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Comment by mitchty | Hacker News Reader

Wouldn't dragon need the rockets to land on Mars? Parachutes aren't gonna do jack in Mars atmosphere.

I thought that was the bigger reason for the retro rockets.

That's not true, Curiosity used a parachute for part of its landing - it just needs to be pretty large to have an effect in atmosphere so thin:

https://www.space.com/16878-mars-rover-landing-sky-crane-gui...

SpaceX pivoted to building an entirely new rocket for Mars called Starship.

SpaceX has no plans to send Dragon to mars

SpaceX had plans to send Dragon to mars, that were cancelled at about the same time propulsive landing was cancelled for commercial crew. It's likely that cancelling it for commercial crew also cancelled the mars plans.

I think they cancelled it because it was a hair brained scheme from the get go. You need a lot of juice to not only land on mars, but then to propulse back off of it.

Red Dragon was always going to be one way, as a lander for some notional science payload. It was still a bit harebrained because it wasn't clear how to deploy that payload, whether rover or other, through the hatch. (I think I once saw a slide that had a subsurface drill deployed from inside the capsule by drilling through the heat shield.)

If propulsive landing was a harebrained scheme, we'd have never had the lunar landings.

Propulsive landing is awesome if the surface can stand up to it. Propulsive landing on mars with only a crew capsule is a little bonkers.

It takes ~3,000 m/s of dV to slow down, and do a powered landing on the Moon.

It takes takes ~6.400 m/s of dV to slow down, and do a powered landing on Mars.

Thanks to the tyranny of the rocket equation, it takes ~5x more fuel to do a powered landing on Mars, than it does on the Moon. For the entire mission. That's five Saturn V rockets, if you want to get a similar payload to Mars and back.

Shaving most of the dV requirements of the 'land on Mars' leg of the journey will reduce that requirement, from ~5x to ~2x.

As I understand the numbers something like 99.9% of that 6400 m/s dV is done via aerobreaking. Which makes it actually much easier to land on mars than on the moon energy wise.

The falcon heavy and dragon were almost definitely up to the task, given that SpaceX was for a short period of time actively looking for customers.

Source on the 99.9% is this talk (about starship, but I doubt red dragon would have been substantially different in terms of delta v): https://www.youtube.com/watch?v=bysu8XN5OfY

>99% is possible for the Starship because it has a lot of pitch control, and can fly a trajectory in the atmosphere where it maximizes lift for as long as it can.

A capsule doing a simpler landing generally needs to get rid of about 400m/s using other methods, based on NASA's past missions.

> It takes ~3,000 m/s of dV to slow down, and do a powered landing on the Moon.

With generous reserves, I guess. The speed on a low Moon orbit is about 1700 m/s, Apollo's LEMs had about (less than?) 2500 m/s delta-V for landing.

> It takes takes ~6.400 m/s of dV to slow down, and do a powered landing on Mars.

Seems too generous. Escape velocity for Mars is 5.027 km/s, so low orbital speed is about 3.6 km/s; more importantly, there is atmosphere - which maybe too thin to land safely on parachutes, but it definitely is going to reduce speed from orbital to some extent.

Propulsive landing on Mars doesn't take anywhere near that much. The atmosphere is thick enough that you can bleed 90% of your energy with a heat shield.

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