[AR] reentry Isp (was Re: SSTO)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Mon, 12 Feb 2018 00:40:02 -0500 (EST)
On Mon, 12 Feb 2018, William Claybaugh wrote:
Henry:
This is interesting; could you be explicit about how you calculate
effective Isp for rentry system mass?
Same as for a rocket: Isp = deltaV / ln(massbefore/massafter) / g0.
For example, a late-model Apollo CM at start of reentry weighed roughly
13klb, and the Block II TPS was about 1.5klb of that (ref: TN D-7564, the
Apollo Experience Report on TPS), which took off essentially all of the
CM's ~11km/s return velocity, so 11000 / ln(13.0/11.5) / g0 gives an
effective Isp of about 9150s.
(Either I just misremembered the number, or the 8000s I cited was figured
for the early lunar CMs, which were a bit lighter on reentry.)
And this is a large underestimate because no real reentry consumed more
than a fraction of the TPS thickness on the base, and the TPS on the upper
cone was almost certainly quite unnecessary. The basic design was fixed
at a time when heatshield scaling laws weren't fully understood, so it was
deliberately very conservative, and it also made assumptions about
worst-case reentry profile that ended up being quite unrealistic.
Henry
Other related posts: