure is an option here. 

Failure is an option, but it also means an extra synod before you can do anything interesting.

Again, you're going to use existing imaging assets and existing relay satellites to get your primary data. The black box is the plan B on top of this in case your scaremogering technical concerns about SpaceX not being able to find a root cause despite all that data turn out on the pessimistic side.

And regardless, at the subsequent synod after a hypothetical failure of the Plan B of the Plan B ( ), it's not like they'll do something conservative like send the same design of Starship again. That's not the SpaceX way. So there will certainly be "interesting" developments regardless.

If your concerns-upon-concerns all work out for the worst, and SpaceX can "only" narrow the failure mode down to 2 or 3 root causes, then.......... SpaceX will just beef up one or two other things that perhaps didn't need to be strengthened (and perhaps needed to be strengthened anyway and averts a second failure).

Forgive me if I am unbothered by this, and believe it unwise to devote millions upon millions of dollars in R&D to make a slightly better black box.

I'd put the the chances of a successful Mars EDL the first time out at under 20%.  So the data is the payload--and that payload has to be clear of the Ship before it burns up / tumbles / fails structurally / crashes / hard-lands / just plain loses its mind.  Spending some time to both optimize that payload and ensure its delivery seems well worth it.

"Some time" has been spent, and overspent.

At some point it's not worth it, and the returns diminish. I say "transmit before impact" is past that point already. SpaceX probably shouldn't bother with the black box at all, and they certainly shouldn't flush money on anything even more elaborate and overengineered than that.

In the end of the day it's a distraction, wasted effort. Starship lands and you spent all that money for nothing. TRM seems to say yes to every imaginable preparatory step before landing on Mars... except landing on Mars.

When you say 20% chance, is that "20% chance of the mission reaching a successful landing," or is that "if the Starship survives until Mars entry interface, it has a 20% chance of landing success?"  This actually makes a big difference for the risk/benefit calculation.

[meekGee]

4) it fails. That's fine. Remember, this was a Plan B on top of a Plan B anyway! Repeat after me: failure is an option here. 

Failure is an option, but it also means an extra synod before you can do anything interesting.

I'd put the the chances of a successful Mars EDL the first time out at under 20%.  So the data is the payload--and that payload has to be clear of the Ship before it burns up / tumbles / fails structurally / crashes / hard-lands / just plain loses its mind.  Spending some time to both optimize that payload and ensure its delivery seems well worth it.

If they have time for it, can a higher altitude pass through earth's atmosphere emulate Mars reentry?

[xvel]

Earth atmosphere wasn't primarily composed of carbon dioxide last time I checked, so no.

[MickQ]

Why should that matter ??  RF works equally in both atmospheres AFAIK.

[crandles57]

If they have time for it, can a higher altitude pass through earth's atmosphere emulate Mars reentry?

I assume there is going to trials for fuel transfer. When this is successfully done, what are you then going to do with the refuelled Starship?

I suppose you might want to immediately recover it to check for any damage or anything that might impede further refuelling.

However, it would also be a first opportunity to send to a lunar free return to test GNC, TLI, and a higher speed re-entry/aerobraking test.

Would it be SpaceX like to try to test all of the above at the first opportunity? So they will have an opportunity without it eating into whatever timeline they are working towards? (As long as they are prepared for success of the fuel transfer tests?)

[TheRadicalModerate]

If they have time for it, can a higher altitude pass through earth's atmosphere emulate Mars reentry?

There are three things that aren't easy to simulate at full-up system scale:

1) Lots of CO2 means lots of free oxygen in the plasma stream.  This can be simulated in lab experiments on individual tiles, which will yield lots of data, but it's unclear if all heating and oxidation effects can be captured for the entire aerosurface.

2) Mars's lower gravity means that the Starship needs to generate negative lift to stay in the entry corridor without skipping back out into interplanetary space.  It also requires transitioning from negative lift early in the entry to positive lift later on, which will require rotating the Starship 180� around the flight path.  Both of these can be simulated to some extent in an Earth entry, but it's a very strange Earth entry, given Earth's atmosphere's lower scale height.

3) The flip and burn for landing occurs much higher, much faster, with the burn at much higher thrust.  The flip likely occurs at supersonic speeds.  I guess you could test a supersonic flip during and Earth EDL, but it'll take a lot more propellant, and may not be representative of the same maneuver on Mars.

So yeah, there are ways to reduce risk.  I don't think that's a good argument for not collecting a boatload of data during the actual Mars EDL.

[TheRadicalModerate]

If they have time for it, can a higher altitude pass through earth's atmosphere emulate Mars reentry?

I assume there is going to trials for fuel transfer. When this is successfully done, what are you then going to do with the refuelled Starship?

A bit off-topic, but I'd guess:

1) For a single Starship-to-Starship transfer test, maybe a boost into an HEEO, which should allow testing entry at 12-13km/s.

2) For a depot accumulation test followed by a depot-to-Starship refueling, that sounds like the precursor to the Option A demo test.

[TheRadicalModerate]

Probably not relevant for Starship PNT, but:  Blue Origin offers Mars communications orbiter concept.

The concept, called the Mars Telecommunications Orbiter (MTO), would provide what Blue Origin calls �multiple, steerable high-rate links� along with wide-area coverage to provide communications support to spacecraft on the Martian surface or in orbit. That would be supplemented by several relay spacecraft in low Mars orbit with UHF links for �legacy� spacecraft and to relay entry, descent and landing telemetry.

Sounds kinda familiar from this discussion.