In order to evaluate the "adequacy" of current situation of how much more heat rejection capability Raptor would need, we should cover a few principals.
- Heat transfer from a heat exchanger is roughly proportional to air flow through and roughly proportional to temperature difference between heat source (coolant) and heat sink (ambient air);
- Air flow through heat exchangers is roughly proportional to dynamic pressure across them, which means it is also roughly proportional to airspeed squared;
- Heat rejected by an engine is roughly proportional to engine power;
- Thermostats for car engines typically open and then regulate top water temperature at 192F;
- If you have more than one effect, you multiply them together.
So how does this apply here? Well, we can postulate some numbers to get some ballpark estimates on each effect:
- Dynamic pressure - If current version of Raptor is adequate at 120 knots cruise, it would need 144% of current capability at a 100 knot climb speed;
- Ambient air effect - If Raptor is at 198F on a 50 F day, current delta T is 148F. If Raptor is to cool to 192 F (and thus be just barely regulating temperature) on a 100F day, it would need enough cooling when at a delta T of 92 F, or 161% of current capability;
- If Raptor is to climb at 100% power, that is roughly 100/60 = 167% of current capability;
- To do all of them simultaneously, that is 144%*161%*167% = 387%.
To climb at a reasonable airspeed, at full power, on a hot day, Raptor must have about four times the current ability.
But does PM need to do all that now? Well, if he is going to continue to climb on modest power, we can skip the higher power, but we have to keep the lower climb airspeed or it won't really climb. And he is going to have temperatures go up 50F over the next four months, so we have to keep the delta T. So that is still 232% of current heat rejection.
We now have a data based engineering estimate that Raptor will require the cooling of 2-1/2 to 4 times the current cooling. Put another way, the Raptor currently has 25% to 40% of the needed cooling right now, depending upon how you view the mission and power needs. Big shortfall to correct.
Now we can evaluate the idea of a second little heat exchanger in the nose. Yes, it might double the current forward heat exchanger capability, but what does it do to the whole system. Will that be enough? Hmmm, doubling 40% is 80% of needed rejection. Nope, it will still overheat.
Doubtless the heat exchangers aft, while inadequate, were doing something. Let's say the first forward heat exchanger is as capable as the aft stuff. We are at 40% of needed capacity now with both, so we were about at 20% before. Hmm adding another of those exchangers will add another 20% of needed capacity. That does not sound like much, does it? That would put Raptor at 60% of needed capacity to climb on 60% power on a hot day. Way inadequate....
Now are these numbers really accurate? Nope, not intended to be either. But as an indicator towards how much additional cooling is required, I think it points the way.
Billski
I guess this is evidence of why he's found himself alone (except for his adoring Youtube audience).
