Unfortunately, it's incredibly hard to convince an iPhone to actually send an SMS to a specific number, at least if the contact is registered for iMessage...
If you have no data connection it will fall back to SMS.
What if I know that the recipient doesn’t have a data connection, but I do?
This is all incredibly clunky and one of the weakest points of iOS in my view. How can it be so hard to provide a switch that lets me use SMS proactively?
If I’m reading the docs correctly the satellites will offer SMS service directly so you won’t need an app, you’ll just need your carrier or roaming provider to support it.