Efficiency in Space Capsule Docking Process

Question

I'm watching the first operational Crew Dragon Capsule proceed through the regimented, and VERY careful International Space Station (ISS) docking process, which was turned on after pausing another episode of The Expanse. That's only relevant insofar as making it even more aware of how this docking process is like watching paint dry.

With respect to the current global space faring organizations, both government and private sector, are there any discussions or development efforts amongst the those who develop the operational SOPs to reduce the time it currently takes to dock a spacecraft to another structure in orbit?

I can think of a few justifications supporting the development of a more efficient launch to dock duration: safety, quick turn requirements, emergency response, etc., I suspect there are more. I'm a lifelong aviation careerist, so safety is front and center for me every day, and get that there are lives at stake. But with Elon Musk's dream of space travel becoming as routine as air travel today, I'm wondering if turning up the dial with the automated flight guidance systems which are relied upon today.

Or is there simply no interest in speeding up the process, at least at this point in the game?

Answer 1

Several political and practical factors are in play. The first is that the ISS is a multinational construction, so whole of craft risks from a high speed docking need to be signed off on by all parties, including those who are using other types of craft to deliver supplies (and therefore have no benefit from the risks being taken). It is perfectly possible to parallel park using a handbrake turn, but if it is your car in the next bay up you may have an opinion on people doing so.

If a station is owned by a single entity, and that entity is the one benefiting from the increased risks of a faster approach it becomes much easier to get sign off.

There is a rich history of thruster/control system problems in space flight in ways that would make a high speed docking fatal so it is a very known risk.

During docking hot exhaust gases from thrusters will be sent towards the station, this is a hazard to sensors, solar panels and even the base structure. For this reason approaches are flown that minimise the volume and intensity of the thruster impingement. Flying an approach at say 100kmh would require a substantial amount of rocket blast directly at the station during final docking to drop off the speed where current profile has all but the very final stages of approach thrusting off axis.

There is also orbital dynamics in play, per the linked video the approach is flown similar to a Hohmann transfer both for efficiency and because it makes things fail safe for most of approach. This means that the major maneuver to commence approach will happen half an orbit period before docking, or about 45 minutes in LEO (which is fixed no mater how small the distance to close). It is certainly possible to brute force faster intercepts using more fuel but suspect burning fuel to save a couple of minutes will always be a challenging sell in space risk managment.

Answer 2

@GremlinWrangler talked about the approach aspect, but I'm going to mention the docking aspect.

Docking is fundamentally a controlled, low-speed collision between two large pieces of hardware (ISS comes in at around 1M pounds, or around 450,000 kg). The soft capture system of the docking mechanism has to serve as alignment, capture, and shock absorber for this collision before retracting to complete hard capture.

Why is it so slow? Well, let's take a look at the requirements for the international docking standard. Paragraph 3.3.1 of the International Docking Standard Specification Interface Definition Document, rev. E (available at that link) outlines the relevant aspects of the soft capture system.

Here are the key limiting values:

• Approach velocity must be between 5 and 10 cm/s. Too slow and the vehicle may bounce off without engaging the docking latch. Too fast and the soft capture system won't be able to dampen the motion.
• Sideways drift must be no more than 4 cm/s.
• Attitude must be steady within 0.2 degrees/sec.
• The approaching vehicle can be no more than 10 cm off center or misaligned by more than 4 degrees at first contact.

Docking is slow because the hardware requires it. You can't meet this envelope without going painfully slow.