<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>sandworm101</author><text>Mach 5 at 28km and mach 6 at 70km are not all that different.  Mach slows down with altitude.  And specific altitude numbers dont matter much if you are aiming for orbit.  All that really matters is horizontal velocity.  The first stages of both schemes are relatively slow.  Both get the bulk of thier orbital velocity from thier second stages.&lt;p&gt;SpaceX takes a very non-aggressive approach with its first stages.  When they detach they arent moving nealy as fast as other rocket first stages.  That is done so they dont slam back into the lower atmosphere but it means relying upon second&amp;#x2F;third stages to do the horizontal work.  By the raw physics, it isnt the most energy efficient approach.  It is a compromise to facilitate recovery.</text><parent_chain><item><author>trhway</author><text>&amp;gt;After shutting the inlet cone off at Mach 5.14, and at an altitude of 28.5 km, the system continues as a closed-cycle high-performance rocket engine burning liquid oxygen and liquid hydrogen from on-board fuel tanks&lt;p&gt;the benchmark of today - SpaceX - separates the first stage at Mach6-8 at 65-80km height. Thus, the SpaceX second stage has  less delta-v to add and don&amp;#x27;t have to carry all that weight - the SABRE would naturally be bigger than the 2nd stage for the same payload. The economical space related case of HOTOL&amp;#x2F;SABRE single-stage-to-orbit pretty much disappeared for a foreseeable future when SpaceX started to recover the 1st stage. That not to say about suborbital flights SF-Shanghai though. Or even &amp;quot;just&amp;quot; hypersonic as SABRE tech seems to successfully deal with one of the major&amp;#x2F;key obstacles for it (and that is huge, i&amp;#x27;m in no way trying to dismiss their work). And one can see how SABRE can be used in some cases as a  convenient 1st stage, i.e. by being able to launch from any big airport instead of only specially designed spaceports (such a 1st stage would probably be better than that &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;Pegasus_(rocket)#Carrier_aircraft&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;Pegasus_(rocket)#Carrier_aircr...&lt;/a&gt;).</text></item><item><author>nickpinkston</author><text>In case anyone was confused by the article&amp;#x27;s description, here&amp;#x27;s the Wikipedia:&lt;p&gt;&amp;quot;SABRE (Synergetic Air Breathing Rocket Engine) is a concept under development by Reaction Engines Limited for a hypersonic precooled hybrid air-breathing rocket engine. The engine is being designed to achieve single-stage-to-orbit capability, propelling the proposed Skylon spaceplane to low Earth orbit. SABRE is an evolution of Alan Bond&amp;#x27;s series of liquid air cycle engine (LACE) and LACE-like designs that started in the early&amp;#x2F;mid-1980s for the HOTOL project.&lt;p&gt;The design comprises a single combined cycle rocket engine with two modes of operation. The air-breathing mode combines a turbo-compressor with a lightweight air precooler positioned just behind the inlet cone. At high speeds this precooler cools the hot, ram-compressed air, which would otherwise reach a temperature that the engine could not withstand, leading to a very high pressure ratio within the engine. The compressed air is subsequently fed into the rocket combustion chamber where it is ignited along with stored liquid hydrogen. The high pressure ratio allows the engine to provide high thrust at very high speeds and altitudes. The low temperature of the air permits light alloy construction to be employed and allow a very lightweight engine—essential for reaching orbit. In addition, unlike the LACE concept, SABRE&amp;#x27;s precooler does not liquefy the air, letting it run more efficiently.&lt;p&gt;After shutting the inlet cone off at Mach 5.14, and at an altitude of 28.5 km, the system continues as a closed-cycle high-performance rocket engine burning liquid oxygen and liquid hydrogen from on-board fuel tanks, potentially allowing a hybrid spaceplane concept like Skylon to reach orbital velocity after leaving the atmosphere on a steep climb.&amp;quot;&lt;p&gt;&lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;SABRE_(rocket_engine)&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;SABRE_(rocket_engine)&lt;/a&gt;</text></item></parent_chain></comment><story><title>Skylon’s SABRE Engine Passes a Big Test</title><url>https://www.universetoday.com/143810/skylons-sabre-engine-passes-a-big-test/</url></story>
<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>SECProto</author><text>Imagine putting SABRE on the first stage though - the engines would be heavier, but using air instead of carrying oxygen, for the full burn of the first stage. The payload mass fraction would be dramatically higher without carrying the oxidizer mass on board.</text><parent_chain><item><author>trhway</author><text>&amp;gt;After shutting the inlet cone off at Mach 5.14, and at an altitude of 28.5 km, the system continues as a closed-cycle high-performance rocket engine burning liquid oxygen and liquid hydrogen from on-board fuel tanks&lt;p&gt;the benchmark of today - SpaceX - separates the first stage at Mach6-8 at 65-80km height. Thus, the SpaceX second stage has  less delta-v to add and don&amp;#x27;t have to carry all that weight - the SABRE would naturally be bigger than the 2nd stage for the same payload. The economical space related case of HOTOL&amp;#x2F;SABRE single-stage-to-orbit pretty much disappeared for a foreseeable future when SpaceX started to recover the 1st stage. That not to say about suborbital flights SF-Shanghai though. Or even &amp;quot;just&amp;quot; hypersonic as SABRE tech seems to successfully deal with one of the major&amp;#x2F;key obstacles for it (and that is huge, i&amp;#x27;m in no way trying to dismiss their work). And one can see how SABRE can be used in some cases as a  convenient 1st stage, i.e. by being able to launch from any big airport instead of only specially designed spaceports (such a 1st stage would probably be better than that &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;Pegasus_(rocket)#Carrier_aircraft&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;Pegasus_(rocket)#Carrier_aircr...&lt;/a&gt;).</text></item><item><author>nickpinkston</author><text>In case anyone was confused by the article&amp;#x27;s description, here&amp;#x27;s the Wikipedia:&lt;p&gt;&amp;quot;SABRE (Synergetic Air Breathing Rocket Engine) is a concept under development by Reaction Engines Limited for a hypersonic precooled hybrid air-breathing rocket engine. The engine is being designed to achieve single-stage-to-orbit capability, propelling the proposed Skylon spaceplane to low Earth orbit. SABRE is an evolution of Alan Bond&amp;#x27;s series of liquid air cycle engine (LACE) and LACE-like designs that started in the early&amp;#x2F;mid-1980s for the HOTOL project.&lt;p&gt;The design comprises a single combined cycle rocket engine with two modes of operation. The air-breathing mode combines a turbo-compressor with a lightweight air precooler positioned just behind the inlet cone. At high speeds this precooler cools the hot, ram-compressed air, which would otherwise reach a temperature that the engine could not withstand, leading to a very high pressure ratio within the engine. The compressed air is subsequently fed into the rocket combustion chamber where it is ignited along with stored liquid hydrogen. The high pressure ratio allows the engine to provide high thrust at very high speeds and altitudes. The low temperature of the air permits light alloy construction to be employed and allow a very lightweight engine—essential for reaching orbit. In addition, unlike the LACE concept, SABRE&amp;#x27;s precooler does not liquefy the air, letting it run more efficiently.&lt;p&gt;After shutting the inlet cone off at Mach 5.14, and at an altitude of 28.5 km, the system continues as a closed-cycle high-performance rocket engine burning liquid oxygen and liquid hydrogen from on-board fuel tanks, potentially allowing a hybrid spaceplane concept like Skylon to reach orbital velocity after leaving the atmosphere on a steep climb.&amp;quot;&lt;p&gt;&lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;SABRE_(rocket_engine)&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;en.wikipedia.org&amp;#x2F;wiki&amp;#x2F;SABRE_(rocket_engine)&lt;/a&gt;</text></item></parent_chain></comment><story><title>Skylon’s SABRE Engine Passes a Big Test</title><url>https://www.universetoday.com/143810/skylons-sabre-engine-passes-a-big-test/</url></story>