<story><title>DARPA moving forward with nuclear thermal engine design</title><url>https://orbitalindex.com/archive/2022-05-25-Issue-170/</url></story><parent_chain><item><author>jhgb</author><text>I don&amp;#x27;t see how an NTR helps you in any way to get to Mars or back. Heavy engine, voluminous tanks (~70 kg&amp;#x2F;m³), criminally wasted ISRU material (you have to throw away 88.9% of the water that you mine on site, whereas a hydrolox or methalox system uses almost all of it and the methalox system can even mix it with considerable amount of CO₂ for better system performance). The performance figures for such a system will be &lt;i&gt;terrible&lt;/i&gt;. At best a LANTR (not just an NTR) might be somewhat useful for cislunar uses. For Mars flights not even LANTR may be useful.</text></item><item><author>robonerd</author><text>The PDFs here have a lot more information: &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;sam.gov&amp;#x2F;opp&amp;#x2F;af490b568d2a438498afa1e80bce63e5&amp;#x2F;view&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;sam.gov&amp;#x2F;opp&amp;#x2F;af490b568d2a438498afa1e80bce63e5&amp;#x2F;view&lt;/a&gt;&lt;p&gt;A few takeaways; they intend for such an engine to eventually support long duration human spaceflight (going to Mars.)  The propellant for the NTR engine to be liquid hydrogen.   One of the problems DARPA anticipates with using such an engine for such a mission is needing to store liquid hydrogen longer than the present state of the art.&lt;p&gt;The PDF doesn&amp;#x27;t seem to mention it, but I think the Advanced Cryogenic Evolved Stage (ACES) is probably relevant to this project.  Does anybody know what kind of duration they expect to get from ACES?  I&amp;#x27;m not sure but I think it&amp;#x27;s weeks, not months.</text></item></parent_chain><comment><author>robonerd</author><text>I&amp;#x27;m skeptical too, but DARPA is saying the DRACO program is for getting to&amp;#x2F;from Mars quickly:&lt;p&gt;&amp;gt; &lt;i&gt;The DRACO program intends to develop novel nuclear thermal propulsion (NTP) technology to
enable time-critical missions over vast distances in cislunar space. Unlike propulsion
technologies in use today, NTP can achieve high thrust-to-weights similar to chemical
propulsion but with two to five times the efficiency. This enables NTP systems to be both faster
and smaller than electric and chemical systems, respectively. The propulsive capabilities
afforded by NTP will enable the United States to maintain its interests in space, and to expand
possibilities for the National Aeronautics and Space Administration (NASA)’s long-duration
human spaceflight missions (i.e., to Mars). Because of the ability to transit space faster than
other propulsion systems, the NTR engine can return astronauts to Earth much faster in case of
an emergency and similarly ensure reduction of overall trip time and exposure to deleterious
impacts to astronaut health which come with long-term spaceflight.&lt;/i&gt;</text></comment>
<story><title>DARPA moving forward with nuclear thermal engine design</title><url>https://orbitalindex.com/archive/2022-05-25-Issue-170/</url></story><parent_chain><item><author>jhgb</author><text>I don&amp;#x27;t see how an NTR helps you in any way to get to Mars or back. Heavy engine, voluminous tanks (~70 kg&amp;#x2F;m³), criminally wasted ISRU material (you have to throw away 88.9% of the water that you mine on site, whereas a hydrolox or methalox system uses almost all of it and the methalox system can even mix it with considerable amount of CO₂ for better system performance). The performance figures for such a system will be &lt;i&gt;terrible&lt;/i&gt;. At best a LANTR (not just an NTR) might be somewhat useful for cislunar uses. For Mars flights not even LANTR may be useful.</text></item><item><author>robonerd</author><text>The PDFs here have a lot more information: &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;sam.gov&amp;#x2F;opp&amp;#x2F;af490b568d2a438498afa1e80bce63e5&amp;#x2F;view&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;sam.gov&amp;#x2F;opp&amp;#x2F;af490b568d2a438498afa1e80bce63e5&amp;#x2F;view&lt;/a&gt;&lt;p&gt;A few takeaways; they intend for such an engine to eventually support long duration human spaceflight (going to Mars.)  The propellant for the NTR engine to be liquid hydrogen.   One of the problems DARPA anticipates with using such an engine for such a mission is needing to store liquid hydrogen longer than the present state of the art.&lt;p&gt;The PDF doesn&amp;#x27;t seem to mention it, but I think the Advanced Cryogenic Evolved Stage (ACES) is probably relevant to this project.  Does anybody know what kind of duration they expect to get from ACES?  I&amp;#x27;m not sure but I think it&amp;#x27;s weeks, not months.</text></item></parent_chain><comment><author>Symmetry</author><text>This is more the sort of engine you develop if you&amp;#x27;re going for an Apollo style mission where there&amp;#x27;s a mother craft that goes into orbit and a separate lander goes down to the surface. A NTR&amp;#x27;s poor TWR compared to conventional combustion rockets means it would be a bad ascent stage.&lt;p&gt;I wouldn&amp;#x27;t assume the plan relies on ISRU at all but if it do having to carry the resulting hydrogen up to orbit on the ascent stage will be a big limiting factor so not keeping the oxygen isn&amp;#x27;t so large a flaw.  And if you&amp;#x27;re carrying the fuel to orbit on another rocket you want to get as high an ISP as you can manage with what you bring up.&lt;p&gt;All of which isn&amp;#x27;t to say this would be a good plan.  I&amp;#x27;ve drunk the SpaceX koolaid on the topic.  But if it&amp;#x27;s a bad plan at least it isn&amp;#x27;t a stupid one and there are reasons behind things.</text></comment>