▁contract ▁for ▁the ▁Av ro ▁ 7 2 0 ▁was ▁eventually ▁cancel led . ▁One ▁of ▁the ▁reasons ▁for ▁prefer ring ▁the ▁SR . 5 3 ▁was ▁although ▁the ▁aircraft ▁was ▁development ally ▁behind , ▁its ▁use ▁of ▁hydro gen ▁per ox ide ▁as ▁an ▁ox id iser ▁was ▁viewed ▁as ▁less ▁problem atic ▁than ▁the ▁Av ro ▁ 7 2 0 ' s ▁use ▁of ▁liquid ▁o xygen , ▁which ▁pos ed ▁a ▁fire ▁ha z ard . ▁According ▁to ▁Wood , ▁there ▁was ▁a ▁preference ▁for ▁the ▁HT P ▁approach , ▁as ▁used ▁by ▁the ▁SR . 5 3 , ▁alongside ▁an ▁unw illing ness ▁to ▁continue ▁to ▁support ▁the ▁operation ▁of ▁two ▁different ▁fuel ▁program mes . ▁However , ▁the ▁valid ity ▁of ▁this ▁reason ▁is ▁und erm ined ▁somewhat ▁by ▁the ▁fact ▁that ▁Av ro ▁had ▁been ▁propos ing ▁to ▁switch ▁to ▁using ▁HT P ▁and ▁the ▁Spect re , ▁the ▁same ▁engine ▁that ▁was ▁used ▁by ▁the ▁SR . 5 3 . ▁Another ▁factor ▁that ▁had ▁influenced ▁the ▁can cellation ▁was ▁the ▁hes it ancy ▁of ▁the ▁R AF ▁to ▁back ▁either ▁project , ▁the ▁service ▁had ▁apparently ▁wanted ▁to ▁wait ▁until ▁after ▁flight ▁evalu ations ▁had ▁been ▁conducted ▁before ▁it ▁was ▁to ▁make ▁any ▁determ ination ▁on ▁its ▁preference . ▁ ▁During ▁late ▁ 1 9 5 3 , ▁Sau nd ers - R oe ▁comm enced ▁work ▁upon ▁a ▁derivative ▁design , ▁which ▁was ▁designated ▁as ▁the ▁SR . 1 7 7 . ▁B ren nan