PrepTest 143, Section 3, Question 17
Rocket engines are most effective when exhaust gases(66 more words not shown)
Which one of the following(6 more words not shown)
- AEquipping a rocket's(20 more words not shown)
- BAt some point(12 more words not shown)
- CA rocket with(10 more words not shown)
- DFor a rocket(23 more words not shown)
- EFor a rocket(28 more words not shown)
Reading the argument
A short nozzle works best low down, and a long one works better high up. Having both sounds sensible. But does that require two separate nozzles?
What if a nozzle could change length during flight? It could be short in the dense lower atmosphere and extend in the thin upper atmosphere. I'm not claiming engineers can build that design; it's a possibility the argument hasn't ruled out. It makes me question the leap from needing two lengths to needing both kinds of nozzle on every rocket.
What we’re looking for
For the required assumption, I might expect an answer ruling out that adjustable design. But that isn't the only possible gap. We need to read the choices for something the argument depends on, even if it differs from our first objection.
Working the answers
- A
A design could be much harder to build and still be required for the best performance. The argument compares how effectively the engines work, not how easy different arrangements are to install. It doesn't need both nozzle lengths to be nearly as easy to provide as equal-length nozzles.
- BCorrect
Here's a different opening in the argument: do all rockets even reach the conditions for which a long nozzle is needed? The author concludes that all rockets need both lengths for maximum effectiveness during their ascents. But the stated reason for a long nozzle concerns the thin upper atmosphere.
If a rocket's entire ascent stayed below that region, this argument would give us no reason it needed a long nozzle. B closes that gap by saying every rocket passes through the upper atmosphere at some point. It doesn't answer the adjustable-nozzle objection, but a required assumption only needs to address a condition the argument relies on.
- C
Reaching a high altitude and working most effectively there are different things. A rocket with short nozzles might reach that altitude while operating less effectively than one with long nozzles. The passage doesn't establish impossibility, and the argument doesn't require it.
- D
This changes “most effectively” to “effectively.” The stated pressure relationship describes the best performance. It need not be required for any effective performance at all. An engine could work reasonably well without being at its best.
- E
The argument doesn't require both lengths on the same engine. Imagine a rocket with eight engines, four using short nozzles and four using long ones. It would have both kinds without any single engine having both. E imposes a particular arrangement beyond the conclusion's claim about the rocket's engines collectively.
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