# Challenger disaster: how full was the external tank at the time of destruction?

We have all heard about the Challenger disaster.

The external tank at this point suffered a complete structural failure, the LH2 and LOX tanks rupturing, mixing, and igniting, creating a fireball that enveloped the whole stack.

I know it is not the explosion which destroyed the whole vehicle but aerodynamic forces, but I wonder:

• How much liquid fuel and oxidizer were left in the external tank at the time of its destruction?

Bonus question:

• How many seconds were left before SRB separation?

I did a crude spreadsheet sim using the Rogers Commission report to get throttle times, to wit:

• Throttle down to 94% at 24 seconds
• Throttle down to 65% at 42 seconds
• Throttle up to 104% at 65 seconds

I neglected startup propellant consumption and assumed step function throttling. I took liftoff O2 load to be 1,387,457 lb and H2 load to be 234,265 lb.

I used 154 lb/sec/engine for H2 flow at 104% and 925 lb/sec/engine for O2 flow. I assumed flow rates scale linearly with throttle setting - this is a decent assumption from what I remember.

Given that, I got ~ 87.7% O2 load at 73 seconds and ~ 87.9% H2 load at the same time.

With my 1 sec step time and googled liftoff loading numbers, it's approximate - real approximate - but you can see that there was a lot of prop left in the ET.

SRB Sep time is ~ 2:03 so they were ~60% burned out.

Note: Calculations in my answer to How much propellant is used up until liftoff? show that ~ 0.5% of the ET propellant is used up pre-liftoff, which would bring the percentages at 73 seconds down to ~87.2% O2 and ~87.4% H2. I did not redo the spreadsheet.

• Thanks, that was going to be bugging me and the only thing I could think of to work out the result was taking a rendered plot of the throttle schedule and counting pixels under the curve. ;) – Russell Borogove Feb 24 '17 at 2:38

The breakup of Challenger occurred about 73 seconds into flight. Main engine cutoff normally occurs about 510 seconds into flight, implying that about 86% of the fuel would be remaining. (Many sources give 480 seconds, but I suspect that's a simple division of the tankage mass by the full-throttle consumption rate; looking at actual mission reports supports the ~510 second main engine burn figure.)

However, the throttle schedule of the main engines isn't constant, so it would have been somewhat more than that -- the accident came shortly after the end of the max-Q "throttle bucket", during which time less fuel would be used, but there's also a scheduled throttle-down toward the end of the main engine burn which partially cancels that out.

In any case, at that point, the tank would be holding something more than 540 tons of liquid oxygen and 91 tons of liquid hydrogen.

The boosters should burn for about 127 seconds, so they were more than halfway there, with 54 seconds remaining.

• You are correct, nominal MECO is ~ 8 minutes 30 seconds. The Space Shuttle Missions Summary lists MECO time for all flights. google.com/… – Organic Marble Feb 24 '17 at 1:38

A few facts:

• SRB Burn Time is 127 seconds
• Start of Challenger Incident- 64 s
• Vehicle breakup- 72 s
• Nominal time to orbit- 510 s.

So the SRBs were about half-way done with their burn time before the vehicle started to break up. The Space Shuttle Main Engine actually produced less than 1 g thrust until about the time of SRB separation. If the SRBs could have somehow been safely released, it is possible the Space Shuttle could have turned around and returned to land at Kennedy Space Center, but the SRBs could not have have been released.

Also, the Main Tank had the vast majority of it's fuel. For much of the time with the SRBs attached, the Main Engine was not throttled at 100%, to not hit too high of resistance. I don't have an exact number, but I would expect that greater than 90% of the fuel in the tank was still there at the time of vehicle breakup.