Burn by Cara Loren

Overview

Burn is a subscription fitness app for Cara Loren, a fitness influencer with an audience of over two million. Subscribers pick a goal (build muscle, lose weight, or stay active) and get guided workout programs that progress toward it.

I joined Smash Creative in March 2021, took Burn over from the previous developer, and rebuilt it from the ground up. That version launched in June 2022. From then until I left in January 2026, I was the engineer responsible for it: features, releases, the backend it depends on, and later the contractors that were brought on to help.

How it's built

React Native on Expo, in TypeScript, with Firebase for auth. Content started on a Strapi backend I built. When that became the bottleneck for the content team, I moved it to Sanity with no downtime, writing the migration scripts and keeping the app stable through the cutover. After that, workout updates shipped without an engineering ticket.

Subscriptions run through RevenueCat. Its webhooks feed a retention pipeline I helped design: it catches trial conversions, dedupes them in Redis, and sends a physical thank-you letter through Scribeless. Daily churn dropped 9% after it went in. A goal-based progression engine gives each goal its own difficulty curve, and MRR grew 24% (from $22.8K to $28.2K) over the three months after it shipped.

Running it

Releases go through a GitHub Actions pipeline: tests and typechecks gate production, tags cut staging and production builds, the database backs up hourly, and Slack gets an alert when anything fails. Sentry, error boundaries, and a long tail of fixes brought the crash rate from 2.9 a day to 0.9. Three major Expo SDK upgrades (50 to 52, React Native 0.74 to 0.76) went through with the app staying stable across each one.

Afterward

In 2025 I rebuilt Burn from scratch on my own time to see what I'd do differently with everything I'd learned since. That version never shipped. It has its own page.