lancelot Wed Jun 02, 2021 12:15 am
Big_Gazza wrote:IMHO the crewed Dragon is a death trap due to the absurd decision to integrate escape rockets within the crewed capsule rather than on a jettisonable tower. I can't for the life of me understand how they could possibly risk assess the hazard of maintaining hypergolic propellents within 4 enclosures built into the walls of the capsule. Heck, we've already seen one test capsule get blown to smithereens during a post-recovery Draco test, and that nicely illustrated the nature of the hazard. On the other hand, these are the same fucking geniuses that decided to perform fuelling of the stack after the crew board the vehicle... hardly keeping risks ALARP (as low as reasonably practical).
The only question in my mind is how missions will be flown until a crewed Dragon fails catastrophically and kills its crew. Its a case of when, not if.
The idea is to cut cost and save weight. The decision to use those engines also reduces the black zones where you can't abort.
The escape tower needs to be ejected some time after takeoff. After that there is a time period where aborting is pretty much impossible.
SpaceX has no experience with solid rockets or the facilities to produce them.
Outsourcing the solid rockets in the escape tower would be cost prohibitive. Try reading about the cost of the escape tower on the Orion capsule for example.
Using liquid engines in this configuration means you can reuse them together with the capsule.
It was supposed to save weight because you could basically use the same engines and fuel you would use in the escape tower for the propulsive landing.
For what it's worth the Soviet Union had plans to make a Zarya capsule that was to be launched on the Zenit rocket as a replacement for Soyuz which used a similar system.
The main difference Dragon V2 has is it has a parachute large enough to brake the capsule even without propulsive landing.
I agree that having a hygroscopic propellant like hydrazine in a vehicle which does water landings is a disaster waiting to happen though.
For what it's worth the Shuttle also used hypergolics in the OMS/RCS system and AFAIK it never caused a major problem.
Although it made upkeep of the Shuttle hideously expensive because of the complex ground handling required to handle the fuel.