So listening to the bit about electric power production made me wonder about the potential for hybrid electric propulsion. If a distributed electric propulsion element could be integrated with the SABRE hypersonic platform it might improve takeoff noise and thrust and allow improved subsonic efficiency.SABRE is an engine for an LV. For this to be a net win you have to factor in what the extra weight will buy you over the whole trajectory.
According to REL " SABRE - Synergetic Air Breathing Rocket Engine - is a new class of engine for propelling both high speed aircraft and spacecraft." and " is well suited for a variety of potential high-speed mission areas. SABRE enables more capable high-speed vehicles able to operate within and between expansive and unique flight envelopes, which improves many critical mission characteristics. " so from their point of view SABRE is no longer exclusively an LV engine and I was talking explicitly about their " Hypersonic Mission Applications", I highly doubt that the extra mass would pay for itself over a launch trajectory, although you never know until you caculate this stuff.
It would be an interesting question to ask REL how they see the relation between SABRE, SABRE class and Scimitar in light of where they are now with the technology and their current business model?
Do they see Scimitar as just a SABRE class engine or is it still a second generation engine?
Given they're trying to sell SABRE as an aircraft solution do they now have higher reliability goals?
It occurred to me that the SABRE cycle would integrate well with a a solid oxide fuel cell given that it produces high temperature hydrogen in the right temperature range and that it has excess hydrogen at points in the cycle and that the cryogenic hydrogen would be available to cool superconducting motors.And these motors are doing what exactly?
As I understand it current designs for next generation transport aircraft revolve around improving fuel efficiency and noise profile by increasing the effective bypass ratio through moving to hybrid electric distributed propulsion where rather than having a jet engine powering a single large bypass fan it powers a generator which feeds a large number of electric motors which collectively have a much larger bypass area than a single fan could practically have and thus have a greater efficiency at subsonic speeds. As part of this design space SOFC's are sometimes integrated into the jet engine for power generation and superconducting motors are often used for the powerful bypass fans, but without a cryogenic heat sink they aren't yet very practical.
A hypersonic platform that could loiter efficiently at subsonic speeds might be useful, or indeed one that could takeoff quietly and fly efficiently over land.Then you be looking at the Scimitar work REL did for LAPCAT.
Again, not if you listen to what REL is saying today and if you consider Scimitar as just a SABRE class engine. The question with Scimitar was wether you could just safely leave the bypass fan to open rotor in a hypersonic airstream, perhaps with distributed propulsion you can have your subsonic fan bypass without having to leave it open to the air stream.
The question I guess is could distributed motors be integrated in such a way that they can be protected at hypersonic speeds.Such motors would have to be integrated into the wing of the vehicle, not SABRE. Skylon's wing is sized for high speed takeoff because that's what REL trade studies indicate is the best way to save overall weight on the whole trajectory.
The major issue with fuel cells is that they are tough if you don't have a supply of H2. Once you've a supply there are about half a dozen approaches to choose from and Skylon is H2 powered to begin with. REL expect Skylon to use fuel cells for on orbit power anyway for reasons I don't quite understand why, given that water (useful for a human crew) is just a waste product for an uncrewed vehicle that has to be either stored or vented overboard.
Skylon is just REL's reference design for a SSTO launcher, for the hypersonic mission space we've already seen BAE's single engine design and according to the lecture LM are having fun with theirs so the question is could there be a viable design that could utilise this. Perhaps with a single engine design the fans could be wrapped around the mid section shrouding its rocket exhaust with a larger mass of slower moving air providing some muffling.